Science with SRT

Proper motion study of the 6.7 GHz methanol maser rings. I. A sample of sources with little variation

(Published in Astronomy & Astrophysics, Volume 686, id.A275, 23 pp. (2024)

by A. Bartkiewicz, A. Sanna, M. Szymczak, L. Moscadelli, H.J. van Langevelde, P. Wolak, A. Kobak and M. Durjasz

Context. Methanol masers at 6.7 GHz are well-known signposts of high-mass star-forming regions. Due to their high brightness, they enable us to derive the three-dimensional gas kinematics near protostars and young stars. Aims: We aim to understand the origin of the ring-like structures outlined by methanol maser emission in a number of sources. This emission could be, a priori, spatially associated with an outflow and/or disc around a high-mass protostar. In cases of expansion or rotation, maser proper motions should be, for instance, diverging from the ring centre or perpendicular to the ring radius. Methods: Using sensitive, three-epoch observations spanning over 8 yr with the European VLBI Network, we have started the most direct investigations of maser rings using very accurate proper motion measurements with uncertainties below ~1 km s−1. Results: We present full results for the five targets of our sample, G23.207−00.377, G23.389+00.185, G28.817+00.365, G31.047+00.356, and G31.581+00.077, where proper motions show similar characteristics; maser cloudlets do not move inwards towards the centre of the rings but rather outwards. Conclusions: In five of the six targets, expansion motions prevail. Only in the case of G31.581+00.077 can a scenario of disc-like rotation not be excluded. Complementary observations of thermal tracers as well as searching for ultra-compact H II regions in the same sources are needed. Although the overall morphology of the maser emission has remained stable, the intensities of individual maser cloudlets varied from epoch to epoch, suggesting internal instabilities.

High-resolution radio imaging of the two particle-accelerating colliding-wind binaries HD 167971 and HD 168112

(Published in Astronomy & Astrophysics, Volume 682, id.A160, 8 pp. (2024)

by M. De Becker, B. Marcote, T. Furst, P. Benaglia

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole. The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from general relativity. With a baseline of 17 years of observations, there was a shift in the jet’s transverse position, possibly arising from an 8- to 10-year quasi-periodicity. However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years for the variation in the position angle of the jet. We infer that we are seeing a spinning black hole that induces the Lense–Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation.

Weighted Stacking of Radio Images Affected by Noise and Interfering Signals

(Published in The Astrophysical Journal Supplement Series, Volume 272, Issue 1, id.10, 12 pp. (2024)

by G. Murgia and S. Fatigoni

We implement an algorithm based on the weighted stacking of astronomical images that can combine different observations of the same region of the sky removing the interfering signals. We develop a C++ code that takes as input a set of spectral cubes and computes the local weights of the intensity for each pixel of every channel. The weights are calculated as the inverse variance of the nearby pixels and are used to compute the weighted merge of the input files. Astronomical sources, present in all cubes, are preserved by the weighted average. However, interfering signals, present in specific cubes and a certain frequency range, are down-weighted in the average and removed from the output spectral cube. We present the results obtained by analyzing simulated spectral cubes containing astronomical sources, noise, and a random set of interferences of different intensities and spectral occupations. The performance of the algorithm is evaluated by comparing the output result with the input sky-model image. Finally, we present the results obtained by applying the method to a set of real data consisting of observations of the Andromeda galaxy, Messier 31 (M31), at 6.6 GHz obtained with the Sardinia Radio Telescope.

Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes II. Evidence of coronal emission

(Published in Astronomy & Astrophysics, Volume 684, id.A123, 12 pp. (2024)

by M. Marongiu, A. Pellizzoni, S. Righini, S. Mulas, R. Nesti, A. Burtovoi, M. Romoli, G. Serra, G. Valente, E. Egron, G. Murtas, M.N. Iacolina, A. Melis, S.L. Guglielmino, S. Loru, P. Zucca, A. Zanichelli, M. Bachetti, A. Bemporad, F. Buffa, R. Concu, G.L. Deiana, C. Karakotia, A. Ladu, A. Maccaferri, P. Marongiu, M. Messerotti, A. Navarrini, A. Orfei, P. Ortu, M. Pili, T. Pisanu, G. Pupillo, P. Romano, A. Saba, L. Schirru, C. Tiburzi, L. Abbo, F. Frassati, M. Giarrusso, G. Jerse, F. Landini, M. Pancrazzi, G. Russano, C. Sasso, and R. Susino

We aim to constrain the temperature (T) and density (N) distributions of the solar atmosphere through observations in the centimetric radio domain. We employed single-dish observations from two of the INAF radio telescopes at the K-band frequencies (18 – 26 GHz). We investigated the origin of the significant brightness temperature (TB) detected up to the upper corona (at an altitude of ∼ 800 Mm with respect to the photospheric solar surface). To probe the physical origin of the atmospheric emission and to constrain instrumental biases, we reproduced the solar signal by convolving specific 2D antenna beam models. We performed an analysis of the solar atmosphere by adopting a physical model that assumes the thermal bremsstrahlung as the emission mechanism, with specific T and N distributions. We compared the modelled TB profiles with those observed by averaging solar maps obtained at 18.3 and 25.8 GHz during the minimum of solar activity (2018 – 2020). We probed any possible discrepancies between the T and N distributions assumed from the model and those derived from our measurements. The T and N distributions are compatible (within a 25% of uncertainty) with the model up to ∼ 60 Mm and ∼ 100 Mm in altitude, respectively. Our analysis of the role of the antenna beam pattern on our solar maps proves the physical nature of the atmospheric emission in our images up to the coronal tails seen in our TB profiles. Our results suggest that the modelled T and N distributions are in good agreement (within 25% of uncertainty) with our solar maps up to altitudes of ∼ 100 Mm. A subsequent, more challenging analysis of the coronal radio emission at higher altitudes, together with the data from satellite instruments, will require further multi-frequency measurements.

Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes I. Solar radius

(Published in Astronomy & Astrophysics, Volume 684, id.A122, 18 pp.(2024)

by M. Marongiu, A. Pellizzoni, S. Mulas, S. Righini, R. Nesti, G. Murtas, E. Egron, M. N. Iacolina, A. Melis, G. Valente, G. Serra, S. L. Guglielmino, A. Zanichelli, P. Romano, S. Loru, M. Bachetti, A. Bemporad, F. Buffa, R. Concu, G. L. Deiana, C. Karakotia, A. Ladu, A. Maccaferri, P. Marongiu, M. Messerotti, A. Navarrini, A. Orfei, P. Ortu, M. Pili, T. Pisanu, G. Pupillo, A. Saba, L. Schirru, C. Tiburzi, and P. Zucca

We determine the mean, equatorial, and polar radii of the Sun in the frequency range 18.1 – 26.1 GHz. We employed single-dish observations from the newly appointed Medicina Gavril Grueff Radio Telescope and the Sardinia Radio Telescope (SRT) in five years, from 2018 to mid-2023, in the framework of the SunDish project for solar monitoring. Two methods for calculating the radius at radio frequencies were employed and compared: the half-power, and the inflection point. To assess the quality of our radius determinations, we also analysed the possible degrading effects of the antenna beam pattern on our solar maps using two 2D models (ECB and 2GECB). We carried out a correlation analysis with the evolution of the solar cycle by calculating Pearson’s correlation coefficient ρ in the 13-month running means. We obtained several values for the solar radius, ranging between 959 and 994 arcsec, and ρ, with typical errors of a few arcseconds. These values constrain the correlation between the solar radius and solar activity, and they allow us to estimate the level of solar prolatness in the centimeter frequency range. Our measurements are consistent with the values reported in the literature, and they provide refined estimates in the centimeter range. The results suggest a weak prolateness of the solar limb (equatorial radius > polar radius), although the equatorial and polar radii are statistically compatible within 3σ errors.

Multiwavelength observations of the lensed quasar PKS 1830-211 during the 2019 γ-ray flare

(Published in Monthly Notices of the Royal Astronomical Society, Volume 527, Issue 3, pp.5717-5731 (2024))

by S. Vercellone, I. Donnarumma, C. Pittori, F. Capitanio, A. De Rosa, L. Di Gesu, S. Kiehlmann, M.N. Iacolina, P.A. Pellizzoni, E. Egron, L. Pacciani, G. Piano, S. Puccetti, S. Righini, G. Valente, F. Verrecchia, V. Vittorini, M. Tavani, E. Brocato, A.W. Chen, T. Hovatta, A. Melis, W. Max-Moerbeck, D. Perrodin, M. Pilia, M. Pili, A.C.S. Readhead, R. Reeves, A. Ridolfi, F. Vitali, A. Bulgarelli, P.W. Cattaneo, F. Lucarelli, A. Morselli, A. Trois

PKS 1830 -211 is a γ-ray emitting, high-redshift (z =2.507 ± 0.002), lensed flat-spectrum radio quasar. During the period 2019 mid-February to mid-April, this source underwent a series of strong γ-ray flares that were detected by both AGILE-GRID (Gamma-Ray Imaging Detector) and Fermi Large Area Telescope (Fermi-LAT). Here, we report on a coordinated campaign from both on-ground [Medicina, Owens Valley Radio Observatory (OVRO), Rapid Eye Mount (REM), and Sardinia Radio Telescope (SRT)] and orbiting facilities (AGILE, Fermi, INTEGRAL, NuSTAR, Swift, and Chandra), with the aim of investigating the multiwavelength properties of PKS 1830-211 through nearly simultaneous observations presented here for the first time. We find a possible break in the radio spectra in different epochs above 15 GHz, and a clear maximum of the 15 GHz data approximately 110 d after the γ-ray main activity periods. The spectral energy distribution shows a very pronounced Compton dominance (> 200) which challenges the canonical one-component emission model. Therefore, we propose that the cooled electrons of the first component are re-accelerated to a second component by, for example, kink or tearing instability during the γ-ray flaring periods. We also note that PKS 1830-211 could be a promising candidate for future observations with both Compton satellites [e.g. enhanced ASTROGAM (e-ASTROGAM)] and Cherenkov arrays [Cherenkov Telescope Array Observatory (CTAO)] which will help, thanks to their improved sensitivity, in extending the data availability in energy bands currently uncovered.

Sardinia Radio Telescope observations of the Coma cluster

(Published in MNRAS, Volume 528, Issue 4, Pages 6470–6495 (2024)

by M. Murgia, F. Govoni, V. Vacca, F. Loi, L. Feretti, G. Giovannini, A. Melis, R. Concu, E. Carretti, S. Poppi, G.  Valente, A. Bonafede, G. Bernardi, W. Boschin, M. Brienza, T.E. Clarke, F. de Gasperin, T.A. Ensslin, C. Ferrari, F. Gastaldello, M. Girardi, L. Gregorini, M. Johnston-Hollitt, E. Orru', P. Parma, R.A. Perley, G.B Taylor, P. Marchegiani

We present deep total intensity and polarization observations of the Coma cluster at 1.4 and 6.6 GHz performed with the Sardinia Radio Telescope. By combining the single-dish 1.4 GHz data with archival Very Large Array observations we obtain new images of the central radio halo and of the peripheral radio relic where we properly recover the brightness from the large scale structures. At 6.6 GHz we detect both the relic and the central part of the halo in total intensity and polarization. These are the highest frequency images available to date for these radio sources in this galaxy cluster. In the halo, we find a localized spot of polarized signal, with fractional polarization of about 45 per cent. The polarized emission possibly extends along the north-east side of the diffuse emission. The relic is highly polarized, up to 55 per cent, as usually found for these sources. We confirm the halo spectrum is curved, in agreement with previous single-dish results. The spectral index is α = 1.48 ± 0.07 at a reference frequency of 1 GHz and varies from α ≃ 1.1, at 0.1 GHz, up to α ≃ 1.8, at 10 GHz. We compare the Coma radio halo surface brightness profile at 1.4 GHz (central brightness and e-folding radius) with the same properties of the other halos, and we find that it has one of the lowest emissivities observed so far. Reanalyzing the relic’s spectrum in the light of the new data, we obtain a refined radio Mach number of M = 2.9 ± 0.1.

Monitoring of the polarized H2O maser emission around the massive protostars W75N(B)-VLA 1 and W75N(B)-VLA 2

(Published in Astronomy & Astrophysics, Volume 673, id.A10, 32 pp. (2023))

by G. Surcis, W. H. T. Vlemmings, C. Goddi, J. M. Torrelles, J. F. Gómez, A. Rodríguez-Kamenetzky, C. Carrasco-González, S. Curiel, S.-W. Kim, J.-S. Kim and H. J. van Langevelde

 Several radio sources have been detected in the high-mass star-forming region W75N(B), with the massive young stellar objects VLA 1 and VLA 2 shown to be of particular interest among them. These objects are thought to be at different evolutionary stages: VLA 1 is in the early stage of photoionization and driving a thermal radio jet, while VLA 2 is a thermal, collimated ionized wind surrounded by a dusty disk or envelope. In both sources, 22 GHz H2O masers have been detected in the past. Those around VLA 1 show a persistent linear distribution along the thermal radio jet, while those around VLA 2 have traced the evolution from a non-collimated to a collimated outflow over a period of ∼20 yr. The magnetic field inferred from the H2O masers has shown an orientation rotation following the direction of the major-axis of the shell around VLA 2, whereas it is immutable around VLA 1. By monitoring the polarized emission of the 22 GHz H2O masers around both VLA 1 and VLA 2 over a period of six years, we aim to determine whether the H2O maser distributions show any variation over time and whether the magnetic field behaves accordingly. The European VLBI Network was used in full polarization and phase-reference mode in order to determine the absolute positions of the 22 GHz H2O masers with a beam size of ∼1 mas and to determine the orientation and the strength of the magnetic field. We observed four epochs separated by two years from 2014 to 2020. We detected polarized emission from the H2O masers around both VLA 1 and VLA 2 in all the epochs.

The magnetic field of a magnetohydrodynamic disk wind: Water maser observations and simulations

(Published in Astronomy & Astrophysics, Volume 680, id.A107, 14 pp. (2023))

by L. Moscadelli, A. Oliva, G. Surcis, A. Sanna, M. T. Beltrán and R. Kuiper

Although star-formation models predict that the magnetic field plays an important role in regulating disk-mediated accretion and launching and collimating protostellar jets, observations of the magnetic field close enough (within a few 100 au) to the forming stars are still sparse. Our goal is to measure and model the magnetic field distribution in the disk wind of the young stellar object (YSO) IRAS 21078+5211. We performed sensitive global very long baseline interferometry observations of the polarized emission of the 22 GHz water masers tracing individual streamlines of the magnetohydrodynamic (MHD) disk wind in IRAS 21078+5211. Our resistive-radiativegravito-MHD simulations of a jet around a forming massive star are able to closely reproduce the observed maser kinematics in the inner jet cavity. Results: We measure a weak level of 0.3-3.2% of linear and circular polarization in 24 and 8 water masers, respectively. The detected polarized masers sample the direction and the strength of the magnetic field along five distinct streamlines within the inner 100 au region of the disk wind. Along the four streamlines at smaller radii from the jet axis (≤25 au), the sky-projected direction of the magnetic field forms, in most cases, a small offset angle of ≤30º with the tangent to the streamline. Along the stream at larger radii (50-100 au), the magnetic field is sampled at only three separated positions, and it is found to be approximately perpendicular to the streamline tangent at heights of ≈10 and 40 au, and parallel to the tangent at ≈70 au. According to our simulations, the magnetic field lines should coincide with the flow streamlines in the inner jet cavity. The small tilt in the magnetic field direction observed along the inner streams can be well explained by Faraday rotation, assuming a realistic low level of ionization for the molecular shell of the jet of namely ~10−2.

