A Statistical Survey of Ultra-luminous X-Ray Sources

Chandra and previous X-ray observations have revealed in other galaxies the existence of extra-nuclear X-ray point sources with X-ray luminosities that exceed the Eddington luminosity of normal binary star systems, i.e., ultra-luminous X-ray sources (ULXs). To study the observational properties of these objects, we developed this catalog of 120 ULXs in 62 nearby galaxies within 40 Mpc derived from the archive ROSAT HRI observations. The positions of the ULXs have been improved using Chandra observations, reaching an accuracy of 0.3 arcseconds in some cases with the help of optical/IR/radio counterparts. The ULXs in our sample show very different long term variability and diverse identifications, including background QSO/AGNs, foreground Galactic stars, recent SN/SNRs, HII regions, stellar clusters, and radio sources, implying that ULXs selected by the luminosity criterion is a heterogeneous class. A statistical study of our ULX sample shows a strong preference of ULXs in spiral galaxies, especially on spiral arms and dark lanes, and in contrast to the scarcity of ULXs in normal early-type galaxies. For spirals, the ULX rate is 0.29±0.08 ULXs per 10¹⁰ L_⊙, but only 0.02±0.05 ULXs per 10¹⁰ L_⊙ for early-type galaxies, an order of magnitude lower. The luminosity functions of ULXs in all galaxies and in spirals can be fitted by power-laws, whose slopes are consistent with the low mass X-ray binary populations in external galaxies such as NGC 4697 and NGC 1553, suggesting a connection between the ULXs and the LMXB population.