We propose a methodology to derive black hole mass of ultraluminous X-ray sources (ULXs) from X-ray spectral fitting by using the extended disk blackbody model (extended DBB) where $T_{\rm eff} \propto r^{-p}$ is assumed (Mineshige et al. 1994).
We fit the theoretical slim disk spectra by Watarai et al. (2005) with the extended DBB model and estimate the black hole mass from the value of $r_{\rm in}$ from the fitting in the conventional manner in which $r_{\rm in}=3r_{\rm g} (\propto M)$ is assumed.
We calculate a correction factor, $M/M_{\rm x}$, where $M$ is the actual
black hole mass and $M_{\rm x}$ is the derived mass based on the conventional manner as mentioned above.
We found a mass correction factor ranges from 0.87 -- 1.83, depending on the black hole mass. |