arXiv subjects

G. Giovannini

Publications and source records attributed to G. Giovannini.

At least 181 records · Page 10Linked to original sources

arXiv1999

Radio Halo and Relic Candidates from the NRAO VLA Sky Survey

We present the first results of the search of new halo and relic candidates in the NRAO VLA Sky Survey. We have inspected a sample of 205 clusters from the X-ray-brightest Abell-type clusters presented by Ebeling et al (1996), and found 29 candidates. Out of them, 11 clusters are already known from the literature to contain a diffuse cluster-wide source, while in 18 clusters this is the first indication of the existence of this type of sources. We classify these sources as halos or relics according to their location in the cluster center or periphery, respectively. We find that the occurrence of cluster halos and relics is higher in clusters with high X-ray luminosity and high temperature. We also confirm the correlation between the absence of a cooling flow and the presence of a radio halo at the cluster center.

astro-ph↗

arXiv1999

B2 1144+35: A Giant Low Power Radio Galaxy with Superluminal Motion

We report on centimeter VLA and VLBI observations of the giant, low power radio galaxy 1144+35. These observations are sensitive to structures on scales from less than 1 parsec to greater than 1 megaparsec. Diffuse steep spectrum lobes on the megaparsec scale are consistent with an age of $\sim$ 10$^8$ years. On the parsec scale, a complex jet component is seen to move away from the center of activity with an apparent velocity 2.7 h$^{-1}_{50}$ c. It shows a central spine -- shear layer morphology. A faint parsec scale counterjet is detected and an intrinsic jet velocity of 0.95 c and angle to the line of sight of 25$^\circ$ are derived, consistent with an intrinsically symmetric ejection. The central spine in the parsec scale jet is expected to move at a higher velocity and a Lorentz factor $γ$ $\sim$ 15 has been estimated near the core.The age of this inner VLBI structure is $\sim$ 300 years. Assuming a constant angle to the line-of-sight, the jet velocity is found to decrease from 0.95 c at 20 mas (32 pc on the plane of the sky) to 0.02 c at 15 arcsec (24 kpc on the plane of the sky). These findings lend credence to the claim that (1) even the jets of low power radio galaxies start out relativistic; and (2) these jets are decelerated to subrelativistic velocities by the time they reach kiloparsec scales.

astro-ph↗

arXiv1999

A Parsec Scale Accelerating Radio Jet in the Giant Radio Galaxy NGC315

Observations of the core of the giant radio galaxy NGC315 made with VLBI interferometers are discussed in the context of a relativistic jet. The sidedness asymmetry suggests Doppler favoritism from a relativistic jet. The presence of moving features in the jet as well as jet counter--jet brightness ratios hint at an accelerating, relativistic jet. An increasing jet velocity is also supported by a comparison of the jet's observed properties with the predictions of an adiabatic expansion model. On the parsec scale, the jet is unpolarized at a wavelength of 6 cm to a very high degree in clear distinction to the high polarization seen on the kiloparsec scale.

astro-ph↗

arXiv1999

The radio galaxies and the magnetic field in Abell 119

We present new, multiwavelength Very Large Array observations of the 3 radio galaxies in the cluster Abell 119: 0053-015, 0053-016 and 3C 29. The first two radio galaxies, which lie close to the cluster center, show a narrow-angle-tailed structure, with many twists in the tails. The third radio source is located at the cluster periphery, and shows an undistorted FR I morphology. All three radio sources are strongly polarized at the highest frequencies, and all three show both more depolarization and higher Faraday Rotation Measures with increasing proximity to the cluster center. We interpret this polarization behaviour as induced by the magneto-ionized intracluster medium, whose magnetic field is estimated to be in the range 5-10 microG.

