A candidate supermassive binary black hole system in the brightest cluster galaxy of RBS 797

The radio source at the center of the cool core galaxy cluster RBS 797 (z=0.35) is known to exhibit a misalignment of its radio jets and lobes observed at different VLA-scale, with the innermost kpc-scale jets being almost orthogonal to the radio emission which extends for tens of kpc filling the X-ray cavities. Gitti et al. suggested that this peculiar radio morphology may indicate a recurrent activity of the central radio source, where the jet orientation is changing between the different outbursts due to the effects of supermassive binary black holes (SMBBHs). We aim at unveiling the nuclear radio properties of the brightest cluster galaxy (BCG) in RBS 797 and at investigating the presence of a SMBBH system in its center. We have performed new high-resolution observations at 5 GHz with the European VLBI Network (EVN), reaching an angular resolution of 9x5 mas^2 and a sensitivity of 36 microJy/beam. We report the EVN detection of two compact components in the BCG of RBS 797, with a projected separation of ~77 pc. We can envisage two possible scenarios: the two components are two different nuclei in a close binary system, or they are the core and a knot of its jet. Both interpretations are consistent with the presence of SMBBHs. Our re-analysis of VLA archival data seems to favor the first scenario, as we detect two pairs of radio jets misaligned by ~90 degrees on the same kpc scale emanating from the central radio core. If the two outbursts are almost contemporaneous, this is clear evidence of the presence of two active SMBHs, whose radio nuclei are unresolved at VLA resolution. The nature of the double source detected by our EVN observations in the BCG of RBS 797 can be established only by future sensitive, multi-frequency VLBI observations. If confirmed, RBS 797 would be the first SMBBH system observed at medium-high redshift at VLBI resolution. (abridged)Comment: 4 pages, 2 figures, A&A Letter in pres

Chandra Observation of the Cluster Environment of a WAT Radio Source in Abell 1446

Wide-angle tail (WAT) radio sources are often found in the centers of galaxy clusters where intracluster medium (ICM) ram pressure may bend the lobes into their characteristic C-shape. We examine the low redshift (z=0.1035) cluster Abell 1446, host to the WAT radio source 1159+583. The cluster exhibits possible evidence for a small-scale cluster-subcluster merger as a cause of the WAT radio source morphology. This evidence includes the presence of temperature and pressure substructure along the line that bisects the WAT as well as a possible wake of stripped interstellar material or a disrupted cool core to the southeast of the host galaxy. A filament to the north may represent cool, infalling gas that's contributing to the WAT bending while spectroscopically determined redshifts of member galaxies may indicate some component of a merger occurring along the line-of-sight. The WAT model of high flow velocity and low lobe density is examined as another scenario for the bending of 1159+583. It has been argued that such a model would allow the ram pressure due to the galaxy's slow motion through the ICM to shape the WAT source. A temperature profile shows that the cluster is isothermal (kT= 4.0 keV) in a series of annuli reaching a radius of 400 kpc. There is no evidence of an ongoing cooling flow. Temperature, abundance, pressure, density, and mass profiles, as well as two-dimensional maps of temperature and pressure are presented.Comment: 40 AASTeX pages including 15 postscript figures; accepted for publication in Ap

Comparative analysis of the diffuse radio emission in the galaxy clusters A1835, A2029, and Ophiuchus

