Gabriele Giovannini - INAF-Istituto di Radio Astronomia, Bologna

Gabriele Giovannini
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Contact Details

Name
Gabriele Giovannini
Affiliation
INAF-Istituto di Radio Astronomia, Bologna
City
Bologna
Country
Italy

Pubs By Year

Pub Categories

 
Cosmology and Nongalactic Astrophysics (19)
 
High Energy Astrophysical Phenomena (16)
 
Astrophysics of Galaxies (11)
 
Astrophysics (7)
 
Instrumentation and Methods for Astrophysics (3)
 
High Energy Physics - Phenomenology (1)
 
Solar and Stellar Astrophysics (1)

Publications Authored By Gabriele Giovannini

The mechanism of radio emission in radio-quiet (RQ) active galactic nuclei (AGN) is still debated and might arise from the central AGN, from star formation activity in the host, or from either of these sources. A direct detection of compact and bright radio cores embedded in sources that are classified as RQ can unambiguously determine whether a central AGN significantly contributes to the radio emission. We search for compact, high-surface-brightness radio cores in RQ AGNs that are caused unambiguously by AGN activity. Read More

Deep (103 ks) \chandra\ observations of Abell 665 have revealed rich structures in this merging galaxy cluster, including a strong shock and two cold fronts. The newly discovered shock has a Mach number of $M$ = 3.0 $\pm$ 0. Read More

We report on results from new high-sensitivity, high-resolution 86GHz (3.5 millimeter) observations of the jet base in the nearby radio galaxy M87, obtained by the Very Long Baseline Array in conjunction with the Green Bank Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the highest ever achieved for this jet at this frequency, resolving and imaging a detailed jet formation/collimation structure down to ~10 Schwarzschild radii (Rs). Read More

2015Oct
Affiliations: 1Technion, Israel, 2INAF-Osservatorio di Torino, Italy, 3Universita' di Bologna, Italy

Are the FRI and FRII radio galaxies representative of the radio-loud (RL) AGN population in the local Universe? Recent studies on the local low-luminosity radio sources cast lights on an emerging population of compact radio galaxies which lack extended radio emission. In a pilot JVLA project, we study the high-resolution images of a small but representative sample of this population. The radio maps reveal compact unresolved or slightly resolved radio structures on a scale of 1-3 kpc. Read More

The statistical study of the parsec scale properties of radio sources is crucial to get information on the nature of the central engine and to provide the foundations of the current unified theories, suggesting that the appearance of active galactic nuclei depends strongly on orientation. We started a project to observe at sub-arcsec resolution a complete sample of 94 nearby (z<0.1) radio galaxies, the Bologna Complete Sample, which is not affected by any selection effect on the jet velocity and orientation with respect to the line of sight. Read More

The radio-loud BCG at the center of the cool core cluster RBS 797 is known to exhibit a misalignment of its 5 GHz radio emission observed at different VLA resolutions, with the innermost kpc-scale jets being almost orthogonal to the radio lobes which extends for tens of kpc filling the X-ray cavities seen by Chandra. The different radio directions may be caused by rapid jet reorientation due to interaction with a secondary supermassive black hole (SMBH), or to the presence of two AGN, probably in a merging phase, which are emitting radio jets in different directions. We present the results of new 5 GHz observations performed with the EVN in May 2013. Read More

The radio galaxy 3C84 is a representative of gamma-ray-bright misaligned active galactic nuclei (AGN) and one of the best laboratories to study the radio properties of subparsec scale jets. We discuss here the past and present activity of the nuclear region within the central 1pc and the properties of subparsec-sized components C1, C2 and C3. We compare these results with the high resolution space-VLBI image at 5GHz obtained with the RadioAstron satellite and we shortly discuss the possible correlation of radio emission with the gamma-ray emission. Read More

