X-ray Spectroscopic Evidence for Intermediate-mass Black Holes: Cool Accretion Disks in Two Ultraluminous X-ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two “ultraluminous” X-ray (ULX) sources. We measure isotropic luminosities of ergs s 1 and 40 L p 2.0 # 10 X ergs s 1 for NGC 1313 X-1 and X-2 (0.2–10.0 keV, assuming a distance of 3.7 Mpc). The 39 L p 6.6 # 10 X spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULX sources have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 j level of confidence for X-1; the improvement for X-2 is significant at the 3 j level. The soft components in these ULX spectra are well fitted by multicolor disk blackbody models with color temperatures of eV. This temperature differs markedly from those commonly measured in kT 150 the spectra of stellar mass ( ) black holes in their brightest states ( keV). It is expected that the 10 M kT 1 , temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: . Simple Ed3 M 10 M BH , dington scaling arguments suggest a minimum mass of . NGC 1313 X-1 and X-2 are found in 2 M ∼ 10 M BH , optical nebulae, which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed. Subject headings: black hole physics — X-rays: stars