SPARCS-North Wide-field VLBI Survey: exploring the resolved μJy extragalactic radio source population with EVN + e-MERLIN

(Published in Monthly Notices of the Royal Astronomical Society, Volume 519, Issue 2, Pages 1732–1744 (2023))

by A. Njeri, R.J. Beswick, J. F. Radcliffe, A.P. Thomson, N. Wrigley, T.W.B. Muxlow, M.A. Garrett, R. P. Deane, J. Moldon, R.P. Norris, R. Kothes

The SKA PAthfinder Radio Continuum Surveys (SPARCS) are providing deep-field imaging of the faint (sub-mJy) extragalactic radio source populations through a series of reference surveys. One of the key science goals for SPARCS is to characterize the relative contribution of radio emission associated with active galactic nucleus (AGN) from star formation (SF) in these faint radio source populations, using a combination of high sensitivity and high angular resolution imaging over a range of spatial scales (arcsec to mas). To isolate AGN contribution from SF, we hypothesize that there exists a brightness temperature cut-off point separating pure AGN from SF. We present a multiresolution (10-100 mas) view of the transition between compact AGN and diffuse SF through a deep wide-field EVN + e-MERLIN, multiple phase centre survey of the centre of the Northern SPARCS (SLOAN) reference field at 1.6 GHz. This is the first (and only) VLBI (+ e-MERLIN) milliarcsecond angular resolution observation of this field, and of the wider SPARCS reference field programme. Using these high spatial resolution (9 pc-0.3 kpc at z ~ 1.25) data, 11 milliarcsec-scale sources are detected from a targeted sample of 52 known radio sources from previous observations with the e-MERLIN, giving a VLBI detection fraction of 21%. At spatial scales of 9 pc, these sources show little to no jet structure whilst at 0.3 kpc one-sided and two-sided radio jets begin to emerge on the same sources, indicating a possible transition from pure AGN emissions to AGN and SF systems.

Intermediate-mass black holes: finding of episodic, large-scale, and powerful jet activity in a dwarf galaxy

(Published in Monthly Notices of the Royal Astronomical Society, Volume 520, Issue 4, pp.5964-5973 (2023))

by J. Yang, Z. Paragi, S. Frey, L.I. Gurvits, M. Liao, X. Liu, L. Cui, X. Yang, W. Chen, E. Varenius, J.E. Conway, R. Chen, N. Chang

Dwarf galaxies are characterized by a very low luminosity and low mass. Because of significant accretion and ejection activity of massive black holes, some dwarf galaxies also host low-luminosity active galactic nuclei (AGNs). In a few dwarf AGNs, very long baseline interferometry (VLBI) observations have found faint non-thermal radio emission. SDSS J090613.77+561015.2 is a dwarf AGN owning an intermediate-mass black hole (IMBH) and showing a rarely seen two-component radio structure in its radio nucleus. To further probe their nature, i.e. the IMBH jet activity, we performed additional deep observations with the European VLBI Network (EVN) at 1.66 and 4.99 GHz. We find the more diffuse emission regions and structure details. These new EVN imaging results allow us to reveal a two-sided jet morphology with a size up to about 150 mas (projected length ~140 pc) and a radio luminosity of about 3 × 1038 erg/s. The peak feature has an optically thin radio spectrum and thus more likely represents a relatively young ejecta instead of a jet base. The EVN study on SDSS J090613.77+561015.2 demonstrates the existence of episodic, relatively large-scale, and powerful IMBH jet activity in dwarf AGNs. Moreover, we collected a small sample of VLBI-detected dwarf AGNs and investigated their connections with normal AGNs. We notice that these radio sources in the dwarf AGNs tend to have steep spectra and small linear sizes, and possibly represent ejecta from scaled-down episodic jet activity.

An interferometric SETI observation of Kepler-111 b

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 522, Issue 3, pp.3784-3794 (2023))

by K. Wandia, M. A Garrett, J.F. Radcliffe, S.T. Garrington, J. Fawcett, V. Gajjar, D.H.E. MacMahon, E.Varenius, R.M. Campbell, Z. Paragi, A.P.V. Siemion

We The application of very long baseline interferometry (VLBI) to the search for extraterrestrial intelligence (SETI) has been limited to date, despite the technique offering many advantages over traditional single-dish SETI observations. In order to further develop interferometry for SETI, we used the European VLBI network (EVN) at 21 cm to observe potential secondary phase calibrators in the Kepler field. Unfortunately, no secondary calibrators were detected. However, a VLBA primary calibrator in the field, J1926+4441, offset only ~1.88 arcmin from a nearby exoplanet Kepler-111 b, was correlated with high temporal (0.25s) and spectral (16384×488Hz channels) resolution. During the analysis of the high-resolution data, we identified a spectral feature that was present in both the auto and cross-correlation data with a central frequency of 1420.424 ± 0.0002 MHz and a width of 0.25 MHz. We demonstrate that the feature in the cross-correlations is an artefact in the data, associated with a significant increase in each telescope's noise figure due to the presence of H I in the beam. This would typically go unnoticed in data correlated with standard spectral resolution. We flag (excluded from the subsequent analysis) these channels and phase rotate the data to the location of Kepler-111 b aided by the Gaia catalogue and search for signals with SNR>7. At the time of our observations, we detect no transmitters with an equivalent isotropically radiated power ≳4 × 10^15 W.

The Close AGN Reference Survey (CARS): An Interplay between Radio Jets and AGN Radiation in the Radio-quiet AGN HE0040-1105

(Published in The Astrophysical Journal, Volume 959, Issue 2, id.107, 20 pp. (2023))

by M. Singha, N. Winkel, S. Vaddi, M. Perez Torres, M. Gaspari, I. Smirnova-Pinchukova, C. P. O'Dea, F. Combes, Osase Omoruyi, T. Rose, R. McElroy, B. Husemann, T. A. Davis, S. A. Baum, C. Lawlor-Forsyth, J. Neumann and G. R. Tremblay

We present a case study of HE 0040-1105, an unobscured radio-quiet active galactic nucleus (AGN) at a high accretion rate of λ Edd = 0.19 ± 0.04. This particular AGN hosts an ionized gas outflow with the largest spatial offset from its nucleus compared to all other AGNs in the Close AGN Reference Survey. By combining multiwavelength observations from the Very Large Telescope/MUSE, Hubble Space Telescope/Wide Field Camera 3, Very Large Array, and European VLBI Network, we probe the ionization conditions, gas kinematics, and radio emission from host galaxy scales to the central few parsecs. We detect four kinematically distinct components, one of which is a spatially unresolved AGN-driven outflow located within the central 500 pc, where it locally dominates the interstellar medium conditions. Its velocity is too low to escape the host galaxy's gravitational potential, and may be re-accreted onto the central black hole via chaotic cold accretion. We detect compact radio emission in HE 0040-1105 within the region covered by the outflow, varying on a timescale of ~20 yr. We show that neither AGN coronal emission nor star formation processes wholly explain the radio morphology/spectrum. The spatial alignment between the outflowing ionized gas and the radio continuum emission on 100 pc scales is consistent with a weak jet morphology rather than diffuse radio emission produced by AGN winds.

Precessing jet nozzle connecting to a spinning black hole in M87

(Published in Nature, Volume 621, pages 711–715 (2023)

by Y. Cui, K. Hada, T. Kawashima, M. Kino, W. Lin, Y. Mizuno, H., Ro, M. Honma, K. Yi, J. Yu, J. Park, W. Jiang, Z. Shen, E. Kravchenko, J.C. Algaba, X. Cheng, I. Cho, G. Giovannini, M. Giroletti, T. Jung, R.S. Lu, K. Niinuma, J. Oh, K. Ohsuga, S. Sawada-Satoh, B. Won Sohn, H.R. Takahashi, M. Takamura, F. Tazaki, S. Trippe, K. Wajima, K. Akiyama, T. An, K. Asada, S. Buttaccio, D.Y. Byun, L. Cui, Y. Hagiwara, T. Hirota, J. Hodgson, N. Kawaguchi, J.Y. Kim, S.S. Lee, J. Won Lee, J. Ae Lee, G. Maccaferri, A. Melis, A. Melnikov, C. Mignon, S.J Oh et al.

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole. The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from general relativity. With a baseline of 17 years of observations, there was a shift in the jet’s transverse position, possibly arising from an 8- to 10-year quasi-periodicity. However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years for the variation in the position angle of the jet. We infer that we are seeing a spinning black hole that induces the Lense–Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation.

Searching for continuous Gravitational Waves in the second data release of the International Pulsar Timing Array

(Published in The Monthly Notices of the Royal Astronomical Society, Volume 521, Issue 4, Pages 5077–5086 (2023))

by M. Falxa, S. Babak, P. T. Baker, B. Bécsy, A. Chalumeau, S. Chen, Z. Chen, N. J. Cornish, L. Guillemot, J. S. Hazboun, C. M. F. Mingarelli, A. Parthasarathy, A. Petiteau, IPTA Collaboration

The International Pulsar Timing Array 2nd data release is the combination of data sets from worldwide collaborations. In this study, we search for continuous waves: gravitational wave signals produced by individual supermassive black hole binaries in the local universe. We consider binaries on circular orbits and neglect the evolution of orbital frequency over the observational span. We find no evidence for such signals and set sky averaged 95 per cent upper limits on their amplitude h95. The most sensitive frequency is 10 nHz with h95 = 9.1 × 10−15. We achieved the best upper limit to date at low and high frequencies of the PTA band thanks to improved effective cadence of observations. In our analysis, we have taken into account the recently discovered common red noise process, which has an impact at low frequencies. We also find that the peculiar noise features present in some pulsars data must be taken into account to reduce the false alarm. We show that using custom noise models is essential in searching for continuous gravitational wave signals and setting the upper limit.

IC 485: A new candidate disk-maser galaxy at ∼100 Mpc distance. Milliarcsecond resolution study of the galaxy nucleus and of the H2O megamaser

(Published in Astronomy & Astrophysics, Volume 682, id.A25, 13 pp. (2023))

by E. Ladu, A. Tarchi, P. Castangia, G. Surcis, J.A. Braatz, F. Panessa, D. Pesce

Masers allow to investigate the emitting gas in the innermost regions of AGNs and to map accretion disks and tori orbiting around supermassive black holes. IC485, which is classified as a LINER or Seyfert galaxy, hosts a bright water maser whose nature is still under debate. Indeed, this might be either a nuclear disk maser, a jet/outflow maser, or even the very first `inclined water maser disk'. We aim to investigate the nature of the maser by determining the location and the distribution of the maser emission at mas resolution and by associating it with the main nuclear components of IC485. We observed in 2018 the nuclear region of IC485 in continuum and spectral-line mode with the VLBA and the EVN at L, C, and K bands (linear scales from ~3 to 0.2 pc). We detected 2 water maser components separated in velocity by 472 km/s, with one centred at the systemic velocity of the nuclear region and the other at a red-shifted velocity. We measured for the first time the absolute positions of these components with an accuracy of ~0.1 mas. Assuming a maser associated with an edge-on disk in Keplerian rotation, the estimated enclosed mass is M_BH = 1.2 x 10^7 M_sun, consistent with the expected mass for a SMBH in a LINER or Seyfert galaxy. The linear distribution of the maser components and a comparison with the high sensitivity GBT spectrum strongly suggest that the bulk of the maser emission is associated with an edge-on accretion disk. This makes IC485 a new candidate for a disk-maser galaxy at the distance of 122 Mpc. In particular, thanks to the upcoming radio facilities (e.g., the SKA and the ngVLA) IC485 will play an important role in our understanding of AGNs in an unexplored volume of Universe.

A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar

(Published in Nature Astronomy, Volume 7, pages 557–568 (2023))

by R.A. Burns, Y. Uno, N. Sakai, J. Blanchard, Z. Fazil, G. Orosz, Y. Yonekura, Y. Tanabe, K. Sugiyama, T. Hirota, Kee-Tae Kim, A. Aberfelds, A. E. Volvach, A. Bartkiewicz, A. Caratti o Garatti, A. M. Sobolev, B. Stecklum, C. Brogan, C. Phillips, D. A. Ladeyschikov, D. Johnstone, G. Surcis, G. C. MacLeod, H. Linz, J. O. Chibueze, J. Brand, J. Eislöffel, L. Hyland, L. Uscanga, M. Olech, M. Durjasz, O. Bayandina, S. Breen, S. P. Ellingsen, S. P. van den Heever, T. R. Hunter & X. Chen

High-mass protostars are thought to gain the majority of their mass via short, intense bursts of growth. This episodic accretion is thought to be facilitated by gravitationally unstable and subsequently inhomogeneous accretion disks. Limitations of observational capabilities, paired with a lack of observed accretion burst events, have withheld affirmative confirmation of the association between disk accretion, instability and the accretion burst phenomenon in high-mass protostars. Following its 2019 accretion burst, a heatwave driven by a burst of radiation propagated outward from the high-mass protostar G358.93-0.03-MM1. Six very long baseline interferometry observations of the radiatively pumped 6.7 GHz methanol maser were conducted during this period, tracing ever increasing disk radii as the heatwave propagated outward. Concatenating the very long baseline interferometry maps provided a sparsely sampled, milliarcsecond view of the spatio-kinematics of the accretion disk covering a physical range of ~50–900 AU. We term this observational approach ‘heatwave mapping’. We report the discovery of a Keplerian accretion disk with a spatially resolved four-arm spiral pattern around G358.93-0.03-MM1. This result positively implicates disk accretion and spiral arm instabilities into the episodic accretion high-mass star formation paradigm.

Modelling annual scintillation arc variations in PSR J1643-1224 using the Large European Array for Pulsars

(Published in The Monthly Notices of the Royal Astronomical Society, Volume 511, Issue 1, pp.1104-1114 (2022))

by G. Mall, R. A. Main, J. Antoniadis, C. G. Bassa, M. Burgay, S. Chen, I. Cognard, R. Concu, A. Corongiu, M. Gaikwad, H. Hu, G. H. Janssen, R. Karuppusamy, M. Kramer, K. J. Lee, K. Liu, J. W. McKee, A. Melis, M B Mickaliger, D Perrodin, M Pilia, A Possenti, D J Reardon, S A Sanidas, T Sprenger, B W Stappers, L. Wang, O. Wucknitz, W. W. Zhu

In this work, we study variations in the parabolic scintillation arcs of the binary millisecond pulsar PSR J1643−1224 over five years using the Large European Array for Pulsars (LEAP). The two-dimensional (2D) power spectrum of scintillation, called the secondary spectrum, often shows a parabolic distribution of power, where the arc curvature encodes the relative velocities and distances of the pulsar, ionized interstellar medium, and Earth. We observe a clear parabolic scintillation arc, which varies in curvature throughout the year. The distribution of power in the secondary spectra is inconsistent with a single scattering screen, which is fully 1D or entirely isotropic. We fit the observed arc curvature variations with two models: an isotropic scattering screen and a model with two independent 1D screens. We measure the distance to the scattering screen to be in the range 114–223 pc, depending on the model, consistent with the known distance of the foreground large-diameter H II region Sh 2-27 (112 ± 17 pc), suggesting that it is the dominant source of scattering. We obtain only weak constraints on the pulsar’s orbital inclination and longitude of ascending node, since the scintillation pattern is not very sensitive to the pulsar’s motion and the screen is much closer to the Earth than the pulsar.

Detection of quasi-periodic micro-structure in three millisecond pulsars with the Large European Array for Pulsars

(Published in The Monthly Notices of the Royal Astronomical Society, Volume 513, Issue 3, Pages 4037–4044 (2022))

by K. Liu, J. Antoniadis, C. G. Bassa, S. Chen, I. Cognard, M. Gaikwad, H. Hu, J. Jang, G. H. Janssen, R. Karuppusamy, M. Kramer, K. J. Lee, R. A. Main, G. Mall, J. W. McKee, M. B. Mickaliger, D. Perrodin, S. A. Sanidas, B. W. Stappers, L. Wang, W. W. Zhu, M. Burgay, R. Concu, A. Corongiu, A. Melis, M. Pilia, A. Possenti

We report on the detection of quasi-periodic micro-structure in three millisecond pulsars (MSPs), PSRs J1022+1001, J2145−0750, and J1744−1134, using high time resolution data acquired with the Large European Array for Pulsars at a radio frequency of 1.4 GHz. The occurrence rate of quasi-periodic micro-structure is consistent among pulses with different peak flux densities. Using an auto-correlation analysis, we measure the periodicity and width of the micro-structure in these three pulsars. The detected micro-structure from PSRs J1022+1001 and J1744−1134 is often highly linearly polarized. In PSR J1022+1001, the linear polarization position angles of micro-structure pulses are in general flat with a small degree of variation. Using these results, we further examine the frequency and rotational period dependency of micro-structure properties established in previous work, along with the angular beaming and temporal modulation models that explain the appearance of micro-structure. We also discuss a possible link of micro-structure to the properties of some of the recently discovered fast radio bursts which exhibit a very similar emission morphology.