The relativistic jet in M87 offers a unique opportunity for understanding the detailed jet structure and emission processes due to its proximity. In particular, the peculiar jet region HST-1 at ~1 arcsecond (or 80 pc, projected) from the nucleus has attracted a great deal of interest in the last decade because of its superluminal motion and broadband radio-to-X-ray outbursts, which may be further connected to the gamma-ray productions up to TeV energies. Over the last five years, we have been doing an intensive monitoring of HST-1 with EVN at 5GHz in order to examine the detailed structural evolution and its possible connection to high-energy activities. Read More

2015Feb
Affiliations: 1SISSA, Trieste, 2INAF - Osservatorio Astrofisico di Torino, 3INAF-Istituto di Radio Astronomia, Bologna

We present the results of a pilot JVLA project aimed at studying the bulk of the radio-emitting AGN population, unveiled by the NVSS/FIRST and SDSS surveys.We obtained A-array observations at the JVLA at 1.4, 4. Read More

The presence of magnetic fields in galaxy clusters has been well established in recent years, and their importance for the understanding of the physical processes at work in the Intra Cluster Medium has been recognized. Halo and relic sources have been detected in several tens clusters. A strong correlation is present between the halo and relic radio power and the X-ray luminosity. Read More

To better understand the origin and properties of cosmological magnetic fields, a detailed knowledge of magnetic fields in the large-scale structure of the Universe (galaxy clusters, filaments) is crucial. We propose a new statistical approach to study magnetic fields on large scales with the rotation measure grid data that will be obtained with the new generation of radio interferometers. Read More

Relativistic jets in active galactic nuclei (AGN) are among the most powerful astrophysical objects discovered to date. Indeed, jetted AGN studies have been considered a prominent science case for SKA, and were included in several different chapters of the previous SKA Science Book (Carilli & Rawlings 2004). Most of the fundamental questions about the physics of relativistic jets still remain unanswered, and await high-sensitivity radio instruments such as SKA to solve them. Read More

Galaxy clusters are known to host a variety of extended radio sources: tailed radio galaxies whose shape is modelled by the interaction with the intra-cluster medium (ICM); radio bubbles filling cavities in the ICM distribution and rising buoyantly through the thermal gas; diffuse giant radio sources ("halos" and "relics") revealing the presence of relativistic electrons and magnetic fields in the intra-cluster volume. It is currently the subject of an active debate how the non-thermal components that we observe at radio wavelengths affect the physical properties of the ICM and depend on the dynamical state of galaxy clusters. In this work we start our SKA1 feasibility study of the "radio cluster zoo" through simulations of a typical radio-loud cluster, hosting several bright tailed radio galaxies and a diffuse radio halo. Read More

2014Dec

Adding VLBI capability to the SKA arrays will greatly broaden the science of the SKA, and is feasible within the current specifications. SKA-VLBI can be initially implemented by providing phased-array outputs for SKA1-MID and SKA1-SUR and using these extremely sensitive stations with other radio telescopes, and in SKA2 by realising a distributed configuration providing baselines up to thousands of km, merging it with existing VLBI networks. The motivation for and the possible realization of SKA-VLBI is described in this paper. Read More

In about 70% of the population of relaxed, cool-core galaxy clusters, the brightest cluster galaxy (BCG) is radio loud, showing non-thermal radio jets and lobes ejected by the central active galactic nucleus (AGN). In recent years such relativistic plasma has been unambiguously shown to interact with the surrounding thermal intra-cluster medium (ICM) thanks to spectacular images where the lobe radio emission is observed to fill the cavities in the X-ray-emitting gas. This `radio feedback' phenomenon is widespread and is critical to understand the physics of the inner regions of galaxy clusters and the properties of the central BCG. Read More

2014Dec
Affiliations: 1SISSA, Trieste, 2INAF-Istituto di Radio Astronomia, Bologna, 3INAF - Osservatorio Astrofisico di Torino, 4INAF-Istituto di Radio Astronomia, Bologna, 5INAF-Istituto di Radio Astronomia, Bologna, 6Instituto de Astrofisica de Andalucia, Granada, 7University of Tsukuba

