arXiv subjects

Gabriele Giovannini

Publications and source records attributed to Gabriele Giovannini.

At least 19 recordsLinked to original sources

arXiv2026

Study of Polarized Emission in Radio Halos and Filaments in the SKA Telescopes Era

Synchrotron diffuse emission in merging galaxy clusters and along filaments connecting them demonstrates the presence of relativistic particles and magnetic fields in these environments. The study of the polarized signal associated with this emission represents a powerful tool to constrain the properties of intracluster magnetic fields and the physics of acceleration and transport of relativistic particles. Despite technological progress, detecting this polarized signal is still very challenging. In order to shed light on the capabilities of the SKA telescopes to study this emission, we use the data of cosmological magneto-hydro-dynamic simulations to predict the expected polarized surface brightness of diffuse synchrotron sources from the center of galaxy clusters to filaments of the cosmic web at 1.4 GHz. We explore the possibility to detect these sources with a polarization survey with SKA-Mid with AA4 telescopes and compare the results with those from pointed observations corresponding to longer exposure times. These simulations provide precious information to understand the potential of the SKA telescopes for studying the origin and evolution of cosmological magnetic fields. We discuss how these observations can be used in order to characterize the magnetic field and the distribution and energy content of the radio emitting plasma and to shed light on the link between non-thermal and thermal properties and the dynamical state of the system.

astro-ph.CO↗

arXiv2026

The twin-jet system in the FRII radio galaxy 3C 452: A sub-parsec scale VLBI study

We present a comprehensive multifrequency VLBI analysis of the FRII, high-excitation radio galaxy 3C 452, aiming to resolve and analyze for the first time its twin-jet structure on sub-parsec scales. Our data set comprises High Sensitivity Array (HSA) observations at 4.9, 8.4, 15.4, 23.6, and 43.2 GHz. Through fitting methods performed in both the visibility and the image plane, we trace the jet expansion from scales of a few thousand to nearly $10^5$ Schwarzschild radii ($R_S$) on both the approaching and receding jets. Additionally, we derive the core brightness temperatures and Doppler factors to constrain the jet's orientation and intrinsic speed. Our study provides the first detailed description of the twin-jet system in 3C 452 on VLBI scales, confirming it as a rare FRII source with jets detected down to millimeter wavelengths. We resolve both jet and counter-jet down to scales of a few thousand $R_S$, revealing a symmetric, parabolically expanding structure with power-law indices $k \approx 0.66$ (jet) and $k \approx 0.47$ (counter-jet). The brightness temperature analysis yields low Doppler factors ($δ\sim 0.03$-$0.83$), indicative of Doppler de-boosting due to the large viewing angle ($θ\approx 70^\circ$) and/or a magnetically dominated jet base. A spectral index analysis reveals a strongly inverted core spectrum ($α> 2$) with additional absorption at the highest frequencies, followed by a sharp steepening ($α\sim -2.5$) to optically thin values in the innermost jet. Finally, a comparison between broad- and narrow-line high-excitation radio galaxies shows that jets in narrow-line sources such as 3C 452 and Cygnus A complete collimation at $\leq 10^5 R_S$, whereas broad-line sources exhibit shape transitions at $10^6$-$10^7 R_S$, suggesting that orientation plays an important role in the observed collimation scales.

astro-ph.HE↗

arXiv2024

Revealing faint compact radio jets at redshifts above 5 with very long baseline interferometry

Over the past two decades, our knowledge of the high-redshift (z > 5) radio quasars has expanded, thanks to dedicated high-resolution very long baseline interferometry (VLBI) observations. Distant quasars provide unique information about the formation and evolution of the first galaxies and supermassive black holes in the Universe. Powerful relativistic jets are likely to have played an essential role in these processes. However, the sample of VLBI-observed radio quasars is still too small to allow meaningful statistical conclusions. We extend the list of the VLBI observed radio quasars to investigate how the source structure and physical parameters are related to radio loudness. We assembled a sample of 10 faint radio quasars located at 5 < z < 6 with their radio-loudness indices spanning between 0.9-76. We observed the selected targets with the European VLBI Network (EVN) at 1.7 GHz. The milliarcsecond-scale resolution of VLBI at this frequency allows us to probe the compact innermost parts of radio-emitting relativistic jets. In addition to the single-band VLBI observations, we collected single-dish and low-resolution radio interferometric data to investigate the spectral properties and variability of our sources. The detection rate of this high-redshift, low-flux-density sample is 90%, with only one target (J0306+1853) remaining undetected. The other 9 sources appear core-dominated and show a single, faint and compact radio core on this angular scale. The derived radio powers are typical of FRII radio galaxies and quasars. By extending our sample with other VLBI-detected z > 5 sources from the literature, we found that the core brightness temperatures and monochromatic radio powers tend to increase with radio loudness.

