2024MNRAS.535.3732K

The multiwavelength outburst activity in the BL Lacertae source OJ 287 has sparked a lot of controversy about whether the source contains one or two black holes (BHs) and what characteristics of this BH binary would be. In this article, we present the results of analysis of the X-ray flaring activity of OJ 287 using the data of Swift/XRT observations. We discovered that the energy spectra in all spectral states can be adequately fit with the XSPEC bulk-motion Comptonization (BMC) model. As a result, we found that the X-ray photon index of the BMC model, $\Gamma$ correlates with the mass accretion rate, $\dot{M}$. We found the photon index $\Gamma$ to increase monotonically with accretion rate $\dot{M}$ from $\Gamma \sim 2$ in the intermediate state (IS) to $\Gamma \sim 2.5$ the high/soft state (HSS) with subsequent saturation at $\Gamma \sim$ 2.6 level at higher luminosities. This type of behaviour of the spectral index is remarkably similar to the pattern observed in a number of established stellar-mass BH candidates. Assuming the universality of the observed pattern of the correlation between the photon index and mass accretion rate and using the well-studied stellar-mass binaries GX 339-4 and XTE J1859-226 to calibrate the model, we estimate a BH mass $\sim 2\times 10^8$ solar masses, which likely pertains to the secondary component of the BH binary in OJ 287.