  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.
