Swift J164449.3+573451 and Swift J2058.4+0516: Black hole mass estimates for tidal disruption event sources

Abstract

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 J164449.3+573451 and Swift J2058.4+0516 (hereafter Swift J1644+57 and Swift J2058+05, respectively) 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 Γ varies from 1.1 to 1.8. We have firmly established the saturation of the photon index versus mass accretion rate at Γ_sat about 1.7-1.8. The saturation of Γ 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 Γ_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 1543-47as reference sources and found that the BH mass in Swift J1644+57 is M_BH ≥ 7 × 10⁶M_⊙ assuming the distance to this source of 1.5 Gpc. For Swift J2058+05 we obtain M_BH ≥ 2 × 10⁷M_⊙ assuming the distance to this source of 3.7 Gpc. We have also found that the seed (disk) photon temperatures are quite low, of order of 100-400 eV, in both of the sources, which are consistent with the estimated BH masses.