The Origin of the Ultraluminous X-Ray Sources

Recently, several ultraluminous X-ray (ULX) sources were shown to host a neutron star (NS) accretor. We perform a suite of evolutionary calculations, which show that, in fact, NSs are the dominant type of ULX accretor. Although black holes (BH) dominate early epochs after the star-formation burst, NSs outweigh them after a few 100 Myr and may appear as late as a few gigayears after the end of the star-formation episode. If star formation is a prolonged and continuous event (i.e., not a relatively short burst), NS accretors dominate the ULX population at any time in the solar metallicity environment, whereas BH accretors dominate when the metallicity is sub-solar. Our results show a very clear (and testable) relation between the companion/donor evolutionary stage and the age of the system. A typical NSULX consists of a similar to 1.3 M circle dot NS and similar to 1.0 M circle dot Red Giant. A typical BH ULX consists of a similar to 8 M circle dot BH and similar to 6 M circle dot main-sequence star. Additionally, we find that the very luminous ULXs (L-X greater than or similar to 10(41) erg s(-1)) are predominantly BH systems (similar to 9 M circle dot) with Hertzsprung-gap donors (similar to 2 M circle dot). Nevertheless, some NSULX systems may also reach extremely high X-ray luminosities (greater than or similar to 10(41) erg s(-1)).