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

A Chandra Snapshot Survey for 3C Radio Galaxies with redshifts between 0.3 and 0.5

This paper contains an analysis of short Chandra observations of 19 3C sources with redshifts between 0.3 and 0.5 not previously observed in the X-rays. This sample is part of a project to obtain Chandra data for all of the extragalactic sources in the 3C catalogue. Nuclear X-ray intensities as well as any X-ray emission associated with radio jet knots, hotspots or lobes have been measured in 3 energy bands: soft, medium and hard. Standard X-ray spectral analysis for the 4 brightest nuclei has been also performed. X-ray emission was detected for all the nuclei of the radio sources in the current sample with the exception of 3C 435A. There is one compact steep spectrum (CSS) source while all the others are FRII radio galaxies. X-ray emission from two galaxy clusters (3C 19 and 3C 320); from 6 hotspots in 4 radio galaxies (3C 16, 3C 19, 3C 268.2, 3C 313); and extended X-ray emission on kpc scales in 3C 187 and 3C 313, has been detected.Comment: 33 pages, 22 figures, 7 tables, accepted for publication on the ApJ Supplement Series. arXiv admin note: text overlap with arXiv:1210.602

Exploring the bulk of the BL Lac object population:1. parsec-scale radio structures

Context. The advent of Fermi is changing our understanding on the radio and gamma-ray emission in Active Galactic Nuclei. Contrary to pre-Fermi ideas, BL Lac objects are found to be the most abundant emitters in the gamma-ray band. However, since they are relatively weak radio sources, most of their parsec-scale structure and their multi-frequency properties are poorly understood and/or have not been investigated in a systematically fashion. Aims. Our main goal is to analyze the radio and gamma-ray emission properties of a sample of 42 BL Lacs selected, for the first time in the literature, with no constraint on their radio and gamma-ray flux densities/emission. Methods. Thanks to new Very Long Baseline Array observations at 8 and 15 GHz for the whole sample, we present here fundamental parameters such as radio flux densities, spectral index information, and parsec-scale structure. Moreover, we search for gamma-ray counterparts using data reported in the Second Catalog of Fermi Gamma-ray sources. Results. Parsec-scale radio emission is observed in the majority of the sources at both frequencies. Gamma-ray counterparts are found for 14/42 sources. Conclusions. The comparison between our results in radio and gamma-ray bands points out the presence of a large number of faint BL Lacs showing "non classical" properties such as low source compactness, core dominance, no gamma-ray emission and steep radio spectral indexes. A deeper multiwavelength analysis will be needed.Comment: 19 pages, 6 figures, 6 tables, accepted for publication in A&