Swift J164449.3+573451 and Swift J2058.4+0516: Black hole mass estimates for tidal disruption event sources
Résumé
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 Γsat 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 Γsat values which indicate a high electron (plasma) temperature, kTe ~ 30–40 keV. This is also consistent with high cutoff energies, Ecut ~ 60–80 keV found using best fits of the RXTE spectra. Swift J2058+05 shows a lower electron temperature, kTe ~ 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 MBH ≥ 7 × 106M⊙ assuming the distance to this source of 1.5 Gpc. For Swift J2058+05 we obtain MBH ≥ 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.Key words: accretion, accretion disks / galaxies: individual:Swift J164449.3+573451 / galaxies: individual:Swift J2058.4+0516 / galaxies: general / radiation mechanisms: general / black hole physics