Abstract: | Ultra-luminous x-ray sources are an enigma. Their apparent luminosity exceeds the Eddington limit for the most massive stellar mass black hole predicted by stellar evolution theory and thus they are either more massive than 20 solar masses or, somehow, exceed the Eddington limit. They have a unique set of x-ray spectral and temporal properties different from possible stellar mass or active galaxy analogs. Their optical counterparts are very faint and do not show obvious unusual photometric signatures. I will discuss recent observations of the x-ray spectral and timing behaviors of ULXs and their UV and radio properties. |