Spectral studies of a luminous X-ray source, NGC 253 X21, are presented based on archival Chandra and XMM-Newton data. The Chandra observation on 1999 December 16 detected the source at a bolometric luminosity of 0.3 x 10^39 erg/s (assuming isotropic emission), while an XMM-Newton observation on 2000 June 3 revealed a short-term source variation in the range of (0.6--1.3) x 10^39 erg/s. All spectra from these observations were successfully modeled by emission from an optically thick accretion disk. The average inner disk radius was kept constant at 63 x (cos 60/cos i)^(1/2) km, where i is the disk inclination, and did not vary significantly, while the disk inner temperature changed in the range of 0.9--1.4 keV. Assuming that this object is an accreting Schwarzschild black hole, and that the disk inner radius coincides with its last stable orbit, the mass of the black hole is estimated to be ~ 7 M_sun. The disk luminosity corresponds to (30--120) x (cos 60/cos i) % of the Eddington limit of this black hole. Therefore, this luminous X-ray source, NGC 253 X21, is understood consistently to be an accreting stellar mass black hole in the standard disk state.
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