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L.M. Winter (University of Maryland), R.F. Mushotzky Collaboration, C.S. Reynolds Collaboration
We have conducted an archival XMM-Newton study of the bright X-ray point sources in 32 nearby galaxies. From our list of approximately 100 point sources, we attempt to determine if there is a low-state counterpart to the Ultraluminous X-ray (ULX) population, testing the specific predictions of the IMBH hypothesis. Indeed, 16 sources in our sample match the criteria we set for a low-state ULX, namely, LX > 1038 erg s-1 and a spectrum well fit with an absorbed power law. Further, we find evidence for 26 high-state ULXs which are well fit by a combined blackbody and a power law. As in Galactic black hole systems, the spectral indices, \Gamma, of the low-state objects, as well as the luminosities, tend to be lower than those of the high-state objects. The observed range of blackbody temperatures is 0.1-1 keV with the most luminous systems tending toward the lowest temperatures. We also find a class of object whose properties (luminosity, blackbody temperature, and power law slopes) are very similar to those of galactic stellar mass black holes. In addition, we find a subset of these objects that can be best fit by a Comptonized spectrum similar to that used for Galactic black holes in the ``very high'' state, when they are radiating near the Eddington limit.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.