2008ApJ...687..471B

To test the idea that ultraluminous X-ray sources (ULXs) in external galaxies represent a class of accreting intermediate-mass black holes (IMBHs), we have undertaken a program to identify ULXs and a lower luminosity X-ray comparison sample with the highest quality data in the Chandra archive. We establish as a general property of ULXs that the most X-ray-luminous objects possess the flattest X-ray spectra (in the Chandra bandpass). No prior sample studies have established the general hardening of ULX spectra with luminosity. This hardening occurs at the highest luminosities (absorbed luminosity ≥5x10³⁹ erg/s) and is in line with recent models arguing that ULXs are actually stellar mass black holes. From spectral modeling, we show that the evidence originally taken to mean that ULXs are IMBHs–i.e., the ``simple IMBH model''–is nowhere near as compelling when a large sample of ULXs is looked at properly. During the last couple of years, XMM-Newton spectroscopy of ULXs has to a large extent begun to negate the simple IMBH model based on fewer objects. We confirm and expand these results, which validates the XMM-Newton work in a broader sense with independent X-ray data. We find that (1) cool-disk components are present with roughly equal probability and total flux fraction for any given ULX, regardless of luminosity, and (2) cool-disk components extend below the standard ULX luminosity cutoff of 10³⁹ erg/s, down to our sample limit of 10^38.3 erg/s. The fact that cool-disk components are not correlated with luminosity damages the argument that cool disks indicate IMBHs in ULXs, for which strong statistical support was never found.