The Ultraluminous X‐Ray Source Population from the Chandra Archive of Galaxies

One hundred fifty-four discrete non-nuclear Ultra-Luminous X-ray (ULX)sources, with intrinsic X-ray luminosities >1e39 ergs/s, are identified in 82galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Sourcepositions, X-ray luminosities, and spectral and timing characteristics aretabulated. Statistical comparisons between these X-ray properties and those ofthe weaker discrete sources in the same fields are made. Sources above ~1e38erg/s display similar spatial, spectral, color, and variability distributions.In particular, there is no compelling evidence in the sample for a new anddistinct class of X-ray object such as the intermediate-mass black holes.Comparison of the cumulative X-ray luminosity functions of the ULXs to ChandraDeep Field results suggests ~25% of the sources may be background objectsincluding 14% of the ULX candidates in the sample of spiral galaxies and 44% ofthose in elliptical galaxies implying the elliptical galaxy ULX population isseverely compromised by background active galactic nuclei. Correlations withhost galaxy properties confirm the number and total X-ray luminosity of theULXs are associated with recent star formation and with galaxy merging andinteractions. The preponderance of ULXs in star-forming galaxies as well astheir similarities to less-luminous sources suggest they originate in a youngbut short-lived population such as the high-mass X-ray binaries with a smallercontribution (based on spectral slope) from recent supernovae. The number ofULXs in elliptical galaxies scales with host galaxy mass and can be explainedmost simply as the high-luminosity end of the low-mass X-ray binary population.Comment: 22 pages including 15 figures and 3 tables, to be published in ApJ