The nature of ULX source M101 X-1: Optically thick outflow from a stellar mass black hole

The nature of ultraluminous X-ray sources (ULX_s) has long been plagued by an ambiguity about whether the central compact objects are intermediate-mass (IMBH,≳10³M_⊙) or stellarmass (a few tens M_⊙) black holes (BHs). The high-luminosity ({reversed tilde equals}10³⁹ erg s-¹) and supersoft spectrum (T {reversed tilde equals} 0.1 keV) during the high state of the ULX source X-1 in the galaxy M101 suggest a large emission radius (≳10⁹ cm), consistent with being an IMBH accreting at a sub-Eddington rate. However, recent kinematic measurement of the binary orbit of this source and identification of the secondary as a Wolf-Rayet star suggest a stellar-mass BH primary with a super-Eddington accretion. If that is the case, a hot, optically thick outflow from the BH can account for the large emission radius and the soft spectrum. By considering the interplay of photons' absorption and scattering opacities, we determine the radius and mass density of the emission region of the outflow and constrain the outflow mass-loss rate. The analysis presented here can be potentially applied to other ULX_s with thermally dominated spectra, and to other super-Eddington accreting sources.