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

Context. 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 scales, 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. (2006, A&A, 448, 853) 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). Aims. We aim to reveal the nuclear radio properties of the brightest cluster galaxy (BCG) in RBS 797 and to investigate the presence of a SMBBH system in its center. Methods. We performed new high-resolution observations at 5 GHz with the European VLBI Network (EVN) on May 3, 2013, reaching an angular resolution of ~9 × 5 mas 2 and a sensitivity of 36 μ Jy beam -1 . We also re-analyzed VLA archival data at 4.8 GHz in A- and B-configurations. Results. 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 supermassive black holes 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.

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