A JVLA, LOFAR, e-Merlin, VLBA and EVN study of RBS 797: can binary SMBHs explain the outburst history of the central radio galaxy?

Abstract

We present a multi-frequency (144 MHz - 9 GHz) and multi-scale (5 pc - 50 kpc) investigation of the central radio galaxy in RBS 797, by means of JVLA, LOFAR (with international stations), e-Merlin, VLBA and EVN data. We investigate the morphological and spectral properties of the radio lobes, the jets, and the active core. We confirm the co-spatiality of the radio lobes with the four perpendicular X-ray cavities (see arXiv:2111.03679). The radiative ages of the E-W lobes ($31.4\pm6.6$ Myr) and of the N-S lobes ($32.1\pm9.9$ Myr) support a coeval origin of the perpendicular outbursts, that also have similar active phase duration ($\sim$12 Myr). For the inner N-S jets (on scales of $\leq10$ kpc), we (a) confirm the S-shaped jet morphology; (b) show the presence of two hotspots per jet with a similar spectral index; (c) estimate the age of the twisting jets to be less than $\sim8$ Myr. Based on these results, we determine that jet precession, with period $\sim$9 Myr, half-opening angle $\sim$24$^{\circ}$ and jet speed $\sim$0.01$c$, can explain the properties of the N-S jets. We also find that the synchrotron injection index has steepened from the large, older outbursts ($Γ\sim0.5$) to the younger S-shaped jets ($Γ\sim0.9$), possibly due to a transition from an FR I-like to an FR II-like activity. The VLBI data reveal a single, compact core at the heart of RBS 797, surrounded by extended radio emission whose orientation depends on the spatial scale sampled by the data. We explore several engine-based scenarios to explain these results. Piecing together the available evidence, we argue that RBS 797 likely hosts (or hosted) binary active SMBHs. This is still consistent with the detection of a single component in the VLBI data, since the predicted separation of the binary SMBHs ($\leq$0.6 pc) is an order of magnitude smaller than the resolution of the available radio data (5 pc).