We recently performed a study of a sample of relaxed, cooling core galaxy clusters with deep Very Large Array observations at 1.4 GHz. We find that in the central regions of A1835, A2029, and Ophiuchus the dominant radio galaxy is surrounded by a diffuse low-brightness radio emission that takes the form of a mini-halo. Here we present the results of the analysis of the extended diffuse radio emission in these mini-halos. In order to investigate the morphological properties of the diffuse radio emission in clusters of galaxies we propose to fit their azimuthally averaged brightness profile with an exponential, obtaining the central brightness and the e-folding radius from which the radio emissivity can be calculated. We investigate the radio properties of the mini-halos in A1835, A2029, and Ophiuchus in comparison with the radio properties of a representative sample of mini-halos and halos already known in the literature. We find that radio halos can have quite different length-scales but their emissivity is remarkably similar from one halo to the other. In contrast, mini-halos span a wide range of radio emissivity. Some of them, like the Perseus mini-halos, are characterized by a radio emissivity which is more than 100 times greater than that of radio halos. On the other hand, the new mini-halos in cooling core clusters analyzed in this work, namely A2029, Ophiuchus, and A1835, have a radio emissivity which is much more typical of halos in merging clusters rather than similar to that of the other mini-halos previously known.Comment: 17 pages, 11 figures, A&A in press. For a version with high quality figures, see http://erg.ca.astro.it/preprints/mini_halo_2

BeppoSAX observations of low power radio galaxies: possible detection of obscured nuclei

We present the first results of BeppoSAX observations of a small sample of low brightness FRI radio galaxies. The flux of all the targets is consistent with a thermal spectrum, as due to the presence of hot intracluster gas or galactic corona. Moreover in three sources a non thermal absorbed spectrum can be present in the MECS spectrum at energies larger than 7 keV, while for a fourth object a high energy flux has been detected in the PDS instrument at energies larger than 15 keV. This component could be related to the inner AGN surrounded by an obscuring torus.Comment: 4 pages, LateX, 3 figures (included). Uses espcrc2.sty (included). To appear in: "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy, 21-24 October, 1997, Eds.: L. Scarsi, H. Bradt, P. Giommi and F. Fior

The intracluster magnetic field power spectrum in Abell 665

The goal of this work is to investigate the power spectrum of the magnetic field associated with the giant radio halo in the galaxy cluster A665. For this, we present new deep Very Large Array total intensity and polarization observations at 1.4 GHz. We simulated Gaussian random three-dimensional turbulent magnetic field models to reproduce the observed radio halo emission. By comparing observed and synthetic radio halo images we constrained the strength and structure of the intracluster magnetic field. We assumed that the magnetic field power spectrum is a power law with a Kolmogorov index and we imposed a local equipartition of energy density between relativistic particles and field. Under these assumptions, we find that the radio halo emission in A665 is consistent with a central magnetic field strength of about 1.3 micro-G. To explain the azimuthally averaged radio brightness profile, the magnetic field energy density should decrease following the thermal gas density, leading to an averaged magnetic field strength over the central 1 Mpc^3 of about 0.75 micro-G. From the observed brightness fluctuations of the radio halo, we infer that the outer scale of the magnetic field power spectrum is ~450 kpc, and the corresponding magnetic field auto-correlation length is ~100 kpc.Comment: 12 pages, 6 figures, accepted for publication on A&A, language editing. For a high quality version see http://erg.ca.astro.it/preprints/a665_halo

The spectral index image of the radio halo in the cluster Abell 520 hosting a famous bow shock

Synchrotron radio emission is being detected from an increasing number of galaxy clusters. Spectral index images are a powerful tool to investigate the origin, nature, and connection of these sources with the dynamical state of the cluster. The aim of this work is to investigate the spectral index distribution of the radio halo in the galaxy cluster A520, a complex system from an optical, radio, and X-ray point of view. We present deep Very Large Array observations in total intensity at 325 and 1400 MHz. We produced and analyzed spectral index images of the radio halo in this frequency range at a resolution of 39" and 60" and looked for possible correlations with the thermal properties of the cluster. We find an integrated radio halo spectral index alpha(325-1400) ~ 1.12. No strong radial steepening is present and the spectral index distribution is intrinsically complex with fluctuations only partially due to measurement errors. The radio halo integrated spectral index and the cluster temperature follow the global trend observed in other galaxy clusters although a strong point-to-point correlation between the spectral index and the thermal gas temperature has not been observed. The complex morphology in the spectral index image of the radio halo in A520 is in agreement with the primary models for radio halo formation. The flatness of the radial profile suggests that the merger is still ongoing and is uniformly and continuously (re-) accelerating the population of relativistic electrons responsible of the radio emission even at large (~ 1 Mpc) distances from the cluster center.Comment: 12 pages, 10 figures, A&A accepte

