Spectral Evolution of a Luminous Compact X-Ray Source in NGC 253 with Chandra and XMM-Newton Observatories

Spectral studies of a luminous X-ray source, NGC 253 X21, are presented basedon archival Chandra and XMM-Newton data. The Chandra observation on 1999December 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 3revealed a short-term source variation in the range of (0.6--1.3) x 10^39erg/s. All spectra from these observations were successfully modeled byemission from an optically thick accretion disk. The average inner disk radiuswas kept constant at 63 x (cos 60/cos i)^(1/2) km, where i is the diskinclination, and did not vary significantly, while the disk inner temperaturechanged in the range of 0.9--1.4 keV. Assuming that this object is an accretingSchwarzschild black hole, and that the disk inner radius coincides with itslast stable orbit, the mass of the black hole is estimated to be ~ 7 M_sun. Thedisk luminosity corresponds to (30--120) x (cos 60/cos i) % of the Eddingtonlimit 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 thestandard disk state.Comment: 5 pages, 5 figures, 1 tabl