Astronomy Colloquium

  Ultraluminous X-ray sources (ULXs) are bright, off-nuclear X-ray sources in external galaxies, with luminosities in excess of 10^39 erg/s.  Although they have been suggested as candidates for hosting intermediate-mass black holes, the majority of ULXs are believed to be powered by Eddington-rate accretion of gas onto stellar-mass black holes, similar to the brightest X-ray binaries observed in our own Galaxy.  They therefore provide ideal laboratories for studying the physics of Eddington-rate accretion, which to date is relatively poorly understood.  The small number of Galactic sources reaching the Eddington luminosity and the Galactic absorption affecting soft X-rays have hindered previous studies of this accretion regime.  I will present our recent discovery of an ultraluminous microquasar in our neighboring galaxy M31.  We monitored the X-ray evolution of the source over a 6-month period, and detected bright, compact radio emission at the peak of the outburst.  I will  give an overview of the disc-jet coupling in this system, comparing it to the few known Galactic sources that have reached Eddington accretion rates.  When combined with constraints on the black hole spin from X-ray spectral fitting, the high peak radio luminosity has important implications for the ongoing debate as to whether jets are powered by black hole spin.  I will conclude by presenting some preliminary results from an ongoing VLA monitoring campaign to detect similar transient radio emission from X-ray binary outbursts in nearby spiral galaxies.