Aims. We want to study the amount of molecular gas in a sample of nearby early-type galaxies (ETGs) which host low-luminosity Active Galactic Nuclei (AGN). We look for possible differences between the radio-loud (RL) and radio-quiet (RQ) AGN. Read More

2014Jan

We outline the science case for extended radio emission and polarization in galaxy clusters which would be a scientifically important area of research for an upcoming Jansky Very Large Array Sky Survey. The survey would provide a major contribution in three key areas of the physics of clusters: 1) the active galactic nucleus population and the impact of feedback on the evolution of the intra-cluster medium, 2) the origin and evolution of diffuse cluster radio sources to probe the physics of mergers with implications for cosmology, and 3) the origin and role of magnetic fields in the ICM and in large scale structures. Considering all three areas, a survey must have sufficient spatial resolution to study the tailed galaxies which trace the cluster weather as well as the radio lobes driving energy into the cluster from the central AGN. Read More

To obtain a better understanding of the location and mechanisms for the production of the gamma-ray emission in jets of AGN we present a detailed study of the HST-1 structure, 0.8 arcsec downstream the jet of M87, previously identified as a possible candidate for TeV emission. HST-1 shows a very peculiar structure with superluminal as well as possible stationary sub-components, and appears to be located in the transition from a parabolic to a conical jet shape, presumably leading to the formation of a recollimation shock. Read More

Abridged: We investigated the detailed radio structure of the nucleus of the Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency, phase-referencing VLBA observations. We obtained the VLBI images toward this nucleus, with unprecedented sensitivities and resolutions, at the seven frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the first clear VLBI detections. Read More

We investigated the detailed inner jet structure of M87 using the Very Long Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing on the multi-frequency properties of the radio core at the jet base. Read More

We report on the detailed radio status of the M87 jet during the Very-High-Energy (VHE) gamma-ray flaring event in April 2010, obtained from high-resolution, multi-frequency, phase-referencing VLBA observations. We especially focus on the properties for the jet base (the radio core) and the peculiar knot HST-1, which are currently favored as the gamma-ray emitting sites. During the VHE flaring event, the HST-1 region remains stable in terms of its structure and flux density in the optically thin regime above 2GHz, being consistent with no signs of enhanced activities reported at X-ray for this feature. Read More

We introduce the GENJI program (Gamma-ray Emitting Notable AGN Monitoring by Japanese VLBI), which is a monitoring program of gamma-ray bright AGNs with the VERA array (VLBI Exploration of Radio Astrometry). The GENJI programme aims a dense monitoring at 22 GHz towards the $\gamma$-ray emitting active galactic nuclei (AGNs) to investigate the radio time variation of the core and possible ejection of new radio component, motion of jets, and their relation with the emission at other wavelengths especially in $\gamma$-rays. Currently we are monitoring 8 $\gamma$-ray-emitting notable AGNs (DA 55, 3C 84, M 87, PKS 1510-089, DA 406, NRAO 530, BL Lac, 3C 454. Read More

Radio observations of galaxy clusters show that there are $\mu$G magnetic fields permeating the intra-cluster medium (ICM), but it is hard to accurately constrain the strength and structure of the magnetic fields without the help of advanced computer simulations. We present qualitative comparisons of synthetic VLA observations of simulated galaxy clusters to radio observations of Faraday Rotation Measure (RM) and radio halos. The cluster formation is modeled using adaptive mesh refinement (AMR) magneto-hydrodynamic (MHD) simulations with the assumption that the initial magnetic fields are injected into the ICM by active galactic nuclei (AGNs) at high redshift. Read More

Clusters of galaxies, as the largest virialized systems in the Universe, are ideal laboratories to study the formation and evolution of cosmic structures... Read More

The radio galaxy 3C 84 is a representative of gamma-ray-bright misaligned active galactic nuclei (AGNs) and one of the best laboratories to study the radio properties of the sub-pc jet in connection with the gamma-ray emission. In order to identify possible radio counterparts of the gamma-ray emissions in 3C 84, we study the change in structure within the central 1 pc and the light curve of sub-pc-size components C1, C2, and C3. We search for any correlation between changes in the radio components and the gamma-ray flares by making use of VLBI and single dish data. Read More

