Observing with SRT

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

(Accepted for publication in Astronomy & Astrophysics (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.

The flickering nuclear activity of Fornax A

(Accepted for publication in Astronomy & Astrophysics

by F. 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, 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.

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.

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.

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, 2018)

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.

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.

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.

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.

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 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, 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.