SDSS~J075217.84+193542.2: X-ray weighing of a secondary BH

Abstract

Precise measurements of black hole (BHs) masses are necessary to understand the coevolution of these sources and their host galaxies. Sometimes in the center of a galaxy there is not one, but two BHs. The BH duality of the quasar nucleus SDSS~J075217.84+193542.2 (herein SDSS~J0752) was recently proposed based on the observed strict periodicity of optical emission from the source. We tested this assumption using X-ray observations with Swift/XRT (2008--2010). We fitted the SDSS~J075217 spectrum using a Comptonization model and discovered soft X-ray variability in the 0.3--10~keV energy range. We pursued a scenario in which two supermassive BHs at the center of SDSS~J0752 form a pair; and the less massive (secondary) BH periodically crosses/punctures the disk around the more massive (primary) BH. We associate these periodic crossings with tidal disruptions of the disk and, as a consequence, with an increase in X-rays seen as a flare in SDSS~J0752. During such an X-ray flare event (2008--2010), we discovered a change in the source spectral states and the photon index saturation at the $\Gamma\sim3$ level with mass accretion rate $\dot M$. For BH mass scaling we used sources: OJ~287, M101 ULX--1 and HLX--1 ESO~243--49, as a reference ones, and found that M$_{SDSS}=9\times 10^7$ solar masses, assuming $d_{SDSS}= 500$ Mpc. Thus, we obtained a lower limit to a BH mass due the unknown inclination. In addition, we used the virial mass of the secondary BH based on $H_\alpha$-line measurements and we estimated the binary's inclination at SDSS~J0752, $i=80^{\circ}$, using a scaling technique.