Engineering topics

Giovannini, G.

Publications and source records attributed to Giovannini, G..

5 records

DOE OSTI2023

Optical/X-ray/radio view of Abell 1213: A galaxy cluster with anomalous diffuse radio emission

Context. Abell 1213, a low-richness galaxy system, is known to host an anomalous radio halo detected in data of the Very Large Array (VLA). It is an outlier with regard to the relation between the radio halo power and the X-ray luminosity of the parent clusters. Aims. Our aim is to analyze the cluster in the optical, X-ray, and radio bands to characterize the environment of its diffuse radio emission and to shed new light on its nature. Methods. We used optical data from the Sloan Digital Sky Survey to study the internal dynamics of the cluster. We also analyzed archival XMM-Newton X-ray data to unveil the properties of its hot intracluster medium. Finally, we used recent data from the LOw Frequency ARray (LOFAR) at 144 MHz, together with VLA data at 1.4 GHz, to study the spectral behavior of the diffuse radio source. Results. Both our optical and X-ray analysis reveal that this low-mass cluster exhibits disturbed dynamics. In fact, it is composed of several galaxy groups in the peripheral regions and, in particular, in the core, where we find evidence of substructures oriented in the NE–SW direction, with hints of a merger nearly along the line of sight. The analysis of the X-ray emission adds further evidence that the cluster is in an unrelaxed dynamical state. At radio wavelengths, the LOFAR data show that the diffuse emission is ~510 kpc in size. Moreover, there are hints of low-surface-brightness emission permeating the cluster center. Conclusions. The environment of the diffuse radio emission is not what we would expect for a classical halo. The spectral index map of the radio source is compatible with a relic interpretation, possibly due to a merger in the N–S or NE–SW directions, in agreement with the substructures detected through the optical analysis. The fragmented, diffuse radio emissions at the cluster center could be attributed to the surface brightness peaks of a faint central radio halo.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Discovery of a bright extended X-ray jet in RGB J1512+020A

ABSTRACT We report the discovery of a bright, extended X-ray jet in the quasar RGB J1512+020A (z = 0.22). Chandra observations show the X-ray core and 13 arcsec (∼45 kpc projected) extended emission coincident with the radio jet. The jet stands out as one of brightest X-ray jets (LX ∼ 7 × 1043 erg s−1) at low redshift (z < 0.5) discovered so far, with remarkably large X-ray to radio luminosity ratios (LX/Lr up to ∼50). We identified four main components, two unresolved knots and two extended structures, one being the jet brightest feature (JBF). All jet features are also detected in ALMA archival observations. The radio, sub-mm to X-ray spectra of the two knots can be modelled with a single synchrotron component. For the two resolved structures, the ALMA data unveil a turnover of the low-energy continuum at ∼460 GHz. External Comptonization of cosmic microwave background photons can account for the observed X-ray emission if the jet speed remains highly relativistic, with bulk motion Γbulk > 15, up to tens of kiloparsec from the core. However, the comparison with the spectral energy distribution of similar X-ray detected jets shows that the alternative hypothesis of synchrotron emission from an additional population of ultra-high-energy electrons is also possible. We report a tentative (≳2σ) optical detection of the JBF in images of the Dark Energy Camera Legacy Survey. If confirmed, the optical emission should be either the low-energy tail of the radiative component responsible for the X-ray emission or a third, separate component.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2016

A Strong Merger Shock in Abell 665

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 +/- 0.6, propagating in front of a cold disrupted cloud. This makes Abell 665 the second cluster, after the Bullet cluster, where a strong merger shock of M is approximately 3 has been detected. The shock velocity from jump conditions is consistent with (2.7 +/- 0.7) × 10(exp 3) km s(exp −1). The new data also reveal a prominent southern cold front with potentially heated gas ahead of it. Abell 665 also hosts a giant radio halo. There is a hint of diffuse radio emission extending to the shock at the north, which needs to be examined with better radio data. This new strong shock provides a great opportunity to study the reacceleration model with the X-ray and radio data combined.

galaxies: clusters: general- galaxies: clisters; i↗

NASA NTRS1994

VLBI observations of a complete sample of radio galaxies. 4: The radio galaxies NGC 2484, 3C 109, and 3C 382

We present here new Very Long Base Interferometry (VLBI) observations of one Fanaroff and Riley (F-R) I radio galaxy (NGC 2484) and two broad-line F-R II radio galaxies (3C 109 and 3C 382). For 3C 109 new Very Large Array (VLA) maps are also shown. These sources belong to a complete sample of radio galaxies under study for a better knowledge of their structures at parsec resolution. The parsec structure of these three objects is very similar: asymmetric emission, which we interpret as the core plus a one-side jet. The parsec-scale jet is always on the same side of the main kiloparsec-scale jet. The limit on the jet to counterjet brightness ratio, the ratio of the core radio power to the total radio power and the synchrotron-self Compton model allow us to derive some constraints on the jet velocity and orientation with respect to the line of sight. From these data and from those published on two other sources of our sample, we suggest that parsec-scale jets are relativistic in both F-R I and F-R II radio galaxies and that parsec scale properties in F-R I and F-R II radio galaxies are very similar despite the large difference between these two classes of radio galaxies on the kiloparsec scale.

Giovannini, G.↗

NASA NTRS1990

Confinement of radio galaxies in rich clusters

X-ray observations obtained with the Einstein Observatory are presented for the Abell clusters A 98, A 160, A 278, and A 569, which are characterized by the presence of an extended radio source associated with the brightest cluster galaxy. The X-ray brightness profiles are fitted with a hydrostatic-thermal model to determine the physical parameters of the X-ray emitting gas and the behavior of the thermal pressure in each cluster. The gas thermal pressure is then compared with the nonthermal pressure within the radio sources, and the conditions for pressure balance are analyzed. The radio source components show a minimum pressure lower than the pressure of the ambient gas. Balance between internal and external pressure is obtained in the case that the usual assumptions for the computation of the energy density in the radio source components are not valid (Phi less than 1, k greater than 1) or that physical conditions are out of equipartition.

Feretti, L.↗