Ultraluminous X-ray sources in nearby galaxies

These ultraluminous X-ray sources (ULXs) may be powered by intermediate-mass black holes of a few thousand M_Sun_ or stellar mass black holes with special radiation processes. In this paper, we present a survey of ULXs in 313 nearby galaxies with D_25_>1 within 40Mpc with 467 ROSAT High Resolution Imager (HRI) archival observations.

The HRI observations are reduced with uniform procedures, refined by simulations that help define the point source detection algorithm employed in this survey. A sample of 562 extragalactic X-ray point sources with L_X_=10^38^-10^43^ergs/s is extracted from 173 survey galaxies, including 106 ULX candidates within the D_25_ isophotes of 63 galaxies and 110 ULX candidates between 1D_25_ and 2D_25_ of 64 galaxies, from which a clean sample of 109 ULXs is constructed to minimize the contamination from foreground or background objects.