In all galaxies near enough to be studied in detail with Chandra (and this includes galaxies as far as at least 20Mpc), we observe populations of X-ray sources, with characteristics (time variability and spectra/X-ray colors) consistent with those of the X-ray binary stars found in the Milky Way: these sources are powered by the outer layers of a normal star falling into a neutron star or a black hole; observing different types of galaxies (elliptical, spiral, starburst and interacting), we can begin to understand how these X-ray sources form and evolve and how their history is linked to the evolution of the parent galaxy. Sources are associated with the young stellar population of spiral arms, with the intermediate-age population of the stellar disks of spiral galaxies, and with the old stellar population of bulges, elliptical galaxies, and globular clusters. Typically, more luminous X-ray emission is found in sources associated with younger stellar populations, which are likely to be highly accreting young binary systems with early-type-star donors.
At the high luminosity end of these luminous sources are the Ultra-Luminous X-ray sources (ULXs); some scientists have suggested that ULX may be special sources, different from normal X-ray binaries, and harbor black holes of masses larger than 100 solar masses, which are unlikely to form from the evolution of normal massive stars, and could be the remnants of primordial black hole formation in the early universe. Although the jury is still out on ULXs, it is becoming more and more clear that the majority of these sources are just the extreme examples of normal X-ray binaries.
See Fabbiano, G. 2006, Ann. Rev. A&A, 44, 323
Populations of X-Ray Sources in Galaxies
Pepi Fabbiano, Dong-Woo Kim, Andreas Zezas, Mike Garcia, Andrea Prestwich