ESO 243-49 HLX-1: scaling of X-ray spectral properties and black hole mass determination

¹ Dipartimento di FisicaUniversità di Ferrara, via Saragat 1, 44122 Ferrara, Italy
e-mail: titarchuk@fe.infn.it
² National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409 Moscow, Russia
³ Moscow State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, 119992 Moscow, Russia
e-mail: seif@sai.msu.ru

Received: 26 November 2015
Accepted: 29 August 2016

Abstract

We report the results of Swift/XRT observations (2008−2015) of a hyper-luminous X-ray source, ESO 243-49 HLX-1. We demonstrate a strong observational evidence that ESO 243-49 HLX-1 undergoes spectral transitions from the low/hard state to the high/soft state during these observations. The spectra of ESO 243-49 HLX-1 are well fitted by the so-called bulk motion Comptonization model for all spectral states. We have established the photon index (Γ) saturation level, Γ_sat = 3.0 ± 0.1, in the Γ versus mass accretion rate (Ṁ) correlation. This Γ−Ṁ correlation allows us to estimate black hole (BH) mass in ESO 243-49 HLX-1 to be M_BH ~ 7 × 10⁴ M_⊙ assuming the distance to ESO 243-49 of 95 Mpc. For the BH mass estimate we use the scaling method taking Galactic BHs XTE J1550-564, H 1743-322 and 4U 1630-472, and an extragalactic BH source, M101 ULX-1 as reference sources. The Γ versus Ṁ correlation revealed in ESO 243-49 HLX-1 is similar to those in a number of Galactic and extragalactic BHs and it clearly shows the correlation along with the strong Γ saturation at ≈3. This is a robust observational evidence for the presence of a BH in ESO 243-49 HLX-1. We also find that the seed (disk) photon temperatures are quite low, of order of 50−140 eV which are consistent with high BH mass in ESO 243-49 HLX-1.