The first 7 months of the 2020 X-ray outburst of the magnetar SGR J1935+2154

(Published in The Monthly Notices of the Royal Astronomical Society, Volume 516, Issue 1, Pages 602–616 (2022))

by A. Borghese, F. Coti Zelati, G. L. Israel, M. Pilia, M. Burgay, M. Trudu, S. Zane, R. Turolla, N. Rea, P. Esposito, S. Mereghetti, A. Tiengo, A. Possenti

The magnetar SGR J1935+2154 underwent a new active episode on 2020 April 27–28, when a forest of hundreds of X-ray bursts and a large enhancement of the persistent flux were detected. For the first time, a radio burst with properties similar to those of fast radio bursts and with a X-ray counterpart was observed from this source, showing that magnetars can power at least a group of fast radio bursts. In this paper, we report on the X-ray spectral and timing properties of SGR J1935+2154 based on a long-term monitoring campaign with Chandra, XMM–Newton, NuSTAR, Swift, and NICER covering a time-span of ∼7 months since the outburst onset. Moreover, we report on simultaneous radio observations performed with the Sardinia Radio Telescope. No evidence for periodic or single-pulse radio emission was found.

A long-lived compact jet in the black hole X-ray binary candidate AT2019wey

(Published in Astronomy & Astrophysics, Volume 657, id.A104, 8 pp. (2022))

by H.M. Cao, G. Migliori, M. Giroletti, S. Frey, J. Yang, K.É. Gabányi, L. Cui, T. An, X.Y. Hong and W.D. Zhang

AT2019wey is a transient discovered by the Asteroid Terrestrial-impact Last Alert System survey in December of 2019. Follow-up optical, radio, and X-ray observations led to classification of this source as a Galactic black hole X-ray binary candidate. We carried out one-epoch 6.7 GHz European Very Long Baseline Interferometry Network and two-epoch multi-frequency (1.6, 4.5, 6.7 GHz) Very Long Baseline Array observations within a year after its discovery. These observations reveal a fading and flat-spectrum radio source with no discernible motion. These features suggest the detection of a compact jet. The source appears resolved at milliarcsecond scales, and the source angular size versus frequency trend is consistent with scatter broadening. This allows us to constrain the lower limit of the source distance to 6 kpc if the scattering medium is in a Galactic spiral arm. For a source location at greater than 3 kpc, the estimated upper limit of the peculiar velocity suggests the asymmetric natal kick may have occurred during the black hole formation stage.

Snapshot of a magnetohydrodynamic disk wind traced by water maser observations

(Published in Nature Astronomy, Volume 6, p. 1068-1076 (2022))

by L. Moscadelli, A. Sanna, H. Beuther, A. Oliva and R. Kuiper

The formation of astrophysical objects of different nature, from black holes to gaseous giant planets, involves a disk-jet system, where the disk drives the mass accretion onto a central compact object and the jet is a fast collimated ejection along the disk rotation axis. Magnetohydrodynamic disk winds can provide the link between mass accretion and ejection, which is essential to ensure that the excess angular momentum is removed and accretion can proceed. However, until now, we have been lacking direct observational proof of disk winds. Here we present a direct view of the velocity field of a disk wind around a forming massive star. Achieving a very high spatial resolution of about 0.05 au, our water maser observations trace the velocities of individual streamlines emerging from the disk orbiting the forming star. We find that, at low elevation above the disk midplane, the flow co-rotates with its launch point in the disk, in agreement with magneto-centrifugal acceleration. Beyond the co-rotation point, the flow rises spiralling around the disk rotation axis along a helical magnetic field. We have performed (resistive-radiative-gravito-)magnetohydrodynamic simulations of the formation of a massive star and record the development of a magneto-centrifugally launched jet presenting many properties in agreement with our observations.

Radio-loud Quasars above Redshift 4: Very Long Baseline Interferometry (VLBI) Imaging of an Extended Sample

(Published in The Astrophysical Journal Supplement Series, Volume 260, Issue 2, id.49, 19 pp. (2022))

by M. Krezinger, K. Perger2, K.É. Gabányi, S. Frey, L.I. Gurvits, Z. Paragi, T. An, Y. Zhang, H. Cao and T. Sbarrato

High-redshift radio sources provide plentiful opportunities for studying the formation and evolution of early galaxies and supermassive black holes. However, the number of known radio-loud active galactic nuclei (AGN) above redshift 4 is rather limited. At high redshifts, it appears that blazars, with relativistically beamed jets pointing toward the observer, are in the majority compared to the radio-loud sources with jets misaligned with respect to the line of sight. To find more of these misaligned AGN, milliarcsecond-scale imaging studies carried out with very long baseline interferometry (VLBI) are needed, as they allow us to distinguish between compact-core-jet radio sources and those with more extended emission. Previous high-resolution VLBI studies revealed that some of the radio sources among blazar candidates in fact show unbeamed radio emission on milliarcsecond scales. The most accurate optical coordinates determined with the Gaia astrometric space mission are also useful in the classification process. Here, we report on dual-frequency imaging observations of 13 high-redshift (4 < z < 4.5) quasars at 1.7 and 5 GHz with the European VLBI Network. This sample increases the number of z > 4 radio sources for which VLBI observations are available by about a quarter. Using structural and physical properties, such as radio morphology, spectral index, variability, brightness temperature, as well as optical coordinates, we identified six blazars and six misaligned radio AGNs, with the remaining one tentatively identified as blazar.

High-frequency Radio Imaging of 3CR 403.1 with the Sardinia Radio Telescope

(Published in The Astrophysical Journal, Volume 936, Issue 1, id.10, 13 pp. (2022))

by V. Missaglia, M. Murgia, F. Massaro, A. Paggi, A. Jimenez-Gallardo, W. Forman , R. P. Kraft, and B. Balmaverde

We present multifrequency observations of the radio source 3CR 403.1, a nearby (z=0.055), extended (~ 0.5 Mpc) radio galaxy hosted in a small galaxy group. Using new high frequency radio observations from the Sardinia Radio Telescope (SRT), augmented with archival low frequency radio observations, we investigated radio spectral and polarimetric properties of 3CR 403.1. From the MHz-to-GHz spectral analysis, we computed the equipartition magnetic field in the lobes to be B_eq=2.4 μG and the age of the source to be ~100 Myr. From the spectral analysis of the diffuse X-ray emission we measured the temperature and density of the intracluster medium (ICM). From the SRT observations, we discovered two regions where the radio flux density is below the background value. We computed the Comptonization parameter both from the radio and from the X-ray observations to test if the Sunyaev-Zel'dovich effect is occurring here and found a significant tension between the two estimates. If the negative signal is considered as real, then we speculate that the discrepancy between the two values could be partially caused by the presence of a non-thermal bath of mildly relativistic ghost electrons. From the polarimetric radio images, we find a net asymmetry of the Faraday rotation between the two prominent extended structures of 3CR 403.1, and constrain the magnetic field strength in the ICM to be 1.8-3.5 μG. The position of 3CR 403.1 in the magnetic field-gas density plane is consistent with the trend reported in the literature between central magnetic field and central gas density.

Puzzling large-scale polarization in the galaxy cluster Abell 523

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 514, Issue 4, pp.4969-4981 (2022))

by V. Vacca, F. Govoni, M. Murgia, R. A. Perley, L. Feretti, G. Giovannini, E. Carretti, F. Gastaldello, F. Cova, P. Marchegiani, E. Battistelli, W. Boschin, T. A. Enßlin., M. Girardi, F. Loi, F. Radiconi

Large-scale magnetic fields reveal themselves through diffuse synchrotron sources observed in galaxy clusters such as radio halos. Total intensity filaments of these sources have been observed in polarization as well, but only in three radio halos out of about 100 currently known. In this paper we analyse new polarimetric Very Large Array data of the diffuse emission in the galaxy cluster Abell 523 in the frequency range 1-2 GHz. We find for the first time evidence of polarized emission on scales of ~2.5 Mpc. The total intensity emission is observed only in the central part of the source, likely due to observational limitations. To look for total intensity emission beyond the central region, we combine these data with single-dish observations from the Sardinia Radio Telescope and we compare them with multifrequency total intensity observations obtained with different instruments, including the LOw Frequency ARray and the Murchison Widefield Array. By analysing the rotation measure properties of the system and utilizing numerical simulations, we infer that this polarized emission is associated with filaments of the radio halo located in the outskirts of the system, in the peripheral region closest to the observer.

Solar observations with single-dish INAF radio telescopes: continuum imaging in the 18-26 GHz range

(Published in Solar Physics, Volume 297, Issue 7, article id.86 (2022))

by A. Pellizzoni, S. Righini, M. N. Iacolina, M. Marongiu, S. Mulas, G. Murtas, G. Valente, E. Egron, M. Bachetti, F. Buffa, R. Concu, G. L. Deiana, S. L. Guglielmino, A. Ladu, S. Loru, A. Maccaferri, P. Marongiu, A. Melis, A. Navarrini, A. Orfei, P. Ortu, M. Pili, T. Pisanu, G. Pupillo, A. Saba, L. Schirru, G. Serra, C. Tiburzi, A. Zanichelli, P. Zucca, M. Messerotti

We present a new solar radio imaging system implemented through the upgrade of the large single-dish telescopes of the Italian National Institute for Astrophysics (INAF), not originally conceived for solar observations. During the development and early science phase of the project (2018–2020), we obtained about 170 maps of the entire solar disk in the 18–26 GHz band, filling the observational gap in the field of solar imaging at these frequencies. These solar images have typical resolutions in the 0.7–2 arcmin range and a brightness temperature sensitivity below 10K. Accurate calibration adopting the Supernova Remnant Cas A as a flux reference, provided typical errors below 3% for the estimation of the quiet-Sun level components and for active regions flux measurements. As a first early science result of the project, we present a catalog of radio continuum solar imaging observations with Medicina 32-m and SRT 64-m radio telescopes including the multi-wavelength identification of active regions, their brightness and spectral characterization. The interpretation of the observed emission as thermal bremsstrahlung components combined with gyro-magnetic variable emission pave the way to the use of our system for long-term monitoring of the Sun. We also discuss useful outcomes both for solar physics (e.g. study of the chromospheric network dynamics) and space weather applications (e.g. flare precursors studies).

Burst timescales and luminosities as links between young pulsars and fast radio bursts

(Published in Nature Astronomy, Volume 6, p. 393-401 (2022))

by K. Nimmo, J.W.T. Hessels, F. Kirsten, A. Keimpema, J.M. Cordes, M.P. Snelders, D. M. Hewitt, R. Karuppusamy, A. M. Archibald, V. Bezrukovs, M. Bhardwaj, R. Blaauw, S. T. Buttaccio, T. Cassanelli, J. E. Conway, A. Corongiu, R. Feiler, E. Fonseca, O. Forssen, M. Gawronski, M. Giroletti, M.A. Kharinov, C. Leung, M. Lindqvist, G. Maccaferri, B. Marcote, K. W. Masui, R. Mckinven, A. Melnikov, D. Michilli A. Mikhailov, C. Ng, A. Orbidans, O.S. Ould-Boukattine, Z. Paragi, A.B. Pearlman, E. Petroff, M. Rahman, P. Scholz, K. Shin, K. M. Smith, I. H. Stairs, G. Surcis, S. P. Tendulkar, W. Vlemmings, N. Wang, J. Yang and J. Yuan

Fast radio bursts (FRBs) are extragalactic radio flashes of unknown physical origin. Their high luminosities and short durations require extreme energy densities, such as those found in the vicinity of neutron stars and black holes. Studying the burst intensities and polarimetric properties on a wide range of timescales, from milliseconds down to nanoseconds, is key to understanding the emission mechanism. However, high-time-resolution studies of FRBs are limited by their unpredictable activity levels, available instrumentation and temporal broadening in the intervening ionized medium. Here we show that the repeating FRB 20200120E can produce isolated shots of emission as short as about 60 nanoseconds in duration, with brightness temperatures as high as 3 × 1041 K (excluding relativistic effects), comparable with `nano-shots' from the Crab pulsar. Comparing both the range of timescales and luminosities, we find that FRB 20200120E observationally bridges the gap between known Galactic young pulsars and magnetars and the much more distant extragalactic FRBs. This suggests a common magnetically powered emission mechanism spanning many orders of magnitude in timescale and luminosity. In this Article, we probe a relatively unexplored region of the short-duration transient phase space; we highlight that there probably exists a population of ultrafast radio transients at nanosecond to microsecond timescales, which current FRB searches are insensitive to.

A repeating fast radio burst source in a globular cluster

(Published in Nature, Volume 602, Issue 7898, p.585-589 (2022))

by  F. Kirsten, B. Marcote, K. Nimmo, J.W.T. Hessels, M. Bhardwaj, S.P. Tendulkar, A. Keimpema, J. Yang, M.P. Snelders, P. Scholz, A.B. Pearlman, C.J. Law, W.M. Peters, M. Giroletti, Z. Paragi, C. Bassa, D.M. Hewitt, U. Bach, V. Bezrukovs, M. Burgay, S.T. Buttaccio, J.E. Conway, A. Corongiu, R. Feiler, O. Forssen, M.P. Gawronski, R. Karuppusamy, M.A. Kharinov, M. Lindqvist, G. Maccaferri, A. Melnikov, O.S. Ould-Boukattine, A. Possenti, G. Surcis, N. Wang, J. Yuan, K. Aggarwal, R. Anna-Thomas, G.C. Bower, R. Blaauw, S. Burke-Spolaor, T. Cassanelli, T.E. Clarke, E. Fonseca, B.M. Gaensler, A. Gopinath, V.M. Kaspi, N. Kassim, T. J.W. Lazio, C. Leung, D.Z. Li, H.H. Lin, K.W. Masui, R. Mckinven, D. Michilli, A. Mikhailov, C. Ng, A. Orbidans, U.L. Pen, E. Petroff, M. Rahman, S. M. Ransom, K. Shin, K.M. Smith, I. H. Stairs and W. Vlemmings

Fast radio bursts (FRBs) are flashes of unknown physical origin. The majority of FRBs have been seen only once, although some are known to generate multiple flashes. Many models invoke magnetically powered neutron stars (magnetars) as the source of the emission. Recently, the discovery of another repeater (FRB 20200120E) was announced, in the direction of the nearby galaxy M81, with four potential counterparts at other wavelengths. Here we report observations that localized the FRB to a globular cluster associated with M81, where it is 2 parsecs away from the optical centre of the cluster. Globular clusters host old stellar populations, challenging FRB models that invoke young magnetars formed in a core-collapse supernova. We propose instead that FRB 20200120E originates from a highly magnetized neutron star formed either through the accretion-induced collapse of a white dwarf, or the merger of compact stars in a binary system. Compact binaries are efficiently formed inside globular clusters, so a model invoking them could also be responsible for the observed bursts.

Searching for Anomalous Microwave Emission in nearby galaxies. K-band observations with the Sardinia Radio Telescope

(Published in Astronomy & Astrophysics, Volume 658, id.L8, 11 pp. (2022))

by  S. Bianchi, M. Murgia, A. Melis, V. Casasola, F. Galliano, F. Govoni, A. P. Jones, S. C. Madden, R. Paladino, F. Salvestrini, E. M. Xilouris, and N. Ysard

Aims. We observed four nearby spiral galaxies (NGC 3627, NGC 4254, NGC 4736 and NGC 5055) in the K band with the 64-m Sardinia Radio Telescope, with the aim of detecting the Anomalous Microwave Emission (AME), a radiation component presumably due to spinning dust grains, observed so far in the Milky Way and in a handful of other galaxies only (most notably, M 31). Methods. We mapped the galaxies at 18.6 and 24.6 GHz and studied their global photometry together with other radio-continuum data from the literature, in order to find AME as emission in excess of the synchrotron and thermal components. Results. We only find upper limits for AME. These non-detections, and other upper limits in the literature, are nevertheless consistent with the average AME emissivity from the few detections: it is AME 30 GHz = 2.4±0.4×10^-2 MJy sr^-1 (M pc^-2 ) −1 in units of dust surface density (equivalently, 1.4 ± 0.2 × 10^-18 Jy sr^-1 (H cm^-2 )^-1 in units of H column density). We finally suggest to search for AME in quiescent spirals with relatively low radio luminosity, such as M 31.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions. V. Completion of the flux-limited sample

(Published in Astronomy & Astrophysics, Volume 658, id.A78, 21 pp. (2022))

by G. Surcis, W.H.T. Vlemmings, H.J. van Langevelde, B. Hutawarakorn Kramer, A. Bartkiewicz

Although the role of magnetic fields in launching molecular outflows in massive YSOs has been convincingly demonstrated by theoretical arguments, observationally, the alignment of the magnetic field lines with the molecular outflows is still under debate. We aim to complete the measurements of the direction of the magnetic fields at mas resolution around a sample of massive star-forming regions (MSFRs) to determine whether the magnetic field and outflows are aligned. In 2012, we started a large VLBI campaign with the EVN to measure the magnetic field orientation and strength toward a sample of 31 MSFRs (the flux-limited sample) by analyzing the polarized emission of 6.7GHz CH3OH masers. In the previous papers of the series, we have presented 80% of the sample. Here, we report the linearly and circularly polarized emission of 6.7GHz CH3OH masers toward the last five MSFRs of the flux-limited sample. The sources are G30.70-0.07, G30.76-0.05, G31.28+0.06, G32.03+0.06, and G69.52-0.97. We detected a total of 209 masers, 15% of which show linearly polarized emission (0.07%-16.7%), and 2% of which show circularly polarized emission (0.2%-4.2%). Zeeman splitting was measured toward G30.70-0.07, G32.03+0.06, and G69.52-0.97. The statistical analysis of the entire flux-limited sample shows that the observations are consistent with a bimodal distribution in the difference between the 3D magnetic field direction and the outflow axis, with half the magnetic field directions being perpendicular and the other half being parallel to the outflow. In addition, we determined that typical values of the linear and circular polarization fractions for 6.7 GHz CH3OH masers are Pl=1.0%-2.5% and Pv=0.5%-0.75%, respectively.

