Observing Extremes of Accretion

The X-ray band provides a view of extreme accretion regimes in neutron star and black hole binaries.    I will discuss insights gained from broad band observations of  X-ray binaries, with emphasis on ultraluminous X-ray sources (ULXs).  ULXs, identified as bright off-nuclear X-ray sources in nearby galaxies, have apparent luminosities significantly in excess of the Eddington luminosity for a stellar-remanant black hole.  This diverse population provides a window on regimes of super-Eddington accretion, and a few sources may harbor large black holes.