astro-ph↗

arXiv1999

Hard X-Ray Radiation in the Coma Cluster Spectrum

Hard X-ray radiation has been detected for the first time in the Coma cluster by BeppoSAX. Thanks to the unprecedented sensitivity of the Phoswich Detection System (PDS) instrument, the source has been detected up to ~80 keV. There is clear evidence (4.5 sigma) for non-thermal emission in excess of thermal above ~25 keV. The hard excess is very unlikely due to X Comae, the Seyfert 1 galaxy present in the field of view of the PDS. A hard spectral tail due to inverse Compton on CMB photons is predicted in clusters, like Coma, with radio halos. Combining the present results with radio observations, a volume-averaged intracluster magnetic field of ~0.15 micro G is derived, while the electron energy density of the emitting electrons is ~7x10**-14 erg/cm**3.

astro-ph↗

arXiv1998

The Broad Line Radio Galaxy J2114+820

In the frame of the study of a new sample of large angular size radio galaxies selected from the NRAO VLA Sky Survey, we have made radio observations of J2114+820, a low power radio galaxy with an angular size of 6'. Its radio structure basically consists of a prominent core, a jet directed in north-west direction and two extended S-shaped lobes. We have also observed the optical counterpart of J2114+820, a bright elliptical galaxy with a strong unresolved central component. The optical spectrum shows broad emission lines. This fact, together with its low radio power and FR-I type morphology, renders J2114+820 a non-trivial object from the point of view of the current unification schemes of radio loud active galactic nuclei.

astro-ph↗

arXiv1998

The Radio-Optical Jet in NGC 3862 from Parsec tu Sub-Kiloparsec Scales

We have observed the radio source NGC 3862 (3C264) simultaneously with the EVN and the MERLIN arrays, obtaining detailed images of its radio structure from parsec to sub-kiloparsec scales. 3C264 shows a one-sided jet, with evident variations in its morphological properties with distance. We have analyzed HST optical data of NGC 3862 finding a one-to-one correspondence between radio and optical features in the jet. The radio to optical spectral index is approximately constant along the jet. Synchrotron appears as the most plausible mechanism for the observed jet emission at radio and optical wavelengths. Local particle reacceleration or, alternatively, deviation from the condition of equipartition, are plausible mechanisms to explain the low synchrotron cooling and constant spectral index in the jet of 3C264.

astro-ph↗

arXiv1998

VLBI Observations of Mkn 501 and Mkn 421

We present here two epochs of Space VLBI observations at 18 cm of the BL-Lac type object Mkn 501. Thanks to the high resolution of these new data we have found that the inner jet is centrally brightened at its beginning but becomes extended and limb brightened at ~ 8 mas from the core. Moreover a comparison between the two epochs shows the presence of a possible proper motion with apparent velocity = 6.7c. VLBI data at 6 and 18 cm of the BL-Lac type object Mkn 421 are also presented. Observational data have been used to constrain the jet velocity and orientation.

astro-ph↗

arXiv1998

VLA Observations of the giant Radio Galaxy 3C 449

In this paper we present multifrequency VLA observations of the giant radio galaxy 3C 449, obtained with high sensitivity and resolution in total intensity and polarized flux. These images show the source morphology in great detail, in particular we detect plumes and wiggles in the low brightness lobes. The source is considerably polarized at all frequencies. The spectral index map between 5 GHz and 8.4 GHz shows that the northern region beyond 1' from the core has a much flatter spectrum than the southern one. We analyse the jet dynamics, and conclude that the jets are relativistic at the beginning, and decelerate significantly within 10" (5 kpc) from the core. We compute the rotation measure, and detect significant values within +-50 rad/m^2 of the expected contribution of our Galaxy. The Faraday effect is likely to originate in an external screen, which is most probably the intergalactic medium of the group of galaxies around 3C pap449.

astro-ph↗

arXiv1998

Space VLBI Observations of MKN501

We present here two epochs of Space VLBI Observations at 18 cm of the BL-Lac type object MKN 501. Thanks to the high resolution of these new data, we have found that the inner jet is centrally brightened at its beginning, but becomes extended and limb brightened at ~8 mas from the core. Moreover a comparison between the two epochs shows the presence of a proper motion with apparent velocity = 6.7c. Observational data have been used to constrain the jet velocity and orientation.

