Volume 585, January 2016
|Number of page(s)||11|
|Published online||23 December 2015|
Scaling of the photon index vs. mass accretion rate correlation and estimate of black hole mass in M101 ULX-1
Dipartimento di FisicaUniversità di Ferrara,
via Saragat 1,
2 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409 Moscow, Russia
3 Goddard Space Flight Center, NASA, code 663, Greenbelt, MD 20770, USA
4 Moscow State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, 119992 Moscow, Russia
Received: 18 March 2015
Accepted: 21 October 2015
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, Γsat = 2.8 ± 0.1, in the Γ versus mass accretion rate (Ṁ) correlation. This Γ−Ṁ correlation allows us to evaluate black hole (BH) mass in M101 ULX-1 to be MBH ~ (3.2−4.3) × 104 M⊙, 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 apply the scaling method, using Galactic BHs XTE J1550-564, H 1743-322 and 4U 1630-472 as reference sources. The Γ vs. Ṁ 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 40−100 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 on the order of 104 solar masses.
Key words: accretion, accretion disks / radiation mechanisms: general / black hole physics
© ESO, 2015
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