Noise analysis in the European Pulsar Timing Array data release 2 and its implications on the gravitational-wave background search

(Published in Monthly Notices of the Royal Astronomical Society, Volume 509, Issue 4, Pages 5538–5558 (2022))

by A. Chalumeau, S. Babak, A. Petiteau, S. Chen, A. Samajdar, R. N. Caballero, G. Theureau, L. Guillemot, G. Desvignes, A. Parthasarathy, K. Liu, G. Shaifullah, H. Hu, E. van der Wateren, J. Antoniadis, A.-S. Bak Nielsen, C. G. Bassa, A. Berthereau, M. Burgay, D. J. Champion, I. Cognard, M. Falxa, R. D. Ferdman, P. C. C. Freire, J. R. Gair, E. Graikou, Y. J. Guo, J. Jang, G. H. Janssen, R. Karuppusamy, M. J. Keith, M. Kramer, K. J. Lee, X. J. Liu, A. G. Lyne, R. A. Main, J.W. McKee, M. B. Mickaliger, B. B. P. Perera, D. Perrodin, N. K. Porayko, A. Possenti, S. A. Sanidas, A. Sesana, L. Speri, B. W. Stappers, C. Tiburzi, A. Vecchio, J. P. W. Verbiest, J. Wang, L. Wang and H. Xu

The European Pulsar Timing Array (EPTA) collaboration has recently released an extended data set for six pulsars (DR2) and reported evidence for a common red noise signal. Here we present a noise analysis for each of the six pulsars. We consider several types of noise: (i) radio frequency independent, “achromatic”, and time-correlated red noise; (ii) variations of dispersion measure and scattering; (iii) system and band noise; and (iv) deterministic signals (other than gravitational waves) that could be present in the PTA data. We perform Bayesian model selection to find the optimal combination of noise components for each pulsar. Using these custom models we revisit the presence of the common uncorrelated red noise signal previously reported in the EPTA DR2 and show that the data still supports it with a high statistical significance. Next, we confirm that there is no preference for or against the Hellings-Downs spatial correlations expected for the stochastic gravitational-wave background. The main conclusion of the EPTA DR2 paper remains unchanged despite a very significant change in the noise model of each pulsar. However, modelling the noise is essential for the robust detection of gravitational waves and its impact could be significant when analysing the next EPTA data release, which will include a larger number of pulsars and more precise measurements.

European VLBI Network Observations of the Proposed Dual AGN SDSS J101022.95+141300.9

(Published in The Astrophysical Journal, Volume 922, Issue 2, id.99, 9 pp. (2021))

by P. Milán Veres, K. É. Gabányi, S. Frey, Z. Paragi, E. Kun and T. An

During galaxy merger events, the supermassive black holes in the center of the galaxies may form a pair of active galactic nuclei (AGN) with kiloparsec-scale or even parsec-scale separation. Recently, optical observations revealed a promising dual-AGN candidate at the center of the galaxy SDSS J101022.95+141300.9 (hereafter J1010+1413). The presence of two distinct [O III]-emitting point sources with a projected separation of ~430 pc indicates a dual-AGN system. To search for AGN-dominated radio emission originating from the Hubble Space Telescope (HST) point sources, we carried out very long baseline interferometry observations. We resolved the radio structure of J1010+1413 and detected a single feature offset from the HST point sources and also from the Gaia optical position of the object. Our multiwavelength analysis of J1010+1413 inferred two possible interpretations of the observed properties challenging its proposed dual-AGN classification.

The Fast Radio Burst FRB 20201124A in a star forming region: constraints to the progenitor and multiwavelength counterparts

(Published in Astronomy & Astrophyics letters, Volume 656, id.L15, 11 pp. (2021))

by L. Piro, G. Bruni, E. Troja, B. O’Connor, F. Panessa, R. Ricci, B. Zhang, M. Burgay, S. Dichiara, K.J. Lee, S. Lotti, J.R. Niu, M. Pilia, A. Possenti, M. Trudu, H. Xu, W.W. Zhu, A.S. Kutyrev, S. Veilleux

We present the results of a multiwavelength campaign of FRB 20201124A, the third closest repeating fast radio burst recently localized in a nearby (z = 0.0978) galaxy. Deep VLA observations led to the detection of quiescent radio emission, also marginally visible in X-rays with Chandra. Imaging at 22 GHz allowed us to resolve the source on a scale of > 1" and locate it at the position of the FRB, within an error of 0.2". EVN and e-MERLIN observations sampled small angular scales, from 2 to 100 mas, providing tight upper limits on the presence of a compact source and evidence for diffuse radio emission. We argue that this emission is associated with enhanced star formation activity in the proximity of the FRB, corresponding to a star formation rate of ≈ 10 M yr^−1 . The surface star formation rate at the location of FRB 20201124A is two orders of magnitude larger than typically observed in other precisely localized FRBs. Such a high SFR is indicative of this FRB source being a new-born magnetar produced from a SN explosion of a massive star progenitor. Upper limits to the X-ray counterparts of 49 radio bursts observed in our simultaneous FAST, SRT and Chandra campaign are consistent with a magnetar scenario.

VOLKS2: A Transient Search and Localization Pipeline for VLBI Observations

(Published in The Astronomical Journal, Volume 162, Issue 4, id.159, 8 pp. (2021))

by L. Liu, Z. Xu, Z. Yan, W. Zheng, Y. Huang and Z. Chen

We present VOLKS2, the second release of "VLBI Observation for transient Localization Keen Searcher." The pipeline aims at transient search in regular VLBI observations as well as detection of single pulses from known sources in dedicated VLBI observations. The underlying method takes the idea of geodetic VLBI data processing, including fringe fitting to maximize the signal power and geodetic VLBI solving for localization. By filtering the candidate signals with multiple windows within a baseline and by cross matching with multiple baselines, RFIs are eliminated effectively. Unlike the station autospectrum-based method, RFI flagging is not required in the VOLKS2 pipeline. EVN observation (EL060) is carried out, so as to verify the pipelines detection efficiency and localization accuracy in the whole FoV. The pipeline is parallelized with MPI and further accelerated with GPU, so as to exploit the hardware resources of modern GPU clusters. We can prove that, with proper optimization, VOLKS2 could achieve comparable performance as autospectrum-based pipelines. All the code and documents are publicly available, in the hope that our pipeline is useful for radio transient studies.

Multiwavelength flare observations of the blazar S5 1803+784

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 502, Issue 4, pp.6177-6187 (2021))

by R. Nesci, S. Cutini, C. Stanghellini, F. Martinelli, A. Maselli, V.M. Lipunov, V. Kornilov, R.R. Lopez, A. Siviero, M. Giroletti, M. Orienti

The radio, optical, and γ-ray light curves of the blazar S5 1803+784, from the beginning of the Fermi-Large Area Telescope (LAT) mission in 2008 August-2018 December, are presented. The aim of this work is to look for correlations among different wavelengths useful for further theoretical studies. We analysed all the data collected by Fermi-LAT for this source, taking into account the presence of nearby sources, and we collected optical data from our own observations and public archive data to build the most complete optical and γ-ray light curve possible. Several γ-ray flares (F > 2.3 10-7ph(E > 0.1 GeV) cm-2 s-1) with optical coverage were detected, all but one with corresponding optical enhancement; we also found two optical flares without a γ-ray counterpart. We obtained two Swift Target of Opportunity observations during the strong flare of 2015. Radio observations performed with VLBA and EVN through our proposals in the years 2016-2020 were analysed to search for morphological changes after the major flares. The optical/γ-ray flux ratio at the flare peak varied for each flare. Very minor optical V - I colour changes were detected during the flares. The X-ray spectrum was well fitted by a power law with photon spectral index α = 1.5, nearly independent of the flux level: No clear correlation with the optical or the γ-ray emission was found. The radio maps showed two new components originating from the core and moving outwards, with ejection epochs compatible with the dates of the two largest γ-ray flares.

Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array: Inferences in the stochastic gravitational-wave background search

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 508, Issue 4, pp. 4970-4993 (2021))

by S. Chen, R. N. Caballero, Y. J. Guo, A. Chalumeau, K. Liu, G. Shaifullah, K. J. Lee, S. Babak, G. Desvignes, A. Parthasarathy, H. Hu, E. van der Wateren, J. Antoniadis, A.-S. Bak Nielsen, C. G. Bassa, A. Berthereau, M. Burgay, D. J. Champion, I. Cognard, M. Falxa, R. D. Ferdman, P. C. C. Freire, J. R. Gair, E. Graikou, L. Guillemot, J. Jang, G. H. Janssen, R. Karuppusamy, M. J. Keith, M. Kramer, X. J. Liu, A. G. Lyne, R. A. Main, J. W. McKee, M. B. Mickaliger, B. B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, A. Samajdar, S. A. Sanidas, A. Sesana, L. Speri, B. W. Stappers, G. Theureau C. Tiburzi, A. Vecchio, J. P. W. Verbiest, J. Wang, L. Wang and H. Xu

We present results from the search for a stochastic gravitational-wave background (GWB) as predicted by the theory of General Relativity using six radio millisecond pulsars from the Data Release 2 (DR2) of the European Pulsar Timing Array (EPTA) covering a timespan up to 24 years. A GWB manifests itself as a long-term low-frequency stochastic signal common to all pulsars, a common red signal (CRS), with the characteristic Hellings-Downs (HD) spatial correlation. Our analysis is performed with two independent pipelines, ENTERPRISE and TEMPONEST+FORTYTWO, which produce consistent results. A search for a CRS with simultaneous estimation of its spatial correlations yields spectral properties compatible with theoretical GWB predictions, but does not result in the required measurement of the HD correlation, as required for GWB detection. Further Bayesian model comparison between different types of CRSs, including a GWB, finds the most favoured model to be the common uncorrelated red noise described by a power-law with 𝐴 = 5.13 +4.20 −2.73 × 10^-15 and 𝛾 = 3.78 +0.69 -0.59 (95% credible regions). Fixing the spectral index to 𝛾 = 13/3 as expected from the GWB by circular, inspiralling supermassive black-hole binaries results in an amplitude of 𝐴 = 2.95 +0.89 -0.72 × 10^-15. We implement three different models, BAYESEPHEM, LINIMOSS and EPHEMGP, to address possible Solar-system ephemeris (SSE) systematics and conclude that our results may only marginally depend on these effects. This work builds on the methods and models from the studies on the EPTA DR1. We show that under the same analysis framework the results remain consistent after the data set extension.

The X-ray evolution and geometry of the 2018 outburst of XTE J1810-197

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 504, Issue 4, pp.5244-5257 (2021))

by A. Borghese, N. Rea, R. Turolla, M. Rigoselli, J.A.J. Alford, E.V. Gotthelf, M. Burgay, A. Possenti, S. Zane, F. Coti Zelati, R. Perna, P. Esposito, S. Mereghetti D. Viganó, A. Tiengo, D. Götz, A. Ibrahim, G.L. Israel, J. Pons, R. Sathyaprakash

After 15 years, in late 2018, the magnetar XTE J1810–197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM–Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from ∼ 7.2 × 10−12 s s^-1 to ∼ 1.5 × 10^-11 s s^-1 after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810–197 is well-modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energydependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810–197 has likely evolved relative to that found for the 2003 event.

Evidence of intra-binary shock emission from the redback pulsar PSR J1048+2339

(Published in Astronomy & Astrophysics, Vol 649, A120 (2021))

by A. Miraval Zanon, P. D’Avanzo, A. Ridolfi, F. Coti Zelati, S. Campana, C. Tiburzi, D. de Martino, T. Munoz Darias, C.G. Bassa, L. Zampieri, A. Possenti, F. Ambrosino, A. Papitto, M. C. Baglio, M. Burgay, A. Burtovoi, D. Michilli, P. Ochner, P. Zucca

We present simultaneous multiwavelength observations of the 4.66 ms redback pulsar PSR J1048+2339. We performed phase-resolved spectroscopy with the Very Large Telescope (VLT) searching for signatures of a residual accretion disk or intra-binary shock emission, constraining the companion radial velocity semi-amplitude (K2), and estimating the neutron star mass (MNS). Using the FORS2- VLT intermediate-resolution spectra, we measured a companion velocity of 291 < K2 < 348 km s^−1 and a binary mass ratio of 0.209 < q < 0.250. Combining our results for K2 and q, we constrained the mass of the neutron star and the companion to (1.0 < MNS < 1.6) sin^−3 i M and (0.24 < M2 < 0.33) sin^−3 i M , respectively, where i is the system inclination. The Doppler map of the Hα emission line exhibits a spot feature at the expected position of the companion star and an extended bright spot close to the inner Lagrangian point. We interpret this extended emission as the effect of an intra-binary shock originating from the interaction between the pulsar relativistic wind and the matter leaving the companion star. The mass loss from the secondary star could be either due to Roche-lobe overflow or to the ablation of its outer layer by the energetic pulsar wind. Contrastingly, we find no evidence for an accretion disk. We report on the results of the Sardinia Radio Telescope (SRT) and the Low-Frequency Array (LOFAR) telescope simultaneous radio observations at three different frequencies (150 MHz, 336 MHz, and 1400 MHz). No pulsed radio signal is found in our search. This is probably due to both scintillation and the presence of material expelled from the system which can cause the absorption of the radio signal at low frequencies. The confirmation of this hypothesis is given by another SRT observation (L-band) taken in 2019, in which a pulsed signal is detected. Finally, we report on an attempt to search for optical pulsations using IFI+Iqueye mounted at the 1.2 m Galileo telescope at the Asiago Observatory.

The New Magnetar SGR J1830-0645 in Outburst

(Published in The Astrophysical Journal Letters, Volume 907, Issue 2, id.L34, 9 pp. (2021))

by F. Coti Zelati, A. Borghese, G. L. Israel, N. Rea, P. Esposito, M. Pilia, M. Burgay, A. Possenti, A. Corongiu, A. Ridolfi, C. Dehman, D. Vigano, R. Turolla, S. Zane, A. Tiengo and E. F. Keane

The detection of a short hard X-ray burst and an associated bright soft X-ray source by the Swift satellite in 2020 October heralded a new magnetar in outburst, SGR J1830−0645. Pulsations at a period of ∼10.4 s were detected in prompt follow-up X-ray observations. We present here the analysis of the Swift/BAT burst, of XMM-Newton and the Nuclear Spectroscopic Telescope Array observations performed at the outburst peak, and of a Swift/XRT monitoring campaign over the subsequent month. The burst was single-peaked, lasted ∼6 ms, and released a fluence of ≈ 5 × 10^−9 erg cm^−2 (15–50 keV). The spectrum of the X-ray source at the outburst peak was well described by an absorbed double-blackbody model plus a power-law component detectable up to ∼25 keV. The unabsorbed X-ray flux decreased from ∼ 5 × 10^−11 to ∼ 2.5 × 10^−11 erg cm^−2 s^−1 one month later (0.3–10 keV). Based on our timing analysis, we estimate a dipolar magnetic field ≈ 5.5 × 10^14 G at pole, a spin-down luminosity ≈ 2.4 × 10^32 erg s^−1 , and a characteristic age ≈24 kyr. The spin modulation pattern appears highly pulsed in the soft X-ray band, and becomes smoother at higher energies. Several short X-ray bursts were detected during our campaign. No evidence for periodic or single-pulse emission was found at radio frequencies in observations performed with the Sardinia Radio Telescope and Parkes. According to magnetothermal evolutionary models, the real age of SGR J1830−0645 is close to the characteristic age, and the dipolar magnetic field at birth was slightly larger, ∼10^15 G.

Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz

(Published in Astronomy & Astrophysics, Volume 651, id.A98, 33 pp. (2021))

by S. Fatigoni, F. Radiconi, E. S. Battistelli, M. Murgia, E. Carretti, P. Castangia, R. Concu, P. de Bernardis, J. Fritz, R. Genova-Santos, F. Govoni, F. Guidi, L. Lamagna, S. Masi, A. Melis, R. Paladini, F. M. Perez-Toledo, F. Piacentini, S. Poppi, R. Rebolo, J. A. Rubino-Martin, G. Surcis, A. Tarchi, V. Vacca

The Andromeda galaxy is the best-known large galaxy besides our own Milky Way. Several images and studies exist at all wavelengths from radio to hard X-ray. Nevertheless, only a few observations are available in the microwave range where its average radio emission reaches the minimum. In this paper, we want to study the radio morphology of the galaxy, decouple thermal from nonthermal emission, and extract the star formation rate. We also aim to derive a complete catalog of radio sources for the mapped patch of sky. We observed the Andromeda galaxy with the Sardinia Radio Telescope at 6.6 GHz with very high sensitivity and angular resolution, and an unprecedented sky coverage. Using new 6.6 GHz data and Effelsberg radio telescope ancillary data, we confirm that, globally, the spectral index is ∼0.7−0.8, while in the star forming regions it decreases to ∼0.5. By disentangling (gas) thermal and nonthermal emission, we find that at 6.6 GHz, thermal emission follows the distribution of HII regions around the ring. Nonthermal emission within the ring appears smoother and more uniform than thermal emission because of diffusion of the cosmic ray electrons away from their birthplaces. This causes the magnetic fields to appear almost constant in intensity. Furthermore, we calculated a map of the star formation rate based on the map of thermal emission. Integrating within a radius of Rmax=15kpc, we obtained a total star formation rate of 0.19±0.01M⊙/yr in agreement with previous results in the literature. Finally, we correlated our radio data with infrared images of the Andromeda galaxy. We find an unexpectedly high correlation between nonthermal and mid-infrared data in the central region, with a correlation parameter r=0.93.

Extending the Z^2_n and H Statistics to Generic Pulsed Profiles

(Published in The Astrophysical Journal, Volume 909, Issue 1, id.33, 10 pp. (2021))

by M. Bachetti, M. Pilia, D. Huppenkothen, S.M. Ransom, S. Curatti, A. Ridolfi

The search for astronomical pulsed signals within noisy data, in the radio band, is usually performed through an initial Fourier analysis to find “candidate” frequencies and then refined through the folding of the time series using trial frequencies close to the candidate. In order to establish the significance of the pulsed profiles found at these trial frequencies, pulsed profiles are evaluated with a chi-squared test, to establish how much they depart from a null hypothesis where the signal is consistent with a flat distribution of noisy measurements. In high-energy astronomy, the chi-squared statistic has widely been replaced by the Z^2_n statistic and the H-test as they are more sensitive to extra information such as the harmonic content of the pulsed profile. The Z^2_n statistic and H-test were originally developed for the use with “event data”, composed of arrival times of single photons, leaving it unclear how these methods could be used in radio astronomy. In this paper, we present a version of the Z^2_n statistic and H-test for pulse profiles with Gaussian uncertainties, appropriate for radio or even optical pulse profiles. We show how these statistical indicators provide better sensitivity to low-significance pulsar candidates with respect to the usual chi-squared method, and a straightforward way to discriminate between pulse profile shapes. Moreover, they provide an additional tool for Radio Frequency Interference (RFI) rejection.

A parsec-scale faint jet in the nearby changing-look Seyfert galaxy Mrk 590

(Published in The Monthly Notices of the Royal Astronomical Society: Letters, Volume 502, Issue 1, pp.L61-L65 (2021))

by J. Yang, I. Van Bemmel, Z. Paragi, S. Komossa, F. Yuan, X. Yang, T. An, J.Y. Koay, C. Reynolds, J.B.R. Oonk, X. Liu, Q. Wu

Broad Balmer emission lines in active galactic nuclei (AGN) may display dramatic changes in amplitude, even disappearance and re-appearance in some sources. As a nearby galaxy at a redshift of 𝑧 = 0.0264, Mrk 590 suffered such a cycle of Seyfert type changes between 2006 and 2017. Over the last fifty years, Mrk 590 also underwent a powerful continuum outburst and a slow fading from X-rays to radio wavelengths with a peak bolometric luminosity reaching about ten per cent of the Eddington luminosity. To track its past accretion and ejection activity, we performed very long baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.6 GHz in 2015. The EVN observations reveal a faint (∼1.7 mJy) radio jet extending up to ∼2.8 mas (projected scale ∼1.4 pc) toward north, and probably resulting from the very intensive AGN activity. To date, such a parsec-scale jet is rarely seen in the known changing-look AGN. The finding of the faint jet provides further strong support for variable accretion as the origin of the type changes in Mrk 590.

The nearby extreme accretion and feedback system PDS 456: finding a complex radio-emitting nucleus

(Published in The Monthly Notices of the Royal Astronomical Society, Volume 500, Issue 2, pp.2620-2626 (2021))

by J. Yang, Z. Paragi, E. Nardini, W.A. Baan, L. Fan, P. Mohan, E. Varenius and T. An

When a black hole accretes close to the Eddington limit, the astrophysical jet is often accompanied by radiatively driven, wide-aperture and mildly relativistic winds. Powerful winds can produce significant non-thermal radio emission via shocks. Among the nearby critical accretion quasars, PDS 456 has a very massive black hole (about one billion solar masses), shows a significant star-forming activity (about seventy solar masses per year) and hosts exceptionally energetic X-ray winds (power up to twenty per cent of the Eddington luminosity). To probe the radio activity in this extreme accretion and feedback system, we performed very-long-baseline interferometric (VLBI) observations of PDS 456 at 1.66 GHz with the European VLBI Network (EVN) and the enhanced Multi-Element Remotely Linked Interferometry Network (e-MERLIN). We find a rarely-seen complex radio-emitting nucleus consisting of a collimated jet and an extended non-thermal radio emission region. The diffuse emission region has a size of about 360 pc and a radio luminosity about three times higher than the nearby extreme starburst galaxy Arp 220. The powerful nuclear radio activity could result from either a relic jet with a peculiar geometry (nearly along the line of sight) or more likely from diffuse shocks formed naturally by the existing high-speed winds impacting on high-density star-forming regions.

Investigating the mini and giant radio flare episodes of Cygnus X-3

(Published in The Astrophysical Journal, Volume 906, Issue 1, id.10, 9 pp. (2021))

by E. Egron, A. Pellizzoni, S. Righini, M. Giroletti, K. Koljonen, K. Pottschmidt, S. Trushkin, J. Lobina, M. Pilia, J. Wilms, S. Corbel, V. Grinberg, S. Loru, A. Trois, J. Rodriguez, A. Lähteenmäki, M. Tornikoski, S. Enestam and E. Järvelä

The microquasar Cygnus X-3 underwent a giant radio flare in April 2017, reaching a maximum flux of ∼ 16.5 Jy at 8.5 GHz. We present results from a long monitoring campaign carried out with Medicina at 8.5, 18.6 and 24.1 GHz, in parallel to the Metsahovi radio telescope at 37 GHz, from 4 to 11 April 2017. We observe a spectral steepening from α = 0.2 to 0.5 (with Sν ∝ ν −α) within 6 h around the epoch of the peak maximum of the flare. We further study the radio orbital modulation of Cyg X-3 emission associated with the 2017 giant flare and with six mini-flares observed in 1983, 1985, 1994, 1995, 2002 and 2016. The enhanced emission episodes observed during the decline of the giant flare at 8.5 GHz coincide with the orbital phase φ ∼ 0.5 (orbital inferior conjunction). On the other hand the light curves of the mini-flares observed at 15 − 22 GHz peak at φ ∼ 0, except for the 2016 light curve (obtained with VLBI observations including SRT) which is shifted of 0.5 w.r.t. the other ones. We attribute the apparent phase shift to the variable location of the emitting region along the bent jet. This might be explained by the different accretion states of the flaring episodes (the 2016 mini-flare occurred in the hypersoft X-ray state).

New high-frequency radio observations of the Cygnus Loop supernova remnant with the Italian radio telescopes

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 500, Issue 4, pp.5177-5194 (2021))

by S. Loru, A. Pellizzoni, E. Egron, A. Ingallinera, G. Morlino, S. Celli, G. Umana, C. Trigilio, P. Leto, M.N. Iacolina, S. Righini, P. Reich, S. Mulas, M. Marongiu, M. Pilia, A. Melis, R. Concu, M. Bufano, C. Buemi, F. Cavallaro, S. Riggi, and F. Schillirò

Supernova remnants (SNRs) represent a powerful laboratory to study the Cosmic-Ray acceleration processes at the shocks, and their relation to the properties of the circumstellar medium. With the aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0 to 24.8 GHz with the Medicina and the Sardinia Radio Telescope, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum well fitted by a power-law function, with spectral index α = 0.45 ± 0.05 for NGC 6992 and α = 0.49 ± 0.01 for the southern shell and without any indication of a spectral break. The spectra are significantly flatter than the whole Cygnus Loop spectrum (α = 0.54 ± 0.01), suggesting a departure from the plain shock acceleration mechanisms, which for NGC6992 could be related to the ongoing transition towards a radiative shock. We model the integrated spectrum of the whole SNR considering the evolution of the maximum energy and magnetic field amplification. Through the radio spectral parameters, we infer a magnetic field at the shock of 10 μG. This value is compatible with a pure adiabatic compression of the interstellar magnetic field, suggesting that the amplification process is currently inefficient.

The hyperluminous, dust-obscured quasar W2246-0526 at z = 4.6: detection of parsec-scale radio activity

(Published in The Astrophysical Journal Letters, Volume 905, Issue 2, id.L32, 7 pp. (2020))

by L. Fan, W. Chen, T. An, F.G. Xie, Y. Han, K.K. Knudsen, J. Yang

WISE J224607.56-052634.9 (W2246−0526) is a hyperluminous, dust-obscured and radio-quiet quasar at redshift z = 4.6. It plays a key role in probing the transition stage between dusty starbursts and unobscured quasars in the co-evolution of galaxies and supermassive black holes (SMBHs). To search for the evidence of the jet activity launched by the SMBH in W2246−0526, we performed very long baseline interferometry (VLBI) observations of its radio counterpart with the European VLBI Network (EVN) plus the enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) at 1.66 GHz and the Very Long Baseline Array (VLBA) at 1.44 and 1.66 GHz. The deep EVN plus e-MERLIN observations detect a compact (size ≤ 32 pc) sub-mJy component contributing about ten percent of its total flux density, which spatially coincides with the peak of dust continuum and [C II] emissions. Together with its relatively high brightness temperature (≥ 8×106 K), we interpret the component as a consequence of non-thermal radio activity powered by the central SMBH, which likely originates from a stationary jet base. The resolved-out radio emission possibly come from a diffuse jet, quasar-driven winds, or both, while the contribution by star formation activity is negligible. Moreover, we propose an updated geometry structure of its multi-wavelength active nucleus and shed light on the radio quasar selection bias towards the blazars at z > 4.

A perfect power-law spectrum even at highest frequencies: The Toothbrush relic

(Astronomy & Astrophysics, Volume 642, id.L13, 5 pp. (2020))

by K. Rajpurohit, F. Vazza, M. Hoeft, F. Loi, R. Beck, V. Vacca, M. Kierdorf, R. J. van Weeren, D. Wittor, F. Govoni, M. Murgia, C. J. Riseley, N. Locatelli, A. Drabent and E. Bonnassieux

Radio relics trace shock fronts generated in the intracluster medium (ICM) during cluster mergers. The particle acceleration mechanism at the shock fronts is not yet completely understood. We observed the Toothbrush relic with the Effelsberg and Sardinia Radio Telescope at 14.25 GHz and 18.6 GHz, respectively. Unlike previously claimed, the integrated spectrum of the relic closely follows a power law over almost three orders of magnitude in frequency, with a spectral index of −1.16±0.03. Our finding is consistent with a power-law injection spectrum, as predicted by diffusive shock acceleration theory. The result suggests that there is only little magnetic field strength evolution downstream to the shock. From the lack of spectral steepening, we find that either the Sunyaev-Zeldovich decrement produced by the pressure jump is less extended than ~600 kpc along the line of sight or, conversely, that the relic is located far behind in the cluster. For the first time, we detect linearly polarized emission from the "brush" at 18.6 GHz. Compared to 8.3 GHz, the degree of polarization across the brush increases at 18.6 GHz, suggesting a strong Faraday depolarization towards lower frequencies. The observed depolarization is consistent with an intervening magnetized screen that arise from the dense ICM containing turbulent magnetic fields. The depolarization, corresponding to a standard deviation of the Rotation Measures as high as 212±23rad/m2, suggests that the brush is located in or behind the ICM. Our findings indicate that the Toothbrush can be consistently explained by the standard scenario for relic formation. 

Spectro-polarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening

(Monthly Notices of the Royal Astronomical Society, Volume 498, Issue 2, pp.1628-1637 (2020))

by F. Loi, M. Murgia, V. Vacca, F. Govoni, A. Melis, D. Wittor, R. Beck, M. Kierdorf, A. Bonafede, W. Boschin, M. Brienza, E. Carretti, R. Concu, L. Feretti, F. Gastaldello, R. Paladino, K. Rajpurohit, P. Serra, and F. Vazza

Observations of radio relics at very high frequency (>10 GHz) can help to understand how particles age and are (re-)accelerated in galaxy cluster outskirts and how magnetic fields are amplified in these environments. In this work, we present new single-dish 18.6 GHz Sardinia Radio Telescope and 14.25 GHz Effelsberg observations of the well known northern radio relic of CIZA J2242.8+5301. We detected the relic which shows a length of ∼1.8 Mpc and a flux density equal to 9.5±3.9 mJy and 7.67±0.90 mJy at 14.25 GHz and 18.6 GHz respectively. The resulting best-fit model of the relic spectrum from 145 MHz to 18.6 GHz is a power-law spectrum with spectral index α = 1.12 ± 0.03: no evidence of steepening has been found in the new data presented in this work. For the first time, polarisation properties have been derived at 18.6 GHz, revealing an averaged polarisation fraction of ∼ 40% and a magnetic field aligned with the ’filaments’ or ’sheets’ of the relic.

The Lowest-frequency Fast Radio Bursts: Sardinia Radio Telescope Detection of the Periodic FRB 180916 at 328 MHz

(The Astrophysical Journal Letters, Volume 896, Issue 2, id.L40, 11 pp. (2020))

by M. Pilia, M. Burgay, A. Possenti, A. Ridolfi, V. Gajjar, A. Corongiu, D. Perrodin, G. Bernardi, G. Naldi, G. Pupillo, F. Ambrosino, G. Bianchi, A. Burtovoi, P. Casella, C. Casentini, M. Cecconi, C. Ferrigno, M. Fiori, K. C. Gendreau, A. Ghedina, G. Naletto, L. Nicastro, P. Ochner, E. Palazzi, F. Panessa, A. Papitto, C. Pittori, N. Rea, G. A. Rodriguez Castillo, V. Savchenko, G. Setti, M. Tavani, A. Trois, M. Trudu, M. Turatto, A. Ursi, F. Verrecchia and L. Zampieri

We report on the lowest-frequency detection to date of three bursts from FRB 180916.J0158+65, observed at 328 MHz with the Sardinia Radio Telescope (SRT). The SRT observed the periodic repeater FRB 180916.J0158+65 for five days from Feb. 20, 2020 to Feb. 24, 2020 during a time interval of active radio bursting, and detected the three bursts during the first hour of observations; no more bursts were detected during the remaining ∼ 30 hours. Simultaneous SRT observations at 1548 MHz did not detect any bursts. Burst fluences are in the range 13 to 37 Jy ms. No relevant scattering is observed for these bursts. We also present the results of the multi-wavelength campaign we performed on FRB 180916.J0158+65, during the ∼ 5 days of the active window. Simultaneously with the SRT observations, observations with different time spans were performed with the Northern Cross at 408 MHz, with XMM-Newton, NICER, INTEGRAL, AGILE and with the TNG and two optical telescopes in Asiago, which are equipped with fast photometers.