astro-ph.GA↗

arXiv2023

Precessing jet nozzle connecting to a spinning black hole in M87

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 eight to ten-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 in the position angle variation 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.

astro-ph.HE↗

arXiv2023

A ring-like accretion structure in M87 connecting its black hole and jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation^{1,2}. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole^3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.

astro-ph.HE↗

arXiv2023

The Past and Future of East Asia to Italy: Nearly Global VLBI

We present here the East Asia to Italy Nearly Global VLBI (EATING VLBI) project. How this project started and the evolution of the international collaboration between Korean, Japanese, and Italian researchers to study compact sources with VLBI observations is reported. Problems related to the synchronization of the very different arrays and technical details of the telescopes involved are presented and discussed. The relatively high observation frequency (22 and 43 GHz) and the long baselines between Italy and East Asia produced high-resolution images. We present example images to demonstrate the typical performance of the EATING VLBI array. The results attracted international researchers and the collaboration is growing, now including Chinese and Russian stations. New in progress projects are discussed and future possibilities with a larger number of telescopes and a better frequency coverage are briefly discussed herein.

astro-ph.GA↗

arXiv2022

The thermal and non-thermal components within and between galaxy clusters Abell 399 and Abell 401

We measure the local correlation between radio emission and Compton-$y$ signal across two galaxy clusters, Abell~399 and Abell~401, using maps from the Low-Frequency Array (LOFAR) and the Atacama Cosmology Telescope (ACT) + \Planck. These datasets allow us to make the first measurement of this kind at $\sim$arcminute resolution. We find that the radio brightness scales as $F_{\mathrm{radio}} \propto y^{1.5}$ for Abell~401 and $F_{\mathrm{radio}} \propto y^{2.8}$ for Abell~399. Furthermore, using \XMM data, we derive a sublinear correlation between radio and X-ray brightness for both the clusters ($F_{\mathrm{radio}} \propto F_{\rm X}^{0.7}$). Finally, we correlate the Compton-$y$ and X-ray data, finding that an isothermal model is consistent with the cluster profiles, $y \propto F_{\rm X}^{0.5}$. By adopting an isothermal--$β$ model, we are able, for the first time, to jointly use radio, X-ray, and Compton-$y$ data to estimate the scaling index for the magnetic field profile, $B(r) \propto n_{\mathrm{e}}(r)^η$ in the injection and re-acceleration scenarios. Applying this model, we find that the combined radio and Compton-$y$ signal exhibits a significantly tighter correlation with the X-ray across the clusters than when the datasets are independently correlated. We find $η\sim 0.6{-}0.8$. These results are consistent with the upper limit we derive for the scaling index of the magnetic field using rotation measure values for two radio galaxies in Abell~401. We also measure the radio, Compton-$y$, and X-ray correlations in the filament between the clusters but conclude that deeper data are required for a convincing determination of the correlations in the filament.

astro-ph.CO↗

arXiv2022

Puzzling large-scale polarization in the galaxy cluster Abell 523

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 one hundred currently known. In this paper we analyze 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. 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 multi-frequency 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.