VLBI Observations of a Complete Sample of Radio Galaxies V. 3C346 and 4C31.04: two Unusual CSS Sources

We present observations at 1.7 and 8.4 GHz of two Compact Steep Spectrum (CSS) sources from a complete sample of low-intermediate power radio galaxies. 3C346 shows an asymmetric structure with a one-sided ``jet'' and ``hot spot''. Present observations suggest that the classification of this source as a CSS is inappropriate, and that it is a common radio galaxy at a small angle to the line of sight. Its properties are in agreement with the predictions of unified schemes models. 4C31.04 shows more complex structure with the possibility of a centrally located flat spectrum core in between two close lobes. We suggest that this source could be a low redshift Compact Symmetric Object.Comment: 15 pages, LATEX, uuenconde ps figures To be published in the Astrophysical Journal, October 20th issu

The Effect of a Non-Thermal Tail on the Sunyaev-Zeldovich Effect in clusters of galaxies

We study the spectral distortions of the cosmic microwave background radiation induced by the Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies when the target electrons have a modified Maxwell-Boltzmann distribution with a high-energy non-thermal tail. Bremsstrahlung radiation from this type of \\ electron distribution may explain the supra-thermal X-ray emission observed in some clusters such as the Coma cluster and A2199 and serve as an alternative to the classical but problematic inverse Compton scattering interpretation. We show that the SZ effect can be used as a powerful tool to probe the electron distribution in clusters of galaxies and discriminate among these different interpretations of the X-ray excess. The existence of a non-thermal tail can have important consequences for cluster based estimators of cosmological parameters.Comment: 14 pages, 3 figures, version to be published in ApJ. Let

BeppoSAX observation of Hercules A and MRC 0625-536

We present BeppoSAX observations of the two FR I type radio galaxies Hercules A (3C 348) and MRC 0625-536 in the energy range $0.2 - 200$ keV. Data analysis shows that the X-ray flux from Hercules A is consistent with a diffuse thermal plasma emitting at $T \approx 4-5$ keV with a possible, but somewhat uncertain, contribution of a softer component at $T \approx 3$ keV. The non thermal emission from the active nucleus must be significantly smaller than the thermal one, and no indication of relevant core obscuration by a surrounding torus was detected. The flux from MRC 0625-536 originates form an extended region and has been fitted to a thermal law with $T \approx 5.7$ keV and with a column density consistent with the galactic absorption. A spatially resolved spectral analysis does not show a relevant variation of the temperature and the metallicity across the diffuse emission zone. A non thermal spectral component, related to the nuclear activity, may be present in the innermost region with some possible amount of local obscuration, contributing \lapp 10% to the total luminosity. Hard X-ray emission from MRC 0625-536 has been detected in the PDS (15 - 200 keV) that may be related either to its galactic core or to the intracluster region

A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields

Results are presented of a new VLA-ROSAT study that probes the magnetic field strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1) galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity, and were selected to be free of (unusual) strong radio cluster halos, and widespread cooling flows. Consistent with these criteria, most clusters show a relaxed X-ray morphology and little or no evidence for recent merger activity. Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM excess out to ~0.5 Mpc from cluster centers. The results, combined with RM imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the hot intergalactic gas within these ``normal'' clusters is permeated with a high filling factor by magnetic fields at levels of = 5-10 (l/10 kpc)^{-1/2} microGauss, where l is the field correlation length. These results lead to a global estimate of the total magnetic energy in clusters, and give new insight into the ultimate energy origin, which is likely gravitational. These results also shed some light on the cluster evolutionary conditions that existed at the onset of cooling flows.Comment: 6 pages, 1 figure, uses emulateapj5.sty, accepted by ApJ