2023A&A...671A.159T

We present a study of outburst activity in the BL Lacertae source OJ 287, which has been observed extensively with the X-ray telescope (XRT) on board the Neil Gehrels Swift Observatory. We demonstrate that the results of our analysis of X-ray flaring activity using the Swift/XRT data allow for the key characteristics of the OJ 287 secondary to be refined (its nature and mass). We discover that the energy spectra in all spectral states can be fitted using the XSPEC bulk motion Comptonization (BMC) model. As a result we find that the X-ray photon index of the BMC model, Γ, correlates with the mass accretion rate, Ṁ. We establish that Γ increases monotonically with Ṁ from the low-hard state, Γ ∼ 1.5, to the high-soft state, Γ ∼ 2.8, and finally saturates. The index behavior is similar to that in a number of black hole (BH) candidates in which we show that its saturation is observational evidence of the presence of a BH. Based on this correlation, we applied a scaling method and determine that a secondary BH mass in OJ 287 is about ∼1.25 × 10⁸ solar masses, using the well-studied X-ray BH binaries XTE 1550-564, H 1743-322, 4U 1630-47, and GRS 1915+105 as well as extragalactic BHs ESO 243-49 and M101 ULX-1, as reference sources. Also, using the power spectrum analysis, we infer the size of the Compton cloud L_CC ∼ 10¹³ cm where X-ray spectra were formed. Using this value of L_CC, we confirm that a BH mass of the secondary in OJ 287 was on the order of 10⁸ solar masses as we derived using the index, Γ-correlation (the scaling method) with respect of the mass accretion rate.