On the need of an ultramassive black hole in OJ 287

TL;DR

This paper argues that the primary black hole in OJ 287 can be ultramassive and still produce the observed brightness, challenging previous claims that such a mass would be observationally excluded.

Contribution

It demonstrates that accretion disc brightness in OJ 287 is consistent with an ultramassive black hole, revising previous constraints based on optical brightness observations.

Findings

Accretion disc brightness is unlikely to exceed host galaxy brightness by more than one magnitude.

The primary black hole's mass and accretion rate can be as high as standard models suggest.

Optical brightness constraints do not exclude ultramassive black holes in OJ 287.

Abstract

The highly variable blazar OJ~287 is commonly discussed as an example of a binary black hole system. The 130 year long optical light curve is well explained by a model where the central body is a massive black hole of 18.35$\times$10$^9$ solar mass that supports a thin accretion disc. The secondary black hole of 0.15$\times$10$^9$ solar mass impacts the disc twice during its 12 year orbit, and causes observable flares. Recently, it has been argued that an accretion disc with a typical AGN accretion rate and above mentioned central body mass should be at least six magnitudes brighter than OJ~287's host galaxy and would therefore be observationally excluded. Based on the observations of OJ~287's radio jet, detailed in Marscher and Jorstad (2011), and up-to-date accretion disc models of Azadi et al. (2022), we show that the V-band magnitude of the accretion disc is unlikely to exceed the host galaxy brightness by more than one magnitude, and could well be fainter than the host. This is because accretion power is necessary to launch the jet as well as to create electromagnetic radiation, distributed across many wavelengths, and not concentrated especially on the optical V-band. Further, we note that the claimed V-band concentration of accretion power leads to serious problems while interpreting observations of other Active Galactic Nuclei. Therefore, we infer that the mass of the primary black hole and its accretion rate do not need to be smaller than what is determined in the standard model for OJ~287.