Ultra-luminous X-ray sources
Abstract
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 -10 41 erg/ sec, exceeding the Eddington luminosity for stellar mass black hole X-ray binaries. ULXs may be powered by intermediate mass black holes of 10 2 -10 5 [Special characters omitted.] , 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 10 39 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 10 39 erg/sec, while there is a significant gap between the regular ULXs and the extreme ULXs above 10 40 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 1240 Å 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 10 3 [Special characters omitted.] . 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.- Publication:
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Ph.D. Thesis
- Pub Date:
- 2005
- Bibcode:
- 2005PhDT........12L
- Keywords:
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- X-ray sources;
- Ultraluminous sources;
- Black holes;
- Extragalactic;
- X-ray binaries;
- Astronomy, Astrophysics