A Stellar-Mass Black Hole in the Ultraluminous X-Ray Source M82 X-1?

We have analyzed the archival XMM-Newton data of the bright ultraluminous X-ray source M82 X-1 with a 105 ks exposure when the source was in the steady state. Thanks to the high photon statistics from the large effective area and long exposure, we were able to discriminate different X-ray continuum spectral models. Neither the standard accretion disk model [where the radial dependency of the disk effective temperature is T(r)~r^-3/4] nor a power-law model gives a satisfactory fit. In fact, observed curvature of the M82 X-1 spectrum was just between those of the two models. When the exponent of the radial dependence [p in T(r)~r^-p] of the disk temperature is allowed to be free, we obtained p=0.61^+0.03_-0.02. Such a reduction of p from the standard value 3/4 under extremely high mass accretion rates is predicted from the accretion disk theory as a consequence of the radial energy advection. Thus, the accretion disk in M82 X-1 is considered to be in the slim-disk state, where an optically thick advection-dominated accretion flow is taking place. We have applied a theoretical slim-disk spectral model to M82 X-1 and estimated the black hole mass ~19-32 M_solar. We propose that M82 X-1 is a relatively massive stellar black hole that has been produced through evolution of an extremely massive star, shining at a super-Eddington luminosity by several times the Eddington limit.