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

摘要

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 ( ? ) correlation. This Γ? ? correlation allows us to estimate black hole (BH) mass in ESO 243-49 HLX-1 to be M _(BH) ~ 7 × 10~(4) 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 ? 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.