astro-ph↗

arXiv1998

Beppo-Sax Observation of the Coma cluster

We present first results of the BeppoSAX observation of the Coma Cluster. Thanks to the unprecedented sensitivity of the PDS instrument, the source has been detected up to ~80 keV. There is clear evidence for emission in excess to the thermal one above ~25 keV, very likely of non-thermal origin. We have therefore, for the first time, detected the long sought Inverse Compton emission on CMB photons predicted in clusters, like Coma, with radio halos. Combining X and radio observations, a value of 0.16 micro Gauss for the volume-averaged intracluster magnetic field is derived.

astro-ph↗

arXiv1998

WNB 0313+683: Analysis of a Newly Discovered Giant Radio Galaxy

We present the results of a detailed analysis of the newly discovered 15 arcmin large FRII-type radio galaxy WNB 0313+683. It has a redshift of 0.0901 +/- 0.0002 and a projected linear size of 2.0 Mpc. Using multi-frequency radio data from the WENSS and NVSS surveys, the VLA, the WSRT and the 100m Effelsberg telescope, we have derived the age of the source (1.4 +/- 0.1 x 10^8 yrs) and the particle densities around the two radio lobes (>1.6 x 10^(-6) per cubic cm for the southern lobe and >5.8 x 10^(-7) per cubic cm for the northern). From optical spectroscopy we find that the H$α$ emission line has a broad component, and that the extinction is large (with colour index E(B-V) = 0.98 +/- 0.10 mag), but is probably mostly galactic in origin (b= +9.8 degrees). Further, the radio core has an inverted spectrum with a spectral index of +0.42 +/- 0.03, suggesting a very compact structure. At 10.5 GHz, it contributes 25% of the total flux density of the source, which is extremely high for radio galaxies. WNB 0313+683 has a very large optical emission line flux with respect to its estimated jet power, when compared with the correlation between these two properties found by Rawlings & Saunders (1991). We argue that this, together with the relatively high radio power and the inverted radio spectrum of the radio core, is suggestive of a new phase of radio activity in this source.

astro-ph↗

arXiv1997

Electron Ageing and Polarization in Tailed Radio Galaxies

High-frequency observations of the tailed radio galaxies IC 310, NGC 1265, 3C 129, and 3C 465 have been performed with the Effelsberg 100-m telescope. For the radio galaxies IC 310, NGC 1265 and 3C 465, radio data obtained at low frequencies with the Westerbork Synthesis Radio Telescope are also available. These new radio data allow us to map the extended structure of the radio galaxies and obtain spectral and polarization information in the outermost regions. The multi-frequency spectra were used to study the synchrotron ageing of relativistic electrons with increasing distance from the active nucleus. We found that the spectrum in each radio galaxy progressively steepens with distance, and at each location it is steeper at high frequencies. The spectra are fitted by models involving synchrotron energy losses and the critical frequency is obtained at increasing distance from the core. Assuming that the magnetic field is the equipartition value, we obtain the radiating electron lifetimes and consequently their drift velocities. Our results imply the existence of reacceleration processes or bulk motions along the tails. The polarization data at 10.6 GHz give information on the intrinsic degree of polarized flux and the orientation of the magnetic field. We find that the polarization percentage increases along the tails, reflecting an increase of the degree of ordering of the magnetic field. The magnetic field in the tails is longitudinal.

astro-ph↗

arXiv1997

Diffuse radio emission in the Coma cluster

The Coma cluster is peculiar in the radio domain owing to the presence of cluster-wide diffuse sources: the radio halo Coma C, the relic source 1253+275, and the bridge between them. Diffuse radio sources in clusters are a rare and poorly understood phenomenon. We summarize here the properties of the diffuse radio sources in Coma and their relation to other cluster properties.

astro-ph↗

arXiv1997

VLBI Observations of a Complete Sample of Radio Galaxies: VIII - Proper Motion in 3C338

