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On the nature of ULXs

P.A. Charles & D.L. Foster, SAAO; Vanderbilt

The nature of ultra-luminous X-ray sources (ULXs) that have been discovered by Chandra and XMM surveys of nearby galaxies is highly controversial. Their (accurately) known luminosities imply that they might be the long-sought intermediate mass black holes. We have conducted a systematic reanalysis of their X-ray spectra and present a comparison of ULX properties with those of (lower mass) galactic X-ray binaries. We discuss the implications of this work for ULX mass constraints.