Swift J164449.3+573451 and Swift J2058.4+0516: Black hole mass estimates
  for tidal disruption event sources
  A tidal disruption event (TDE) is an astronomical phenomenon in which a
previously dormant black hole (BH) destroys a star passing too close to its
central part. We analyzed the flaring episode detected from the TDE sources,
Swift~J1644+57 and Swift J2058+05 using RXTE, Swift and Suzaku data. The
spectra are well fitted by the so called Bulk Motion Comptonization model for
which the best-fit photon index Gamma varies from 1.1 to 1.8. We have firmly
established the saturation of Gamma versus mass accretion rate at Gamma_{sat}
about 1.7 -- 1.8. The saturation of Gamma is usually identified as a signature
of a BH now established in Swift~J1644+57 and Swift J2058+05. In Swift~J1644+57
we found the relatively low Gamma_{sat} values which indicate a high electron
(plasma) temperature, kT_e ~ 30 -- 40 keV. This is also consistent with high
cutoff energies, E_{cut} ~ 60 -- 80 keV found using best fits of the RXTE
spectra. Swift~J2058+05 shows a lower electron temperature, kT_e ~ 4-10 keV
than that for Swift~J1644+57. For the BH mass estimate we used the scaling
technique taking the Galactic BHs, GRO J1655--40, GX~339--4, Cyg~X--1 and
4U~1543--47 as reference sources and found that the BH mass in Swift~J1644+57
is M_{BH}> 7x10^6 solar masses assuming the distance to this of 1.5 Gpc. For
Swift J2058+05 we obtain M_{BH}> 2x 10^7 solar masses assuming the distance to
this source of 3.7 Gpc.
