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

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
e-mail: francesco.ubertosi2@unibo.it
² Istituto Nazionale di Astrofisica – Osservatorio di Astrofisica e Scienza dello Spazio (OAS), Via Gobetti 101, 40129 Bologna, Italy
³ Istituto Nazionale di Astrofisica – Istituto di Radioastronomia (IRA), Via Gobetti 101, 40129 Bologna, Italy
⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁵ Italian Alma Regional Center (ARC), Via Piero Gobetti 101, 40129 Bologna, Italy
⁶ INAF – Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 19 December 2023
Accepted: 11 May 2024

Abstract

Aims. The multifaceted central radio galaxy of the cluster RBS 797 shows several episodes of jet activity in multiple directions. We wish to understand the causes behind these dramatic misalignments and measure the timescales of the successive outbursts.

Methods. We present a multifrequency (144 MHz – 9 GHz) and multiscale (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.

Results. We confirm the co-spatiality of the radio lobes with the four perpendicular X-ray cavities previously discovered. The radiative ages of the east-west lobes (31.4 ± 6.6 Myr) and of the north-south lobes (32.1 ± 9.9 Myr) support a coeval origin of the perpendicular outbursts, which also have similar active phase duration (∼12 Myr). Based on the analysis of the inner north-south jets (on scales of ≤10 kpc), we (a) confirm the S-shaped jet morphology; (b) show the presence of two hotspots per jet with a similar spectral index; and (c) estimate the age of the twisting north-south jets to be less than ∼8 Myr. Based on these results, we determine that jet precession, with a period ∼9 Myr, half-opening angle ∼24°, and jet advance speed ∼0.01c, can explain the properties of the north-south jets. We also find that the synchrotron injection index has steepened from the large, older outbursts (Γ ∼ 0.5) to the younger S-shaped jets (Γ ∼ 0.9), possibly due to a transition from FR I-like to FR II-like activity. The e-Merlin, VLBA, and EVN 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.

Conclusions. 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 supermassive black holes (SMBHs). The detection of a single component in the VLBA and EVN data is still consistent with this interpretation, since the predicted separation of the binary SMBHs (≤0.6 pc) is an order of magnitude smaller than the resolution of the available radio data (5 pc).