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

Abstract

Precise measurements of black hole (BH) 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 referred to as 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 obtained using 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 associated these periodic crossings with tidal disruptions of the disk and, as a consequence, with an increase in X-rays observed 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 Γ ∼ 3 level with mass accretion rate Ṁ. For BH mass scaling, we used OJ 287, M101 ULX–1, and HLX–1 ESO 243–49 as reference sources and found that MSDSS = 9 × 107 solar masses, assuming that dSDSS = 500 Mpc. Thus, we obtained a lower limit to a BH mass due to the unknown inclination. In addition, we used the virial mass of the secondary BH based on Hα-line measurements, and we estimated the inclination of the binary at SDSS J0752, i = 80°, using a scaling technique.