Multiwavelength Observations of Ultraluminous X-Ray Sources
  Multiwavelength observations may help us understand the physical nature of the ultraluminous X-ray sources (ULXs) found in external galaxies. Enabled by the arcsecond X-ray source positions now available from Chandra, there has been significant recent progress in the identification of optical and radio counterparts to ULXs. Recent results are reviewed on the identification of optical stellar counterparts to ULXs, the relation between ULXs and regions of active star formation, the study of optical nebular counterparts to ULXs, and the estimation of the luminosity of ULXs via their influence on surrounding nebula. 
Philip Kaaret