| Ultra-luminous X-ray sources (ULXs) have only recently been established as a class of off-nuclear point sources with X-ray luminosities of 10 39-1041 erg/sec, exceeding the Eddington luminosity for stellar mass black hole X-ray binaries. ULXs may be powered by intermediate mass black holes of 102-105 M⊙ , or by stellar mass black holes with special emission mechanisms. To fully understand what ULXs are, how they form, and why they emit at such luminosities, I have worked on different aspects of ULXs, from detailed studies of individual ULXs to large surveys.; A ROSAT HRI survey of extragalactic X-ray point sources in nearby galaxies reveals a statistical preference for ULXs to occur in late-type galaxies, especially in dusty star forming regions such as the spiral arms. The survey suggests most (≥70%) ULXs are associated with recent star formation activities and a young stellar population, while only a small fraction of ULXs, mostly below 2 x 1039 erg/sec, are associated with the old stellar population. The luminosity function shows the regular ULXs below 10 40 erg/sec lie as a smooth extension of the ordinary X-ray binaries below 1039 erg/sec, while there is a significant gap between the regular ULXs and the extreme ULXs above 1040 erg/sec.; The optical study of a sample of ULXs shows that all these ULXs are in very young stellar environments, with typical ages of 30 million years. A few ULXs are identified with unique stellar objects, e.g., NGC3031-ULX is an O8 V star, and NGC5204-ULX is a B0 Ib supergiant with the NV 1240A emission line as an accretion disk signature in its STIS FUV spectrum. The study on the spectral and timing properties for individual ULXs reveals for one ULX in NGC628 a two-hour quasi-periodicity with unique patterns that suggests an intermediate mass black hole of 2-20 x 103 M⊙ .; Combining evidence from X-ray luminosity, timing, spectroscopy, and the luminosity function, we suggest that the regular ULXs are stellar mass black holes like ordinary X-ray binaries but with special emission mechanisms, and the extreme ULXs are intermediate mass black holes. We propose optical spectroscopy with the Hubble Space Telescope and ground-based telescopes as future work to measure the radial velocity curves and ultimately determine the black hole masses. |
