paper

Ultraluminous X-Ray Sources in External Galaxies

arXiv:astro-ph/0104333 · doi:10.1086/320343

Abstract

We investigate models for the class of ultraluminous non-nuclear X-ray sources (ULXs) seen in a number of galaxies and probably associated with star-forming regions. Models where the X-ray emission is assumed to be isotropic run into several difficulties. In particular formation of sufficient numbers of the required ultramassive black-hole X-ray binaries is problematic, and the likely transient behaviour of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a shortlived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer inevitable in many intermediate and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes.

4 pages, no figures; accepted for ApJ Letters

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