A Very Young Radio-loud Magnetar

(Published in The Astrophysical Journal Letters, Volume 896, Issue 2, id.L30, 8 pp. (2020))

by P. Esposito, N. Rea, A. Borghese, F. Coti Zelati, D. Vigano, G. L. Israel, A. Tiengo, A. Ridolfi, A. Possenti, M. Burgay, D. Gotz, F. Pintore, L. Stella, C. Dehman, M. Ronchi, S. Campana, A. Garcia-Garcia, V. Graber, S. Mereghetti, R. Perna, G. A. Rodriguez Castillo, R. Turolla and S. Zane

The magnetar Swift J1818.0–1607 was discovered in March 2020 when Swift detected a 9 ms hard X-ray burst and a long-lived outburst. Prompt X-ray observations revealed a spin period of 1.36 s, soon confirmed by the discovery of radio pulsations. We report here on the analysis of the Swift burst and follow-up X-ray and radio observations. The burst average luminosity was ∼ 2 × 10^39 erg/s (at 4.8 kpc). Simultaneous observations with XMM–Newton and NuSTAR three days after the burst provided a source spectrum well fit by an absorbed blackbody plus a power-law (Γ = 0.0 ± 1.3) in the 1–20 keV band, with a luminosity of ∼8×10^34 erg/s, dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic field B ∼ 7 × 10^14 G, spin-down luminosity ∼ 1.4 × 10^36 erg/s and characteristic age of 240 yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity <5.5 × 10^33 erg/s, lower than the value expected from magnetar cooling models at the source characteristic age. A 1 hr radio observation with the Sardinia Radio Telescope taken about 1 week after the X-ray burst detected a number of strong and short radio pulses at 1.5 GHz, in addition to regular pulsed emission; they were emitted at an average rate 0.9 /min and accounted for ∼50% of the total pulsed radio fluence. We conclude that Swift J1818.0–1607 is a peculiar magnetar belonging to the small, diverse group of young neutron stars with properties straddling those of rotationally and magnetically powered pulsars. Future observations will make a better estimation of the age possible by measuring the spin-down rate in quiescence.

Simultaneous multi-telescope observations of FRB 121102

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 496, Issue 4, pp.4565-4573, 2020)

by M. Caleb, B. W. Stappers, T. D. Abbott, E. D. Barr, M. C. Bezuidenhout, S. J. Buchner, M. Burgay, W. Chen, I. Cognard, L. N. Driessen, R. Fender, G. H. Hilmarsson, J. Hoang, D. M. Horn, F. Jankowski, M. Kramer, D. R. Lorimer, M. Malenta, V. Morello, M. Pilia, E. Platts, A. Possenti, K. M. Rajwade, A. Ridolfi, L. Rhodes, S. Sanidas, M. Serylak, L. G. Spitler, L. J. Townsend, A. Weltman, P. A. Woudt, J. Wu

We present 11 detections of FRB 121102 in ∼3 h of observations during its 'active' period on the 10th of 2019 September. The detections were made using the newly deployed MeerTRAP system and single pulse detection pipeline at the MeerKAT radio telescope in South Africa. Fortuitously, the Nançay radio telescope observations on this day overlapped with the last hour of MeerKAT observations and resulted in four simultaneous detections. The observations with MeerKAT's wide band receiver, which extends down to relatively low frequencies (900-1670 MHz usable L-band range), have allowed us to get a detailed look at the complex frequency structure, intensity variations, and frequency-dependent sub-pulse drifting. The drift rates we measure for the full-band and sub-banded data are consistent with those published between 600 and 6500 MHz with a slope of −0.147 ± 0.014 m/s. Two of the detected bursts exhibit fainter 'precursors' separated from the brighter main pulse by ∼28 and ∼34 ms. A follow-up multi-telescope campaign on the 6th and 8th of 2019 October to better understand these frequency drifts and structures over a wide and continuous band was undertaken. No detections resulted, indicating that the source was 'inactive' over a broad frequency range during this time.

A Space Debris-Dedicated Channel for the P-Band Receiver of the Sardinia Radio Telescope: A Detailed Description and Characterization

(Published in IEEE Antennas and Propagation Magazine, vol. 62, no. 3, pp. 45-57, June 2020)

by G. Muntoni, L. Schirru, G. Montisci, T. Pisanu, G. Valente, P. Ortu, R. Concu, A. Melis, E. Urru, A. Saba, F. Gaudiomonte and G. Bianchi

Space debris (SD) is an important problem for space operations and a hot topic for space surveillance research. A structured network of radar sensors is available worldwide, offering vital information about debris status. Recently, the Sardinia Radio Telescope (SRT), a 64-m dish located in Sardinia, Italy, was introduced in the European plan for SD monitoring. Because the SRT is devoted to radio astronomy applications and is not optimized for SD echo detection, the need for an SD-dedicated channel has emerged. In this article, we present a detailed description and characterization of the new P-band receiving chain for SRT's space debris-monitoring purposes. As a test of its capabilities, the new channel was used to detect the echoes that emanated from the Chinese space station Tiangong-1 when it reentered Earth's atmosphere on its latest mission.

The nature of the methanol maser ring G23.657-00.127. II. Expansion of the maser structure

(Accepted for publication in Astronomy & Astrophysics, Volume 637, id.A15, 12 pp. (2020))

by A. Bartkiewicz, A. Sanna, M. Szymczak, L. Moscadelli, H.J. van Langevelde and P. Wolak

Ring-like distributions of the 6.7 GHz methanol maser spots at milliarcsecond scales represent a family of molecular structures of unknown origin associated with high-mass young stellar objects (HMYSOs). We studied G23.657-00.127, which has a nearly circular ring of the 6.7 GHz methanol masers, and is the most suitable target to test hypotheses on the origin of the maser rings. The European Very Long Baseline Interferometry Network (EVN) was used at three epochs spanning 10.3 yr to derive the spatio-kinematical structure of the 6.7 GHz methanol maser emission in the target. Interferometric data demonstrated that about one quarter of cloudlets show significant variability in their brightness, although the overall spectrum was non-variable in single-dish studies.

Methods for detection and analysis of weak radio sources with single-dish radio telescopes

(Accepted for publication in Experimental Astronomy, Volume 49, Issue 3, p.159-182 (2020))

by M. Marongiu, A. Pellizzoni, E. Egron, T. Laskar, M. Giroletti, S. Loru, A. Melis, G. Carboni, C. Guidorzi, S. Kobayashi, N. Jordana-Mitjans, A. Rossi, C. G. Mundell, R. Concu, R. Martone and L. Nicastro

The detection of mJy/sub-mJy point sources is a significant challenge for single-dish radio telescopes. Detection or upper limits on the faint afterglow from GRBs or other sources at cosmological distances are important means of constraining the source modeling. Using the Sardinia Radio Telescope (SRT), we compare the sensitivity and robustness of three methods applied to the detection of faint radio sources from raster maps around a known source position: the smart ’quick-look’ method, the ’source extraction’ method (typical of high-energy astronomy), and the fit with a 2-D Gaussian. We developed a Python code specific for the analysis of point-like radio sources applied to the SRT C-band (6.9 GHz) observations of both undetected sources (GRB afterglows of 181201A and 190114C) and the detected Galactic X-ray binary GRS 1915+105. Our comparative analysis of the different detection methods made extensive use of simulations as a useful complement to actual radio observations. The best method for the SRT data analysis is the fit with a 2-D Gaussian, as it pushes down the sensitivity limits of single-dish observations - with respect to more traditional techniques - to 1.8 mJy, improving by ∼ 40 % compared with the initial value. This analysis shows that especially for faint sources, good maps of the scanned region pre- or post-outburst are essential.

A two-sided but significantly beamed jet in the supercritical accretion quasar IRAS F11119+3257

(Accepted for publication in the Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 2, pp.1744-1750 (2020))

by J. Yang, Z. Paragi, T. An, W.A. Baan, P. Mohan and X. Liu

Highly accreting quasars are quite luminous in the X-ray and optical regimes. While, they tend to become radio quiet and have optically thin radio spectra. Among the known quasars, IRAS F11119+3257 is a supercritical accretion source because it has a bolometric luminosity above the Eddington limit and extremely powerful X-ray outflows. To probe its radio structure, we investigated its radio spectrum between 0.15 and 96.15 GHz and performed very-long-baseline interferometric (VLBI) observations with the European VLBI Network (EVN) at 1.66 and 4.93 GHz. The deep EVN image at 1.66 GHz shows a two-sided jet with a projected separation about two hundred parsec and a very high flux density ratio of about 290. Together with the best-fit value of the integrated spectral index of -1.31±0.02 in the optically thin part, we infer that the approaching jet has an intrinsic speed at least 0.57 times of the light speed. This is a new record among the known all kinds of super-Eddington accreting sources and unlikely accelerated by the radiation pressure. We propose a scenario in which IRAS F11119+3257 is an unusual compact symmetric object with a small jet viewing angle and a radio spectrum peaking at 0.53±0.06 GHz mainly due to the synchrotron self-absorption.

VLBI observations of the H2O gigamaser in TXS2226-184

(Accepted for publication in Astronomy and Astrophysics, Volume 637, id.A57, 12 pp. (2020))

by G. Surcis, A. Tarchi and P. Castangia

Outside the Milky Way, the most luminous H2O masers at 22 GHz, called 'megamasers' because of their extreme luminosity with respect to the Galactic and extragalactic H2O masers associated with star formation, are mainly detected in active galactic nuclei. In the case of the H2O maser detected in the nuclear region of the galaxy TXS2226-184 for the first time the term 'gigamaser' was used. However, the origin of this very luminous H2O maser emission has never been investigated into details. We study the nature of the H2O gigamaser in TXS2226-184 by measuring for the first time its absolute position at mas resolution, by comparing the morphology and characteristics of the maser emission at the VLBI scales after about 20 years, and by trying to detect its polarized emission. We observed the maser emission towards TXS2226-184 three times, the first time with the VLBA then with the EVN. In this last epoch the Sardinia Radio Telescope was part of the observing array, its participation allowed to decrease the thermal noise and consequently to increase the signal to noise ratio of the observations. All of the water maser features but one are red-shifted with respect to the systemic velocity of TXS2226-184, and for the first time we were able to measure their absolute position with errors below 1 mas. No linear and circular polarization was detected. We were also able to associate the H2O maser features in TXS2226-184 with the most luminous radio continuum clump reported in the literature.

Sardinia Radio Telescope observations of Local Group dwarf galaxies - I. The cases of NGC 6822, IC 1613, and WLM

(Monthly Notices of the Royal Astronomical Society, Volume 492, Issue 1, p.45-57, 2020)

by A. Tarchi, P. Castangia, G. Surcis, A. Brunthaler, C. Henkel, M. Pawlowski, K. M. Menten, A. Melis, S. Casu, M. Murgia, A. Trois, R. Concu and J. Darling

Almost all dwarf galaxies in the Local Group that are not satellites of the Milky Way or M31, belong to either one of two highly-symmetric planes. It is still a matter of debate, whether these planar structures are dynamically stable or whether they only represent a transient alignment. Proper motions, if they could be measured, could help to discriminate between these scenarios. Such motions could be determined with multi-epoch Very Long Baseline Interferometry (VLBI) of sources that show emission from water and methanol at frequencies of 22 and 6.7 GHz, respectively. We present a search for such masers performed with the SRT in three Local Group galaxies, NGC6822, IC1613 and WLM, members of the two above mentioned planes. Radio continuum maps of these galaxies have been obtained and used to identify star forming regions and to derive global galactic star formation rates. No methanol or water maser emission has been confidently detected, down to line luminosity limits of ~0.004 and 0.01 solar luminosities, respectively. This finding is consistent with the small sizes, low SFRs and metallicities of these galaxies.

The flickering nuclear activity of Fornax A

(Astronomy & Astrophysics, Volume 634, id.A9, 26 pp, 2020)

by F. M. Maccagni, M. Murgia, P. Serra, F. Govoni, K. Morokuma-Matsui, D. Kleiner, S. Buchner, G. I. J Józsa, P. Kamphuis, S. Makhathini, D. Cs Molnár, D. A. Prokhorov, A. Ramaila, M. Ramatsoku, K. Thorat and O. Smirnov

We present new observations of Fornax A taken at 1 GHz with the MeerKAT telescope and at 6 GHz with the Sardinia Radio Telescope (SRT). The sensitive (noise ~ 16 micro-Jy beam$^{-1}), high resolution (< 10'') MeerKAT images show that the lobes of Fornax A have a double-shell morphology, where dense filaments are embedded in a diffuse and extended cocoon. We study the spectral properties of these components by combining the MeerKAT and SRT observations with archival data between 84 MHz and 217 GHz. For the first time, we show that multiple episodes of nuclear activity must have formed the extended radio lobes. The modelling of the radio spectrum suggests that the last episode of injection of relativistic particles into the lobes started ~ 24 Myr ago and stopped 12 Myr ago. More recently (~ 3 Myr ago), a less powerful and short (< 1 Myr) phase of nuclear activity generated the central jets. Currently, the core may be in a new active phase. It appears that Fornax A is rapidly flickering. The dense environment in which Fornax A lives has lead to a complex recent merger history for this galaxy, including mergers spanning a range of gas contents and mass ratios, as shown by the analysis of the galaxy's stellar- and cold-gas phases. This complex recent history may be the cause of the rapid, recurrent nuclear activity of Fornax A.

The Nearby Luminous Transient AT2018cow: A Magnetar Formed in a Subrelativistically Expanding Non-jetted Explosion

(The Astrophysical Journal Letters, Volume 888, Issue 2, id.L24, 2020)

by  P. Mohan, T. An and J. Yang

The fast-rising blue optical transient AT2018cow indicated unusual early-phase characteristics unlike relatively better studied explosive transients. Its afterglow may be produced by either a relativistically beamed (jetted) or intrinsically luminous (non-jetted) ejecta and carries observational signatures of the progenitor and environment. High-resolution monitoring can distinguish between these scenarios and clarify the progenitor nature. We present very long baseline interferometry (VLBI) observations of AT2018cow at 5 GHz involving 21 radio telescopes from the European VLBI Network with five sessions spanning ≈1 yr. With an astrometric precision up to 25 μas per epoch, the rapidly fading compact mas-scale source is found to be non-jetted with a proper motion of ≤ 0.15 mas/yr (0.14 c). This and a dense magnetized environment (magnetic field strength ≥0.84 G) are characteristic of a newly formed magnetar driven central engine, originating in the successful explosion of a low-mass star.

Orbit Determination of Resident Space Objects Using the P-Band Mono-Beam Receiver of the Sardinia Radio Telescope

(Published in Applied Science, 9(19), 4092 (2019))

by  M. Losacco and L. Schirru

The population of space debris in near-Earth space is continuously growing and it represents a serious problem for active satellites and spacecraft. A performant ground-based and space-based network of sensors is necessary for space surveillance and consequently to prevent new collisions and monitoring atmospheric reentry of these objects. This paper illustrates the possible role of the Italian ground-based novel bi-static radar sensor, named BIRALET, for space monitoring and resident space objects tracking. The main characteristics of the receiver system, the Sardinia Radio Telescope with its P-band mono-beam receiver, are described in detail. Then, a preliminary analysis of the performance of the sensor is presented, and the results of numerical simulations are shown, providing a general overview on both observation capabilities and orbit determination accuracy achievable with the Sardinia Radio Telescope.

Water masers in Compton-thick AGN. II. The high detection rate and EVN observations of IRAS 15480-0344

(Astronomy & Astrophysics, Volume 629, id.A25, 11 pp, 2019)

by P. Castangia, G. Surcis, A. Tarchi, A. Caccianiga, P. Severgnini and R. Della Ceca

The radio emission from luminous H2O masers, the so-called “megamasers”, constitutes the only way to directly map the molecular gas at sub-parsec distance from supermassive black holes. Studies of megamaser sources not only allow us to constrain accretion disk geometry but also to improve our understanding of the jet (or outflows) interaction with the ISM of the host galaxies. Employing a sensitive array of EVN antennas, which included the SRT, we performed deep images of the nuclear radio continuum emission from the mid-IR-bright Seyfert 2 galaxy IRAS 15480−0344 which hosts a luminous H2O maser. We resolved the radio continuum emission from the innermost regions of IRAS 15480-0344 into two bright components (labeled SW and NE). The properties of these sources (spectral indices, brightness temperatures, dimensions, and radio power) indicate that their radio emission is synchrotron radiation, most likely produced by two weak knots, part of a compact radio jet. Both components show evidences for strong interaction with a dense interstellar medium. VLBI observations allowed us to locate the masers positions w.r.t. the main nuclear components, determining the nature of the water maser. Indeed, the narrow maser line, M1, might trace the position of the core (not visible in the radio continuum images) and be associated with the accretion disk or a nuclear outflow. The broad maser feature, M2, instead, coincides with source NE, suggesting that the maser emission might be produced by the interaction of the jet with the interstellar medium, as it was proposed for the masers in NGC 1068 and Mrk 348, adding a new source to the few confirmed jet-masers reported so far.

