We report the results of Swift and Chandra observations of an ultra-luminous
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 (\Gamma) saturation level,
\Gamma_{sat}=2.8 +/- 0.1, in the \Gamma vs. mass accretion rate (\dot M)
correlation. This \Gamma-\dot M correlation allows us to evaluate black hole
(BH) mass in M101 ULX-1 to be M_{BH}~(3.2 - 4.3)x10^4 solar masses assuming the
spread in distance to M101 (from 6.4+/- 0.5 Mpc to 7.4+/-0.6 Mpc). For this BH
mass estimate we use the scaling method taking Galactic BHs XTE~J1550-564,
H~1743-322 and 4U~1630-472 as reference sources. The Gamma vs. \dot M
correlation revealed in M101~ULX-1 is similar to that in a number of Galactic
BHs and exhibits clearly the correlation along with the strong \Gamma
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
quite low, of order of 40-100 eV 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 of order 10^{4} solar massesComment: 12 pages, 9 figures, accepted by Astronomy & Astrophysic
