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First detection of the secondary black hole in the OJ287 binary system

Presentation #322.03 in the session AGN and Supermassive Black Holes.

Published onJul 01, 2023
First detection of the secondary black hole in the OJ287 binary system

According to the updated binary black hole (BH) model of OJ287, the secondary BH should have impacted on the accretion disk of the primary on January 20, 2022. There are three imminent observational signatures that are expected from this event. These features include: (1) When the disk matter first entered the Roche lobe of the secondary, the secondary jet should have suddenly brightened up (at JD 59531), (2) when the disk gas collided with the secondary jet, a large gamma-ray flux should have been generated (at JD 59600), and (3) when hot plasma burst out of the disk after the impact, it should have appeared as a strong Optical/UV flare (at JD 59638). These signatures were expected successively one after the other at specific epochs. All three signatures have been observed during our 2021/2022 multi-wavelength campaign. A Signature 1 optical flare has never been observed before, and since in theory it can happen only once in about 10 years, and since it lasts only one day, it is easy to miss, and would have been missed also this time except for the dense monitoring organized by one of us (S.Z.). The peak flux of this signature flare is about 1046 ergs per sec. Using Fermi data, we provide evidence for a gamma-ray flare that fits the expectation of the Signature 2 flare and show that a similar gamma-ray flare took place in 2013, at the previous disk impact of the same kind. Other possible gamma-ray flares predate the Fermi Gamma-ray Space Telescope. Finally, we observed the optical/UV flare of Signature 3 and find that its photometric, BVRI spectral index, optical polarisation, X-ray and 37 GHz radio flux properties are similar to the 2005 disk impact flare (Valtonen et al., MNRAS 2023, in press). No other flares of this kind have been reported in the last 50 years of intensive monitoring of OJ287, and none were expected since this type of flare is associated with the relatively rare impacts (about one per 25 years) at the apocenter part of the secondary orbit. We provide arguments that these rare signature features cannot arise in the main jet of the OJ287 binary system, but are likely associated with a secondary BH and especially with its jet. The first two signature flares represent perhaps the first direct detection of the jet of the secondary BH in the OJ287 binary system.

Acknowledgment: The Fermi data used in this work were obtained through the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA’s Goddard Space Flight Center. Part of this work is based on archival data provided by the ASI Science Data Center (ASDC). We furthermore acknowledge use of archival Fermi and ASDC data.

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