2016A&A...585A..94T

We report the results of Swift and Chandra observations of an ultraluminous X-ray source, ULX-1 in M101. We show strong observational evidence that M101 ULX-1 undergoes spectral transitions from the low/hard state to the high/soft state during these observations. The spectra of M101 ULX-1 are well fitted by the so-called bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index (Γ) saturation level, Γ_sat=2.8±0.1, in the Γ versus mass accretion rate ({dot}(M)) correlation. This Γ-{dot}(M) correlation allows us to evaluate black hole (BH) mass in M101 ULX-1 to be M_BH~(3.2-4.3)x10⁴M_☉, assuming the spread in distance to M101 (from 6.4±0.5Mpc to 7.4±0.6Mpc). For this BH mass estimate we apply the scaling method, using Galactic BHs XTE J1550-564, H 1743-322 and 4U 1630-472 as reference sources. The Γ vs. {dot}(M) correlation revealed in M101 ULX-1 is similar to that in a number of Galactic BHs and clearly exhibits the correlation along with the strong Γ saturation at ≃2.8. This is robust observational evidence for the presence of a BH in M101 ULX-1. We also find that the seed (disk) photon temperatures are low, on the order of 40-100eV, which is consistent with high BH mass in M101 ULX-1. Thus, we suggest that the central object in M101 ULX-1 has intermediate BH mass on the order of 10⁴ solar masses.