astro-ph.CO↗

arXiv2022

Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron source historically classified as a radio halo. Its radio power at 1.4 GHz makes it one of the most significant outliers in the scaling relations between observables derived from multi-wavelength observations of galaxy clusters: it has a morphology that is different and offset from the thermal gas, and it has polarized emission at 1.4 GHz typically difficult to observe for this class of sources. A magnetic field fluctuating on large spatial scales (~ 1 Mpc) can explain these peculiarities but the formation mechanism for this source is not yet completely clear. To investigate its formation mechanism, we present new observations obtained with the LOw Frequency ARray at 120-168 MHz and the Jansky Very Large Array at 1-2 GHz, which allow us to study the spectral index distribution of this source. According to our data the source is observed to be more extended at 144 MHz than previously inferred at 1.4 GHz, with a total size of about 1.8 Mpc and a flux density S_144MHz = (1.52 +- 0.31) Jy. The spectral index distribution of the source is patchy with an average spectral index alpha ~ 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index alpha ~ 2.1 has been obtained between 1.410 GHz and 1.782 GHz. A previously unseen patch of steep spectrum emission is clearly detected at 144 MHz in the south of the cluster. Overall, our findings suggest that we are observing an overlapping of different structures, powered by the turbulence associated with the primary and a possible secondary merger.

astro-ph.CO↗

arXiv2021

The eMERLIN and EVN view of FR0 radio galaxies

We present the results from high-resolution observations carried out with the eMERLIN UK-array and the European VLBI network (EVN) for a sample of 15 FR0s, i.e. compact core-dominated radio sources associated with nearby early-type galaxies (ETGs) which represent the bulk of the local radio galaxy population. The 5-GHz eMERLIN observations available for 5 objects exhibit sub-mJy core components, and reveal pc-scale twin jets for 4 out of 5 FR0s once the eMERLIN and JVLA archival visibilities data are combined. The 1.66-GHz EVN observations available for 10 FR0s display one- and two-sided jetted morphologies and compact cores. The pc-scale core emission contributes, on average, to about one tenth of the total extended radio emission, although we note a increasing core contribution for flat/inverted-spectrum sources. We found an unprecedented linear correlation between the pc-scale core luminosity ($\sim$10$^{21.3}$-10$^{23.6}$ W Hz$^{-1}$) and [O III] line luminosity, generally considered as proxy of the accretion power, for a large sample of LINER-type radio-loud low-luminosity active nuclei, all hosted in massive ETGs, which include FR0s and FRIs. This result represents further evidence of a common jet-disc coupling in FR0s and FRIs, despite they differ in kpc-scale radio structure. For our objects and for other FR0 samples reported in the literature, we estimate the jet brightness sidedness ratios, which typically range between 1 and 3. This parameter roughly gauges the jet bulk Lorentz factor $Γ$, which turns out to range between 1 and 2.5 for most of the sample. This corroborates the scenario that FR0s are characterized by mildly-relativistic jets, possibly as a result of lower spinning black holes (BHs) than the highly-spinning BHs of relativistic-jetted radio galaxies, FRIs.

astro-ph.GA↗

arXiv2021

Morphological transition of the compact radio lobe in 3C84 via the strong jet-cloud collision

We report multi-epoch Very Long Baseline Interferometric (VLBI) observations of the compact radio lobe in the radio galaxy 3C84 (NGC1275) during 2016 - 2020. The image sequence of 3C84 reveals that the hotspot in the radio lobe showed the one-year long frustration in 2017 within a compact region of about 0.07 parsec, suggesting a strong collision between the jet and a compact dense cloud with the estimated average density about $(4\-- 6)\times 10^{5}~{\rm cm^{-3}}$. Although the hotspot and the radio lobe began to move south again after its breakout, the radio lobe showed a morphological transition from FR II- to FR I-class radio lobe and its radio flux became fainter. This is the first detection of the dynamical feedback from the cloud to the jet where the cloud located on the jet axis significantly interferes with the jet propagation and evolution at the central one-parsec region in active galactic nucleus.

