Classifying the zoo of ultraluminous X-ray sources

Abstract

Ultraluminous X-ray sources (ULXs) are likely to include different physical types of objects. We discuss some possible subclasses, reviewing the properties of a sample of ULXs recently observed by Chandra and XMM-Newton. Sources with an isotropic X-ray luminosity up to a few times 10^{39} erg/s are consistent with ``normal'' stellar-mass X-ray binaries (mostly high-mass X-ray binaries in star-forming regions). Higher black hole (BH) masses (~ 50-100 M_sun) may be the end product of massive stellar evolution in peculiar environments: they may explain ULXs with luminosities ~ 1-2 x 10^{40} erg/s. Only a handful of ULXs require a true intermediate-mass BH (M >~ 500 M_sun). Finally, a small subclass of ULXs shows flaring or rapid variability in its power-law spectral component.