  We presented a study of outburst activity in the BL Lacertae source OJ~287,
observed extensively with the X-ray telescope (XRT) onboard Neil Gehrels Swift
Observatory. We demonstrated that the results of our analysis of X-ray flaring
activity using the Swift/XRT data allow to refine the key characteristics of
the OJ~287 secondary (its nature and mass). We discovered that the energy
spectra in all spectral states can be fitted using 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. We
established that Gamma increases monotonically with dot from the low-hard
state, Gamma~ 1.5 to the high-soft state, Gamma~2.8 and finally saturates. The
index behavior was similar to that in a number of black hole (BH) candidates in
which we showed that its saturation was an observational evidence of the
presence of a BH. Based on this correlation, we applied a scaling method and
determined that a secondary BH mass in OJ~287 is about ~1.25x10^8 solar masses,
using the well-studied X-ray BH binaries XTE~1550--564, H~1743--322,
4U~1630--47, 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
inferred the size of the Compton cloud L_{CC}~ 10^{13} cm where X-ray spectra
were formed. Using this value of L_{CC} we confirmed that a BH mass of the
secondary in OJ~287 was of order of 10^8 solar masses as we derived using the
index, Gamma-correlation (the scaling method) with respect of the mass
accretion rate.