astro-ph.HE↗

arXiv2021

Jetted radio-quiet quasars at z>5

We report on the JVLA observations of three high redshift Active Galactic Nuclei (AGN), having a black hole mass estimated to be among the largest known. Two of them, SDSS J0100+2802 and SDSS J0306+1853 at redshift 6.326 and 5.363 respectively, are radio-quiet AGN according to the classic definition, while the third (B2 1023+25 at z=5.284) is a powerful blazar. The JVLA data clearly show a radio structure in the first source, and a radio emission with a relatively steep radio spectrum in the second one, demonstrating the presence of a radio jet and a diffuse component. Therefore, being radio-quiet does not exclude the presence of a powerful relativistic jet with important consequences on the population studies and on the ratio between jetted and non-jetted AGN. We can estimate the viewing angle of these jets, and this allows us to find, albeit with some uncertainty, the density of black holes with a mass in excess of $10^{10}M_\odot$ at high redshifts. We found that their density in jetted AGN is very large in the redshift bin 5-6, comparable with the overall AGN population of the same optical luminosity. Jets might thus play a crucial role in the fast formation and evolution of the most massive black holes in the early Universe. They are more common than what expected from wide radio surveys with mJy flux sensitivity. Deeper JVLA or VLBI observations are key to discover a possible relativistic jet population hiding in plain sight at very high-redshift. The discovery of powerful relativistic jets associated with the most massive black holes in the early Universe re-opens the question: is the jet instrumental for a rapid growth of the black hole or instead is the black hole mass the main driver for the jet formation?

astro-ph.HE↗

arXiv2019

Radio spectral properties of cores and extended regions in blazars in the MHz regime

Low-frequency radio surveys allow in-depth studies and new analyses of classes of sources previously known and characterised only in other bands. In recent years, low radio frequency observations of blazars have been available thanks to new surveys, such as the GaLactic and Extragalactic All-sky MWA Survey (GLEAM). We search for gamma-ray blazars in a low frequency ($ν$ < 240MHz) survey, to characterise the spectral properties of the spatial components. We cross-correlate GLEAM with the fourth catalogue of active galactic nuclei (4LAC) detected by the Fermi satellite. This improves over previous works using a low frequency catalogue that is wider, deeper, with a better spectral coverage and the latest and most sensitive gamma-ray source list. In comparison to the previous study based on the commissioning survey, the detection rate increased from 35% to 70%. We include Australia Telescope 20GHz (AT20G) Survey data to extract high-frequency high-angular resolution information on the radio cores of blazars. We find low radio frequency counterparts for 1274 out of 1827 blazars in the 72-231 MHz range. Blazars have at spectrum at $\sim$ 100MHz regime, with a mean spectral index $α$ = -0.44 +-0.01 (assuming S $\propto$ $ν^ α$ ). Low synchrotron peaked objects have a scatter spectrum than high synchrotron peaked objects. Low frequency radio and gamma-ray emission show a significant but scattered correlation. The ratio between lobe and core radio emission in gamma-ray blazars is smaller than previously estimated.

astro-ph.GA↗

arXiv2019

The low-frequency properties of FR0 radio galaxies

Using the Alternative Data Release of the TIFR GMRT Sky Survey (TGSS), we studied the low-frequency properties of FR0 radio galaxies, the large population of compact radio sources associated with red massive early-type galaxies revealed by surveys at 1.4 GHz. We considered TGSS observations from FR0CAT, a sample formed by 104 FR0s at z<0.05: all but one of them are covered by the TGSS, and 43 of them are detected above a 5 sigma limit of 17.5 mJy. No extended emission has been detected around the FR0s, corresponding to a luminosity limit of < 4 10^23 W/Hz over an area of 100 kpc x 100 kpc. All but eight FR0s have a flat or inverted spectral shape (alpha < 0.5) between 150 MHz and 1.4 GHz: this spectral behavior confirms the general paucity of optically thin extended emission within the TGSS beam, as is expected for their compact 1.4 GHz morphology. Data at 5 GHz were used to build their radio spectra, which are also generally flat at higher frequencies. By focusing on a sub-sample of FR0s with flux density > 50 mJy at 1.4 GHz, we found that ~75% of them have a convex spectrum, but with a smaller curvature than the more powerful gigahertz peaked-spectrum sources (GPS). The typical FR0s radio spectrum is better described by a gradual steepening toward high frequencies, rather than to a transition from an optically-thick to an optically-thin regime, possibly observed in only ~15% of the sample.

astro-ph.GA↗

arXiv2019

Rotation Measure synthesis applied on synthetic SKA images of galaxy clusters

Future observations with next generation radio telescopes will help us to understand the presence and the evolution of magnetic fields in galaxy clusters through the determination of the so-called Rotation Measure (RM). In this work, we applied the RM-synthesis technique on synthetic SKA1-MID radio images of a pair of merging galaxy clusters, measured between 950 and 1750 MHz with a resolution of 10 arcsec and a thermal noise of 0.1$μ$Jy/beam. The results of our RM-synthesis analysis are compared to the simulations' input parameters. We study two cases: one with radio haloes at the cluster centres, and another without. We found that the information obtained with the RM-synthesis is in general agreement with the input information. Some discrepancies are however present. We characterise them in this work, with the final goal of determining the potential impact of SKA1-MID on the study of cluster magnetic fields.

