XMM-Newton Archival Study of the Ultraluminous X-Ray Population in Nearby Galaxies

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• Mushotzky, Richard F.
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• Reynolds, Christopher S.

Abstract

We present the results of an archival XMM-Newton study of the bright X-ray point sources (L_X>10³⁸ ergs s^-1) 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, searching for a soft-hard state dichotomy similar to that known for Galactic X-ray binaries and testing the specific predictions of the intermediate-mass black hole (IMBH) hypothesis. To this end, we searched for ``low-state'' objects, which we defined as objects within our sample that had a spectrum well fitted by a simple absorbed power law, and ``high-state'' objects, which we defined as objects better fitted by a combined blackbody and a power law. Assuming that low-state objects accrete at approximately 10% of the Eddington luminosity (as found by Done & Gierlinski) and that high-state objects accrete near the Eddington luminosity, we further divided our sample of sources into low- and high-state ULX sources. We classify 16 sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic BH systems, the spectral indices, Γ, 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 for the high state is 0.1-1 keV, with the most luminous systems tending toward the lowest temperatures. We therefore divide our high-state ULXs into candidate IMBHs (with blackbody temperatures of approximately 0.1 keV) and candidate stellar mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of the candidate stellar mass BHs have spectra that are well fitted by a Comptonization model, a property similar to Galactic BHs radiating in the ``very high'' state near the Eddington limit.