Following the discovery of a new radio component right before the GeV \gamma-ray detection since 2008 August by Fermi Gamma-ray Space Telescope, we present a detailed study of the kinematics and lightcurve on the central sub-pc scale of 3C 84 using the archival VLBA 43-GHz data covering the period between 2002 January to 2008 November. We find that the new component "C3", previously reported by the observations with the VLBI Exploration of Radio Astrometry (VERA), was already formed in 2003. The flux density of C3 increases moderately until 2008, and then it becomes brighter rapidly after 2008. Read More

Using the deepest (370 ksec) Chandra observation of a high-redshift galaxy cluster, we perform a detailed characterization of the intra-cluster medium (ICM) of WARPJ1415.1+3612 at z=1.03. Read More

Results of processing of data of a VLBI experiment titled RAPL01 are presented. These VLBI observations were made on 4th February, 2010 at 6.28 cm between the 100-m antenna of the Max Planck Institute (Effelsberg, Germany), Puschino 22-m antenna (Astro Space Center (ASC), Russia), and two 32-m antennas of the Istituto di Radioastronomia di Bologna (Bologna, Italy) in Noto and Medicina. Read More

The organization of this paper is as follows: the basic formulae used to derive the age of synchrotron sources and the equipartition parameters are presented in Section 2, while the observational properties of diffuse radio sources (Radio halos, Radio relics, and Mini-halos) are presented in Section 3. Then in Section 4 we give a general outline of the models of the relativistic particle origin and re-acceleration; while the current results on cluster magnetic fields are described in Section 5. Finally, Section 6 reports the properties of cluster radio emitting galaxies. Read More

A deep radio observation of the A2255 cluster of galaxies has been carried out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the excellent (u,v) coverage and sensitivity achieved by our observation, the low brightness diffuse extended sources in the cluster (radio halo and relic) have been imaged with unprecedented resolution and dynamic range. Read More

Parsec scale jet properties are shortly presented and discussed. Observational data are used to derive constraints on the jet velocity and orientation, the presence of velocity structures, and the connection between the pc and kpc scale. Two peculiar sources with limb-brightened jets: 1144+35 and Mkn 501 are discussed in detail. Read More

2003Dec
Affiliations: 1IASF/CNR Rome;, 2IASF/CNR Bologna;, 3IRA/CNR Bologna;, 4IRA/CNR Bologna;, 5Astronomy Dept, Bologna Univ, 6IASF/CNR Bologna;, 7Astronomy Dept, Bologna Univ
Category: Astrophysics

We report the hard X-ray spectrum of the Coma cluster obtained using the PDS data of two independent BeppoSAX observations performed with a time interval of about three years. In both the spectra a non thermal excess with respect to the thermal emission is present at a confidence level of ~ 3.4 sigma. Read More

We present new multi-frequency EVN, MERLIN and VLA observations of the radio source 3C264, sensitive to linear scales ranging from the parsec to several kiloparsecs. The observations confirm the existence of regions with different properties in the first kiloparsec of the jet. The most remarkable feature is the transition between a well collimated narrow jet at distances from the core below 80 pc, to a conical-shaped wide jet, with an opening angle of 20 degrees. Read More

The different methods to derive the jet velocity and orientation on the parsec scale are presented. From these methods I will discuss the velocity distribution of parsec scale jets and the possible presence of acceleration or deceleration in the jet velocity. Moreover, evidences of jet velocity structures will be reported. Read More

New sensitive VLA observations confirm the existence of halo and relic sources in 6 Abell clusters where a diffuse emission was found in the NVSS. We find evidence that the frequency of clusters with halos and relics is larger in clusters with high X-ray luminosity. The evidence that the occurrence of a halo source is larger at high redshifts is marginal. Read More