astro-ph.CO↗

arXiv2018

Parabolic Jets from the Spinning Black Hole in M87

The M87 jet is extensively examined by utilizing general relativistic magnetohydrodynamic (GRMHD) simulations as well as the steady axisymmetric force-free electrodynamic (FFE) solution. Quasi-steady funnel jets are obtained in GRMHD simulations up to the scale of $\sim 100$ gravitational radius ($r_{\rm g}$) for various black hole (BH) spins. As is known, the funnel edge is approximately determined by the following equipartitions; i) the magnetic and rest-mass energy densities and ii) the gas and magnetic pressures. Our numerical results give an additional factor that they follow the outermost parabolic streamline of the FFE solution, which is anchored to the event horizon on the equatorial plane. We also identify the matter dominated, non-relativistic corona/wind play a dynamical role in shaping the funnel jet into the parabolic geometry. We confirm a quantitative overlap between the outermost parabolic streamline of the FFE jet and the edge of jet sheath in VLBI observations at $\sim 10^{1}$-$10^{5} \, r_{\rm g}$, suggesting that the M87 jet is likely powered by the spinning BH. Our GRMHD simulations also indicate a lateral stratification of the bulk acceleration (i.e., the spine-sheath structure) as well as an emergence of knotty superluminal features. The spin characterizes the location of the jet stagnation surface inside the funnel. We suggest that the limb-brightened feature could be associated with the nature of the BH-driven jet, if the Doppler beaming is a dominant factor. Our findings can be examined with (sub-)mm VLBI observations, giving a clue for the origin of the M87 jet.

astro-ph.HE↗

arXiv2018

High-resolution VLA observations of FR0 radio galaxies: properties and nature of compact radio sources

We present the results of Karl G. Jansky Very Large Array (VLA) observations to study the properties of FR0 radio galaxies, the compact radio sources associated with early-type galaxies which represent the bulk of the local radio-loud AGN population. We obtained A-array observations at 1.5, 4.5, and 7.5 GHz for 18 FR0s from the FR0CAT sample: these are sources at $z<0.05$, unresolved in the FIRST images and spectroscopically classified as low excitation galaxies (LEG). Although we reach an angular resolution of $\sim$0.3 arcsec, the majority of the 18 FR0s is still unresolved. Only four objects show extended emission. Six have steep radio spectra, 11 are flat cores, while one shows an inverted spectrum. We find that 1) the ratio between core and total emission in FR0s is $\sim$30 times higher than in FRI and 2) FR0s share the same properties with FRIs from the nuclear and host point of view. FR0s differ from FRIs only for the paucity of extended radio emission. Different scenarios were investigated: 1) the possibility that all FR0s are young sources eventually evolving into extended sources is ruled out by the distribution of radio sizes; 2) similarly, a time-dependent scenario, where a variation of accretion or jet launching prevents the formation of large-scales radio structures, appears to be rather implausible due to the large abundance of sub-kpc objects 3) a scenario in which FR0s are produced by mildly relativistic jets is consistent with the data but requires observations of a larger sample to be properly tested.

astro-ph.GA↗

arXiv2018

VERA monitoring of the radio jet 3C 84 during 2007--2013: detection of non-linear motion

We present a kinematic study of the subparsec-scale radio jet of the radio galaxy 3C 84/NGC 1275 with the VLBI Exploration of Radio Astrometry (VERA) array at 22 GHz for 80 epochs from 2007 October to 2013 December. The averaged radial velocity of the bright component "C3" with reference to the radio core is found to be $0.27 pm 0.02c$ between 2007 October and 2013 December. This constant velocity of C3 is naturally explained by the advancing motion of the head of the mini-radio lobe. We also find a non-linear component in the motion of C3 with respect to the radio core. We briefly discuss possible origins of this non-linear motion.

astro-ph.GA↗