Ultra-luminous X-ray Sources in nearby galaxies from ROSAT HRI observations I. data analysis

Abstract

X-ray observations have revealed in other galaxies a class of extra-nuclear X-ray point sources with X-ray luminosities of $10^{39}$--$10^{41}$ erg/sec, exceeding the Eddington luminosity for stellar mass X-ray binaries. These ultra-luminous X-ray sources (ULXs) may be powered by intermediate mass black hole of a few thousand $M_\odot$ 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^\prime$ within 40 Mpc with 467 ROSAT 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 $L_X = 10^{38}$--$10^{43}$ erg/sec is extracted from 173 survey galaxies, including 106 ULX candidates within the $D_{25}$ isophotes of 63 galaxies and 110 ULX candidates between 1--2$\times D_{25}$ of 64 galaxies, from which a clean sample of 109 ULXs is constructed to minimize the contamination from foreground or background objects. In a subsequent paper, the statistic properties of the survey will be studied to calculate the occurrence frequencies and luminosity functions for ULXs in different types of galaxies to shed light on the nature of these enigmatic sources.