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[7.01] X-ray Spectroscopic Evidence for Intermediate-Mass Black Holes

J. M. Miller, G. Fabbiano (Harvard-Smithsonian CfA), M. C. Miller (University of Maryland), A. C. Fabian (University of Cambridge)

Our analysis of the XMM-Newton spectra of the ultra-luminous X-ray (ULX) sources NGC 1313 X-1 and X-2 reveals cool (kT = 150 eV) accretion disk and hard power-law (coronal) components. These disks are 5-10 times cooler than temperatures measured in Galactic stellar-mass black hole binary systems at high mass accretion rates. X-1 and X-2 were observed at luminosities of 2.0 10⁴⁰ erg/s and 6.6 10³⁹ erg/s (0.2-10.0 keV), respectively -- well above the Eddington limit for 10 solar-mass black holes. As the inner accretion disk temperature around a black hole is expected to fall with increasing mass (T ~ M^-1/4), the observed disk temperatures and luminosities may indicate that NGC 1313 X-1 and X-2 are intermediate mass black holes (100-1000 solar masses). Prior radio and optical observations of X-1 and X-2 indicate that the observed luminosities cannot easily be explained in terms of relativistic beaming or anisotropic emission scenarios. We will discuss our results in terms of prevailing models concerning ULX sources and constraints on models for intermediate-mass black hole formation. Finally, we will discuss other results which indicate that cool accretion disks may be present in ULX sources.

Bulletin of the American Astronomical Society, 35#2
© 2003. The American Astronomical Soceity.