Strong Evidence of Anomalous Microwave Emission from the Flux Density Spectrum of M31

(The Astrophysical Journal Letters, Volume 877, Issue 2, article id. L31, 7 pp, 2019)

by E. S. Battistelli, S. Fatigoni, M. Murgia, A. Buzzelli, E. Carretti, P. Castangia, R. Concu, A. Cruciani, P. de Bernardis, R. Genova-Santos, F. Govoni, F. Guidi, L. Lamagna, G. Luzzi, S. Masi, A. Melis, R. Paladini, F. Piacentini, S. Poppi, F. Radiconi, R. Rebolo, J. A. Rubino-Martin, A. Tarchi, and V. Vacca

The Andromeda galaxy is the largest of the galaxies of the Local Group hosting also our Milky Way. It is a very well studied galaxy at all wavelengths with the exception of the microwave band. Thanks to the observations undertaken with the 64-m Sardinia Radio Telescope, we  completely mapped the Andromeda galaxy in the microwave band at wavelength of 4,5cm (frequency 6.7GHz), with the aim to study its astrophysical emission and the reasons for such emission. Thanks to the state of the art technology implemented in the SRT, as well as to the excellent sensitivity and stability of the observations, it has been observed that, beyond the classical emission mechanisms originated by the interaction of the interstellar medium with the galaxy magnetic field, Andromeda shows the Anomalous emission in excess with respect to what expected which requires a new emission mechanism. The most accepted models foresee that this emission is due to rapidly rotating very small dust grains of the interstellar medium, emitting through electric dipole emission: Spinning Dust.

Imaging of the solar atmosphere in the centimetre-millimetre band through single-dish observations

(Il Nuovo Cimento C, Volume 42, Issue 1, article id. 9, 4 pp, 2019)

by A. Pellizzoni, S. Righini, G. Murtas, F. Buffa, R. Concu, E. Egron, M. N. Iacolina, S. Loru, A. Maccaferri, A. Melis, A. Navarrini, A. Orfei , P. Ortu, T. Pisanu, A. Saba, G. Serra, G. Valente, A. Zanichelli, P. Zucca and M. Messerotti

Solar observations offer both a rich interdisciplinary laboratory on fundamental astrophysics and precious tools for Space Weather applications. The involved plasma processes determine a complex radio emission picture that could be efficiently explored through single-dish imaging at high frequencies. In particular, mapping the brightness temperature of the free-free radio emission in the centimetre and millimetre range is an effective tool to characterise the vertical structure of the solar atmosphere. We are performing continuum imaging of the solar chromosphere in K-band (18-26.5 GHz, spatial resolution ∼1 arcmin) with the 32-m diameter Medicina radio telescope and with the 64-m diameter Sardinia Radio Telescope (SRT), as a first scientific demonstration test for the potentialities of Italian single-dish antennas in this field.

Resolving the Decades-long Transient FIRST J141918.9+394036: An Orphan Long Gamma-Ray Burst or a Young Magnetar Nebula?

(The Astrophysical Journal Letters, Volume 876, Issue 1, article id. L14, 7 pp, 2019)

by B. Marcote, K. Nimmo, K., O.S. Salafia, Z. Paragi, J.W.T. Hessels, E. Petroff and R. Karuppusamy

FIRST J1419+3940 was identified as a radio source sharing similar properties and host galaxy type to the compact, persistent radio source associated with the first known repeating fast radio burst, FRB 121102. FIRST J1419+3940 is a transient source decaying in brightness over the last few decades. One possible interpretation is that it is a nearby analogue to FRB 121102 and that the radio emission represents a young magnetar nebula. Another interpretation is that FIRST J1419+3940 is the afterglow of an ‘orphan’ long gamma-ray burst (GRB). The environment is similar to where most such events are produced. To distinguish between these hypotheses, we conducted radio observations using the European VLBI Network at 1.6 GHz to spatially resolve the emission and to search for millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact radio source with a flux density of 620 ± 20 µJy and a source size of 3.9 ± 0.7 mas (i.e. 1.6 ± 0.3 pc given the angular diameter distance of 83 Mpc). These results confirm that the radio emission is non-thermal and imply an average expansion velocity of (0.10 ± 0.02)c. The source properties and lack of short-duration bursts are consistent with a GRB jet expansion, whereas they disfavor a magnetar birth nebula.

Investigating the high-frequency spectral features of SNRs Tycho, W44 and IC443 with the Sardinia Radio Telescope

(Monthly Notices of the Royal Astronomical Society, Volume 482, Issue 3, p.3857-3867, 2019)

by S. Loru, A. Pellizzoni, E. Egron, S. Righini, M. N. Iacolina, S. Mulas, M. Cardillo, M. Marongiu, R. Ricci, M. Bachetti, M. Pilia, A. Trois, A. Ingallinera, O. Petruk, G. Murtas, G. Serra, F. Buffa, R. Concu, F. Gaudiomonte, A. Melis, A. Navarrini, D. Perrodin, G. Valente

Electron acceleration mechanisms can shape the spectra of Supernova Remnants (SNRs) in specific ways, especially at high radio frequencies. These features are connected to the age and the peculiar conditions of the local interstellar medium interacting with the SNR. Whereas the bulk radio emission is expected at up to 20 − 50 GHz, sensitive high-resolution images of SNRs above 10 GHz are lacking and are not easily achievable, especially in the confused regions of the Galactic Plane. We obtained high-resolution images of SNRs Tycho, W44 and IC443 that provided accurate integrated flux density measurements at 21.4 GHz. We coupled the SRT measurements with radio data available in the literature in order to characterise the integrated and spatially-resolved spectra of these SNRs, and to find significant frequency- and region-dependent spectral slope variations. For the first time, we provide direct evidence of a spectral break in the radio spectral energy distribution of W44 at an exponential cutoff frequency of 15 ± 2 GHz.

VLBI Non-detection of a Candidate Dual AGN in a Galaxy Merger

(Research Notes of the American Astronomical Society, Volume 3, Issue 1, article id. 1, 2019)

by S. Frey, D. Lena, P.G. Jonker, K.E. Gabányi and Z. Paragi

Numerical hydrodynamical simulations show that nuclear activity is experienced in multiple episodes during major galaxy mergers. Simultaneous activity in both nuclei becomes most likely at separations  < 10  kpc. These systems where the central supermassive black holes (SMBHs) are still gravitationally unbound are called dual active galactic nuclei (AGNs). Characterizing a large sample of dual AGNs would help constraining the merger rate of galaxies and SMBHs, understanding the role of galaxy mergers in triggering AGN activity, and making predictions for SMBH coalescence producing gravitational waves. However, securely identified dual AGNs are rare. Confirmation of each case would ideally require AGN indicators at multiple wavebands. Although radio emission is associated with a minority of AGNs, the very long baseline interferometry (VLBI) technique plays a unique role in confirming emission from dual AGNs, owing to its high angular resolution.

Mapping the neutral atomic hydrogen gas outflow in the restarted radio galaxy 3C 236

(Published in Astronomy & Astrophysics, Volume 617, id.A38, 12 pp. (2018))

by R. Schulz, R. Morganti, K. Nyland, Z. Paragi, E. K. Mahony and T. Oosterloo

The energetic feedback that is generated by radio jets in active galactic nuclei (AGNs) has been suggested to be able to produce fast outflows of atomic hydrogen (H I) gas, which can be studied in absorption at high spatial resolution. We have used the Very Large Array (VLA) and a global very long baseline interferometry (VLBI) array to locate and study in detail the H I outflow discovered with the Westerbork Synthesis Radio Telescope (WSRT) in the restarted radio galaxy 3C 236. Based on the VLA data, we confirm a blueshifted wing of the H I with a width of 1000 km/s. This H I outflow is partially recovered by the VLBI observation. In particular, we detect four clouds with masses of 0.28 - 1.5 × 10^4M⊙ with VLBI that do not follow the regular rotation of most of the H I. Three of these clouds are located, in projection, against the nuclear region on scales of ≲40 pc, while the fourth is cospatial to the southeast lobe at a projected distance of 270 pc. Their velocities are between 150 and 640 km/s blueshifted with respect to the velocity of the disk-related H I. These findings suggest that the outflow is at least partly formed by clouds, as predicted by some numerical simulations, and that it originates already in the inner (few tens of pc) region of the radio galaxy. Our results indicate that the entire outflow might consist of many clouds, possibly with comparable properties as those clearly detected, but distributed at larger radii from the nucleus where the lower brightness of the lobe does not allow us to detect them.

High-resolution Radio Image of a Candidate Radio Galaxy at z = 5.72

(Research Notes of the American Astronomical Society, Volume 2, Issue 4, article id. 200, 2018)

by K.E. Gabányi, S. Frey, L. Gurvits, Z. Paragi and K. Perger

Recently, Saxena et al. (2018) reported the discovery of a possible radio galaxy, J1530+1049 at a redshift of z=5.72. We observed the source with the European Very Long Baseline Interferometry Network at 1.7 GHz. We detected two faint radio features with a separation of about 400 mas. The radio power calculated from the VLA flux density by Saxena et al. (2018), and the projected source size derived from our EVN data place J1530+1049 among the medium-sized symmetric objects (MSOs) which are thought to be young counterparts of radio galaxies (An and Baan 2012). Thus, our finding is consistent with a radio galaxy in an early phase of its evolution as proposed by Saxena et al. (2018).

Duty cycle of the radio galaxy B2 0258+35

(Astronomy & Astrophysics, Volume 618, id.A45, 12 pp, 2018)

by M. Brienza, R. Morganti, M. Murgia, N. Vilchez, B. Adebahr, E. Carretti, R. Concu, F. Govoni, J. Harwood, H. Intema, F. Loi, A. Melis, R. Paladino, S. Poppi, A. Shulevski, V. Vacca, G. Valente

We have investigated the duty cycle of the radio source B2 0258+35, which was previously suggested to be a restarted radio galaxy based on its morphology. The radio galaxy consists of a pair of kpc-scale jets embedded in two large-scale lobes (240 kpc) with relaxed shape and very low surface brightness that resemble remnants of a past AGN activity. Thanks to a combination of new SRT observations at 6600 MHz with new LOFAR observations at 145 MHz and archive WSRT data at 1400 MHz, we have investigated the spectral properties of the outer lobes to derive their age. Interestingly, the spectrum of both the Northern and Southern lobes is not ultra-steep or significantly curved over the entire available frequency range as expected for an old ageing plasma. We conclude that either the large-scale lobes are still fuelled by the nuclear engine or the jets have switched off no more than a few tens of Myr ago, allowing us to observe both the inner and outer structure simultaneously. Our study shows the importance of combining morphological and spectral properties to reliably classify the evolutionary stage of low surface brightness, diffuse emission that low frequency observations are revealing around a growing number of radio sources.

The SUrvey for Pulsars and Extragalactic Radio Bursts - II. New FRB discoveries and their follow-up

(Published in the Monthly Notices of the Royal Astronomical Society, Volume 475, Issue 2, p.1427-1446 (2018))

by S. Bhandari, E. F. Keane, E. D. Barr, A. Jameson, E. Petroff, S. Johnston, M. Bailes, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, M. Caleb, R. P. Eatough, C. Flynn, J. A. Green, F. Jankowski, M. Kramer, V. Venkatraman Krishnan, V. Morello, A. Possenti, B. Stappers, C. Tiburzi, W. van Straten, I. Andreoni, T. Butterley, P. Chandra, J. Cooke, A.Corongiu et al.

We report the discovery of four Fast Radio Bursts (FRBs) in the ongoing SUrvey for Pulsars and Extragalactic Radio Bursts at the Parkes Radio Telescope: FRBs 150610, 151206, 151230 and 160102. Our real-time discoveries have enabled us to conduct extensive, rapid multimessenger follow-up at 12 major facilities sensitive to radio, optical, X-ray, gamma-ray photons and neutrinos on time-scales ranging from an hour to a few months post-burst. No counterparts to the FRBs were found and we provide upper limits on afterglow luminosities. None of the FRBs were seen to repeat. Formal fits to all FRBs show hints of scattering while their intrinsic widths are unresolved in time. FRB 151206 is at low Galactic latitude, FRB 151230 shows a sharp spectral cut-off, and FRB 160102 has the highest dispersion measure (DM = 2596.1 ± 0.3 pc cm-3) detected to date. Three of the FRBs have high dispersion measures (DM > 1500 pc cm-3), favouring a scenario where the DM is dominated by contributions from the intergalactic medium.

Observations of a nearby filament of galaxy clusters with the Sardinia Radio Telescope

(Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 1, p.776-806, 2018)

by V. Vacca, M. Murgia, F. Govoni, F. Loi, F. Vazza, A. Finoguenov, E. Carretti, L. Feretti, G. Giovannini, R. Concu, A. Melis, C. Gheller, R. Paladino, S. Poppi, G. Valente, G. Bernardi, W. Boschin, M. Brienza, T. A. Clarke, S. Colafrancesco, T. E. Ensslin, C. Ferrari, F. de Gasperin, F. Gastaldello, M. Girardi, L. Gregorini, M. Johnston-Hollitt, H. Junklewitz, E. Orru, P. Parma, R. Perley, G.B Taylor

We report the detection of diffuse radio emission which might be connected to a large-scale filament of the cosmic web covering a 8deg x 8deg area in the sky, likely associated with a z~0.1 over-density traced by nine massive galaxy clusters. In this work, we present radio observations of this region taken with the SRT. To investigate the presence of large-scale diffuse radio synchrotron emission in and beyond the galaxy clusters in this complex system, we combined the data taken at 1.4GHz with the SRT with higher resolution data taken with the NVSS. We found 28 candidate new sources with a size larger and X-ray emission fainter than known diffuse large-scale synchrotron cluster sources for a given radio power. This new population is potentially the tip of the iceberg of a class of diffuse large-scale synchrotron sources associated with the filaments of the cosmic web.

Radio emission in ultracool dwarfs: The nearby substellar triple system VHS 1256-1257

(Astronomy & Astrophysics, Volume 610, id.A23, 6 pp., 2018)

by J.C. Guirado, R. Azulay, R., B. Gauza, M.A. Pérez-Torres, R. Rebolo, J.B. Climent and M.R. Zapatero Osorio

With the purpose of investigating the radio emission of new ultracool objects, we carried out a targeted search in the recently discovered system VHS J125601.92-125723.9 (hereafter VHS 1256-1257); this system is composed by an equal-mass M7.5 binary and a L7 low-mass substellar object located at only 15.8 pc. We observed VHS 1256-1257 with the VLA at X band and L band and with the European VLBI Network at L band in several epochs during 2015 and 2016. We discovered radio emission at X band spatially coincident with the equal-mass M7.5 binary with a flux density of 60 μJy. We determined a spectral index α = -1.1 ± 0.3 between 8 and 12 GHz, suggesting that non-thermal, optically thin, synchrotron, or gyrosynchrotron radiation is responsible for the observed radio emission. Interestingly, no signal is seen at L band where we set a 3σ upper limit of 20 μJy. This might be explained by strong variability of the binary or self-absorption at this frequency.

The 999th Swift gamma-ray burst: Some like it thermal. A multiwavelength study of GRB 151027A

(Nature Astronomy, Volume 2, p. 118-125 (2018))

by T. An, B. W. Sohn , H. Imai

The very long baseline interferometry (VLBI) technique offers angular resolutions superior to any other instruments at other wavelengths, enabling unique science applications of high-resolution imaging of radio sources and high-precision astrometry. The East Asia VLBI Network (EAVN) is a collaborative effort in the East Asian region. The EAVN currently consists of 21 telescopes with diverse equipment configurations and frequency setups, allowing flexible subarrays for specific science projects. The EAVN provides the highest resolution of 0.5 mas at 22 GHz, allowing the fine imaging of jets in active galactic nuclei, high-accuracy astrometry of masers and pulsars, and precise spacecraft positioning. The soon-to-be-operational Five-hundred meter Aperture Spherical radio Telescope (FAST) will open a new era for the EAVN. This state-of-the-art VLBI array also provides easy access to and crucial training for the burgeoning Asian astronomical community. This Perspective summarizes the status, capabilities and prospects of the EAVN. The synergy between the EAVN and the EVN+eMERLIN provides the excellent opportunity for continuous relay monitoring of fast-varying extended sources in the Milky Way.

