Black hole mass estimate in OJ 287 based on the bulk-motion comptonization model

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, M-center dot. We found the photon index Gamma to increase monotonically with accretion rate M-center dot from Gamma similar to 2 in the intermediate state (IS) to Gamma similar to 2.5 the high/soft state (HSS) with subsequent saturation at Gamma similar to 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 similar to 2x108 solar masses, which likely pertains to the secondary component of the BH binary in OJ 287.