We present new VLA, MERLIN and VLBI images for the radio galaxy 3C 338, and the results of the monitoring of its arcsecond core flux density. Present high sensitivity observations allow us to investigate the radio structure of this source and to confirm the presence of two symmetric parsec scale jets. Morphological changes between different epochs are evident and a proper motion with $β\sim$ 0.4h$^{-1}$ has been derived allowing us to give a lower limit constraint for the Hubble constant. While the steep spectrum large scale structure of 3C 338 could be a relic emission, the small scale structure looks young, similar to the high power MSOs found at high redshift.

astro-ph↗

arXiv1996

The radio and X-ray properties of Abell 2255

New radio and X-ray data are reported for the rich cluster Abell 2255. The cluster radio emission is characterized by the presence of a diffuse halo source, located at the cluster center, and a more peripheral ridge of radio emission which could be a relic. In addition, 6 tailed radio sources and a small double are found to be associated with cluster galaxies. At X-ray wavelengths, the cluster shows elongated emission in the center, indicating an ongoing cluster merger, with the axis in the east-west to southeast-northwest direction. The overall cluster temperature is found from the ROSAT observation to be kT = 3.5 +- 1.5 keV, while the temperature determined from the Einstein MPC data was reported to be 7.3 keV. We interpret this difference as due to the existence in the cluster of several coexisting temperature components. The merger process can provide energy to maintain the radio halo, but we stress the need for tailed radio galaxies orbiting at the cluster center to provide the halo relativistic electrons.

astro-ph↗

arXiv1995

VLBI Observations of a Complete Sample of Radio Galaxies. VI. The Two FR-I Radio Galaxies B2 0836+29 and 3C465

We present 5 GHz global VLBI observations of the two Fanaroff Riley Type I radio galaxies B2 0836+29 and 3C465 (2335+26). For 3C465 we present also 1.7 GHz and 8.4 GHz global VLBI data. In addition VLA observations were used to obtain arsecond resolution continuum and polarization maps at 5 GHz. Both sources are very asymmetric on the parsec-scale, with a core and a one-sided jet, aligned with the main arcsecond scale jet. We place a limit on the milliarcsecond jet to counterjet brightness ratio B$_{jet}$/B$_{cjet}$ \gtsim 20 and \gtsim 30 for B2 0836+29 and 3C465 respectively. For 3C465 the strong asymmetry holds to the kiloparsec scale. The brightness asymmetry and the ratio between the core radio power and total radio power allow us to constrain the jet velocity close to the core and the orientation of the radio structure with respect to the line of sight. The results suggest that the plasma speed is relativistic on the parsec scale for both sources, i.e. v$_{jet}$ \gtsim 0.75c for B2 0836+29 and v$_{jet}$ \gtsim 0.6c for 3C465. While v$_{jet}$ decreases from the parsec to the kiloparsec scale in B2 0836+29, in 3C465 the very high v$_{jet}$ holds all the way to the kiloparsec-scale {\t bright spot}. Our results are in agreement with the unification scheme suggestion that FR-I radio galaxies are the unbeamed poulation of BL-Lac objects. Furthermore, they reinforce the idea that the central engine in FR-I and FR-II radio galaxies must be qualitatively similar. The different radio morphology could then be due either to an intrinsically different nuclear power, which affects the torus geometry or to different conditions in the region beyond the parsec scale, where a significant deceleration in the FR-I jets occurs.

astro-ph↗

arXiv1995

VLBI Observations of Low Power Radio Galaxies

The parsec scale properties of low power radio galaxies are reviewed here, using the available data on 12 FR I galaxies. The most frequent radio structure is an asymmetric parsec-scale morphology, i.e. core and one-sided jet. It is shared by 9 (possibly 10) of the 12 mapped radio galaxies. One (possibly 2) of the other galaxies has (have) a two-sided jet emission. Two sources are known from published data to show a proper motion; we present here evidence for proper motion in two more galaxies. Therefore, in the present sample we have 4 radio galaxies with a measured proper motion. One of these has a very symmetric structure and therefore should be in the plane of the sky. The results discussed here are in agreement with the predictions of the unified scheme models. Moreover, the present data indicate that the parsec scale structure in low and high power radio galaxies is essentially the same.

astro-ph↗