SArdinia Roach2-based Digital Architecture for Radio Astronomy (SARDARA)

(Journal of Astronomical Instrumentation, 07, 1850004, 2018)

by A. Melis, R. Concu, A. Trois, A. Possenti, A. Bocchinu, P. Bolli, M. Burgay, E. Carretti, P. Castangia, S. Casu, C. Cecchi Pestellini, A. Corongiu, N. D’Amico, E. Egron, F. Govoni, M. N. Iacolina, M. Murgia, A. Pellizzoni, D. Perrodin, M. Pilia, T. Pisanu, A. Poddighe, S. Poppi, I. Porceddu, A. Tarchi, V. Vacca, G. Aresu, M. Bachetti, M. Barbaro, A. Casula, A. Ladu, S. Leurini, F. Loi, S. Loru, P. Marongiu, P. Maxia, G. Mazzarella, C. Migoni, G. Montisci, G. Valente, G. Vargiu

In order to improve the scientific capability and cover all the requirements for an advanced single-dish radio telescope, we developed the SArdinia Roach2-based Digital Architecture for Radio Astronomy (SARDARA), a wide-band, multi-feed, general-purpose, and reconfigurable digital platform, whose preliminary setup was used in the early science program of the SRT in 2016. In this paper, we describe the backend both in terms of its scientific motivation and technical design, how it has been interfaced with the telescope environment during its development and, finally, its scientific commissioning in different observing modes with single-feed receivers.

Multi-messenger Observations of a Binary Neutron Star Merger

(The Astrophysical Journal Letters, Volume 848, Issue 2, article id. L12, 2017)

by Abbott et al. (3677 co-authors)

On 2017 August 17, for the first time both gravitational and electromagnetic waves from a single source have been observed. In fact, the detection of a gamma-ray transient 1.7 s after the gravitational wave signal and of a bright optical transient less than 11 hours later led to the localization of the source in the galaxy NGC 4993 located at about 40 Mpc. Following early non-detections, X-ray and radio emission were also discovered at the same celestial location about 9 and 16 days, respectively, after the merger.  The Sardinia Radio Telescope was involved in the large campaign of observations which were performed across the entire electromagnetic spectrum during the 45 days following the event.  The unprecedented data sets collected by more than 80 instruments in the world further supported the interpretation of the gravitational wave event as due to a binary neutron star coalescence.

Observations of the galaxy cluster CIZA J2242+5301 with the Sardinia Radio Telescope

(Monthly Notices of the Royal Astronomical Society, Volume 472, Issue 3, p.3605-3623, 2017)

by F. Loi, M. Murgia, F. Govoni, V. Vacca, L. Feretti, G. Giovannini, E. Carretti, F. Gastaldello, M. Girardi, F. Vazza, R. Concu, A. Melis, R. Paladino, S. Poppi, G. Valente, W. Boschin, T.E. Clarke, S. Colafrancesco, T. Enßlin, C. Ferrari, F. de Gasperin, L. Gregorini, M. Johnston-Hollitt, H. Junklewitz, E. Orrù, P. Parma, R. Perley, G.B Taylor

We observed the galaxy cluster CIZA J2242.8+5301 with SRT to provide new constraints on its spectral properties at high frequency. We conducted observations in three frequency bands centred at 1.4 GHz, 6.6 GHz and 19 GHz, resulting in beam resolutions of 14′, 2.9′ and 1′ respectively. These single-dish data were also combined with archival interferometric observations at 1.4 and 1.7 GHz. Assuming simple diffusive shock acceleration, we interpret measurements of the northern relic with a continuous injection model represented by a broken power-law. Unlike other studies of the same object, no significant steepening of the relic radio emission is seen in data up to 8.35 GHz. Our results provide new insights on the magnetic structure of the relic, but further observations are needed to clarify the nature of the observed Faraday rotation.

The 999th Swift gamma-ray burst: Some like it thermal. A multiwavelength study of GRB 151027A

(The Astrophysical Journal, Volume 842, Issue 1, article id. 17, 10 pp. (2017))

by E. Kundu, P. Lundqvist, M. A. Pérez-Torres, R. Herrero-Illana, and A. Alberdi

We modeled the radio non-detection of two Type Ia supernovae (SNe), SN 2011fe and SN 2014J, considering synchrotron emission from the interaction between SN ejecta and the circumstellar medium. For ejecta whose outer parts have a power-law density structure, we compare synchrotron emission with radio observations. Assuming that 20% of the bulk shock energy is being shared equally between electrons and magnetic fields, we found a very low-density medium around both the SNe. A less tenuous medium with particle density ∼1 cm−3 , which could be expected around both SNe, can be estimated when the magnetic field amplification is less than that presumed for energy equipartition. This conclusion also holds if the progenitor of SN 2014J was a rigidly rotating white dwarf (WD) with a main-sequence (MS) or red giant companion. SRT was part of the EVN observations of SN 2014J.

The 999th Swift gamma-ray burst: Some like it thermal. A multiwavelength study of GRB 151027A

(Astronomy & Astrophysics, Volume 598, id.A23, 17 pp. (2017))

by F. Nappo, A. Pescalli, G. Oganesyan, G. Ghirlanda, M. Giroletti, A. Melandri, S. Campana, G. Ghisellini, O. S. Salafia, P. D’Avanzo, M. G. Bernardini, S. Covino, E. Carretti, A. Celotti, V. D’Elia, L. Nava, E. Palazzi, S. Poppi, I. Prandoni, S. Righini, A. Rossi, R. Salvaterra, G. Tagliaferri, V. Testa, T. Venturi, and S. D. Vergani

We present a multiwavelength study of GRB 151027A. This is the 999th gamma-ray burst detected by the Swift satellite and it has a densely sampled emission in the X-ray and optical band and has been observed and detected in the radio up to 140 days after the prompt. The multiwavelength light curve from 500 s to 140 days can be modelled through a standard forward shock afterglow, but it requires an additional emission component to reproduce the early X-ray and optical emission. Radio observations, performed with the Sardinia Radio Telescope (SRT) and Medicina in single-dish mode and with the European Very Long Baseline Interferometer (VLBI) Network and the Very Long Baseline Array (VLBA), between day 4 and 140 suggest that the burst exploded in an environment characterized by a density profile scaling with the distance from the source (wind profile).

Space debris detection in Low Earth Orbit with the Sardinia Radio Telescope

(Published in Electronics, vol. 6, n°3, pp. 1-16, 2017)

by G. Muntoni, L. Schirru, T. Pisanu, G. Montisci, G. Valente, F. Gaudiomonte, G. Serra, E. Urru, P. Ortu and A. Fanti

Space debris are orbiting objects that represent a major threat for space operations. The most used countermeasure to face this threat is, by far, collision avoidance, namely the set of maneuvers that allow to avoid a collision with the space debris. Since collision avoidance is tightly related to the knowledge of the debris state (position and speed), the observation of the orbital debris is the key of the problem. In this work a bistatic radar configuration named BIRALET (BIstatic RAdar for LEO Tracking) is used to detect a set of space debris at 410 MHz, using the Sardinia Radio Telescope as the receiver antenna. The signal-to-noise ratio, the Doppler shift and the frequency spectrum for each debris are reported.

Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode

(Monthly Notices of the Royal Astronomical Society, Volume 471, Issue 3, p.2703-2714, 2017)

by E. Egron, A. Pellizzoni, M. Giroletti, S. Righini, M. Stagni, A. Orlati, C. Migoni, A. Melis, L. Barbas, S. Buttaccio, P. Cassaro, P. De Vicente, M.P. Gawronski, M. Lindqvist, G. Maccaferri, C. Stanghellini, P. Wolak, J. Yang, A. Navarrini, S. Loru, M. Pilia, M. Bachetti, M.N. Iacolina, M. Buttu, S. Corbel, J. Rodriguez, S. Markoff, J. Wilms, K. Pottschmidt, M. Cadolle Bel, E. Kalemci, T. Belloni, V. Grinberg, M. Marongiu, G.P. Vargiu, A. Trois

In September 2016, the microquasar Cygnus X-3 underwent a giant radio flare, which was monitored for 6 days with the Medicina Radio Astronomical Station and the Sardinia Radio Telescope. Long observations were performed in order to follow the evolution of the flare on a hourly scale, covering six frequency ranges from 1.5 GHz to 25.6 GHz. Rapid flux variations were observed at high radio frequencies at the peak of the flare, together with rapid evolution of the spectral index. This is the first time that such fast variations are observed. Based on the Italian network (Noto, Medicina and SRT) and extended to the European antennas (Torun, Yebes, Onsala), VLBI observations were triggered at 22 GHz on five different occasions, four times prior to the giant flare, and once during its decay phase. Flux variations of 2-hour duration were recorded.

Imaging of SNR IC443 and W44 with the Sardinia Radio Telescope at 1.5 GHz and 7 GHz

(Monthly Notices of the Royal Astronomical Society, Volume 470, Issue 2, p.1329-1341, 2017)

by E. Egron, A. Pellizzoni, M. N. Iacolina, S. Loru, M. Marongiu, S. Righini, M. Cardillo, A. Giuliani, S. Mulas, G. Murtas, D. Simeone, R. Concu, A. Melis, A. Trois, M. Pilia, A. Navarrini, V. Vacca, R. Ricci, G. Serra, M. Bachetti, M. Buttu, D. Perrodin, F. Buffa, G. L. Deiana, F. Gaudiomonte, A. Fara, A. Ladu, F. Loi, P. Marongiu, C. Migoni, T. Pisanu, S. Poppi, A. Saba, E. Urru, G. Valente, G.P. Vargiu

Observations of supernova remnants (SNRs) are a powerful tool for investigating the later stages of stellar evolution, the properties of the ambient interstellar medium, and the physics of particle acceleration and shocks (Cosmic Rays production). Although radio emission is a prime probe for refining models, high-resolution images at frequencies above 5 GHz are surprisingly lacking. In the frameworks of the Early Science Program with the SRT, we provided, for the first time, single-dish deep imaging at 7 GHz of the SNR IC443 and W44 complexes coupled with spatially-resolved spectra in the 1.5-7 GHz frequency range. Our images were obtained through on-the-fly mapping techniques, providing antenna beam oversampling and resulting in accurate continuum flux density measurements.

Sardinia Radio Telescope observations of Abell 194 - the intra-cluster magnetic field power spectrum

(Astronomy & Astrophysics, 2017,  603, A122)

by F. Govoni, M. Murgia, V. Vacca, F. Loi, M. Girardi, F. Gastaldello, G. Giovannini, L. Feretti, R. Paladino, E. Carretti, R. Concu, A. Melis, S. Poppi, G. Valente, G. Bernardi, A. Bonafede, W. Boschin, M. Brienza, T.E. Clarke, S. Colafrancesco, F. de Gasperin, D. Eckert, T.A. Ensslin, C. Ferrari, L. Gregorini, M. Johnston-Hollitt, H. Junklewitz, E. Orru', P. Parma, R. Perley, M. Rossetti, G.B Taylor, F. Vazza

This work focuses on the study of the intra-cluster magnetic field in the poor galaxy cluster Abell 194. New total intensity and polarization observations of Abell 194 obtained with the Sardinia Radio Telescope were combined with archival Very Large Array observations to derive the spectral aging and the rotation measure images of the radio galaxies 3C40A and 3C40B embedded in the cluster. These data in combination with X-ray images are used to constrain the intra-cluster magnetic field properties. To date, the central magnetic field derived for Abell 194 is the weakest ever found using rotation measure data in galaxy cluster.

The Sardinia Radio Telescope: From a Technological Project to a Radio Observatory

(Astronomy & Astrophysics, 2017, A&A, 608, A40

by I. Prandoni, M. Murgia, A. Tarchi, M. Burgay, P. Castangia, E. Egron, F. Govoni, A. Pellizzoni, R. Ricci, S. Righini, M. Bartolini, S. Casu, A. Corongiu, M. N. Iacolina, A. Melis, F. T. Nasir, A. Orlati, D. Perrodin, S. Poppi, A. Trois, V. Vacca, A. Zanichelli, M. Bachetti, M. Buttu, G. Comoretto, R. Concu, A. Fara, F. Gaudiomonte, F. Loi, C. Migoni, A. Orfei, M. Pilia, P. Bolli, E. Carretti, N. D’Amico, D. Guidetti, S. Loru, F. Massi, T. Pisanu, I. Porceddu, A. Ridolfi, G. Serra, C. Stanghellini, C. Tiburzi, S. Tingay and G. Valente

In this paper, we provide an overview of the main science drivers for the SRT, describe the main outcomes from the scientific commissioning of the telescope, and discuss a set of observations demonstrating the SRT's scientific capabilities. The astronomical validation activities were prioritized based on technical readiness and scientific impact. The highest priority was to make the SRT available for joint observations as part of European networks: European VLBI Network and Large European Array for Pulsars (observing sessions in early 2014). The validation of single-dish operations for the suite of SRT first light receivers and backends continued in the following years, and was concluded with the first call for shared-risk/early-science observations issued at the end of 2015.

Sardinia Radio Telescope wide-band spectral-polarimetric observations of the galaxy cluster 3C129

(Monthly Notices of the Royal Astronomical Society, Volume 461, Issue 4, p.3516-3532, 2016)

by M. Murgia, F. Govoni, E. Carretti, A. Melis, R. Concu, A. Trois, F. Loi, V. Vacca, A. Tarchi, P. Castangia, A. Possenti, A. Bocchinu, M. Burgay, S. Casu, A. Pellizzoni, T. Pisanu, A. Poddighe, S. Poppi, N. D'Amico, M. Bachetti, A. Corongiu, E. Egron, N. Iacolina, A. Ladu, P. Marongiu, C. Migoni, D. Perrodin, M. Pilia, G. Valente, G. Vargiu

This paper presents the results of the commissioning of the ROACH2-based backend at the Sardinia Radio Telescope. New observations of the galaxy cluster 3C129 in the frequency range 6000-7200 MHz are used to image the large-angular-scale emission at high-frequency of the radio sources located in this cluster of galaxies, including the tailed radio galaxy 3C129 at its center. These data were used, in combination with literature data at lower frequencies, to derive the variation of the synchrotron spectrum of 3C129 along the tail of the radio source in order to infer its radiative age and its speed.

The host galaxy of a fast radio burst

(Published in Nature, Volume 530, Issue 7591, pp. 453-456 (2016))

by E.F. Keane, S. Johnston, S. Bhandari, E. Barr, N. D. R. Bhat, M. Burgay, M. Caleb, C. Flynn, A. Jameson, M. Kramer, E. Petroff, A. Possenti, W. van Straten, M. Bailes, S. Burke-Spolaor, R. P. Eatough, B. Stappers, T. Totani, M. Honma, H. Furusawa et al.

In recent years, millisecond-duration radio signals originating in distant galaxies appear to have been discovered in the so-called fast radio bursts. These signals are dispersed according to a precise physical law and this dispersion is a key observable quantity, which, in tandem with a redshift measurement, can be used for fundamental physical investigations. Every fast radio burst has a dispersion measurement, but none before now have had a redshift measurement, because of the difficulty in pinpointing their celestial coordinates. Here we report the discovery of a fast radio burst and the identification of a fading radio transient lasting ~6 days after the event, which we use to identify the host galaxy; we measure the galaxy’s redshift to be z = 0.492 ± 0.008. The dispersion measure and redshift, in combination, provide a direct measurement of the cosmic density of ionized baryons in the intergalactic medium of 4.9 ± 1.3 per cent, in agreement with the expectation from the Wilkinson Microwave Anisotropy Probe, and including all of the so-called ‘missing baryons’. The ~6-day radio transient is largely consistent with the radio afterglow of a short γ-ray burst, and its existence and timescale do not support progenitor models such as giant pulses from pulsars, and supernovae. This contrasts with the interpretation of another recently discovered fast radio burst, suggesting that there are at least two classes of bursts.