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

¹ LAPTh, 74941 Annecy-le-Vieux, France
e-mail: seifina@lapth.cnrs.fr
² Lomonosov Moscow State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, 119992 Moscow, Russia
e-mail: seif@sai.msu.ru
³ Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44122 Ferrara, Italy
e-mail: titarchuk@fe.infn.it; virgilli@fe.infn.it
⁴ National Research Nuclear University, MEPhI (Moscow Engineering Physics Institute), Moscow, Russia

Received: 25 March 2017
Accepted: 7 July 2017

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.