AAS 207th Meeting, 8-12 January 2006
Session 102 Black Holes II
Oral, Tuesday, 2:00-3:30pm, January 10, 2006, Wilson A/B

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[102.05] Ultraluminous X-ray sources

J. Liu (CfA), J. Bregman (University of Michigan)

Ultraluminous X-ray sources (ULXs) in nearby galaxies have stirred great interest in the high energy community because of their unresolved nature. ULXs are characterized by their high luminosities of 1E39-1E41 erg/sec, about 10 - 1000 times the Eddington luminosity for a neutron star. They can be intermediate mass black holes (IMBHs; 1E2-1E5 solar masses) emitting at sub-Eddington levels as Galactcic X-ray binaries, or stellar mass black holes with beamed emission. To fully understand what ULXs are, how they form, and why they emit at such luminosities, I have worked on the following topics. (1) A survey of extragalactic X-ray point sources in nearby galaxies with ROSAT HRI, which 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 also suggests most (above 70%) ULXs are associated with recent star formation activities and a young stellar population, while only a small fraction of ULXs, mostly below 2E39 erg/sec, are associated with the old stellar population. (2) The Optical study of a sample of ULXs with HST and Magellan, which shows that all these ULXs are in very young stellar environments with typical ages of 30 million years, and a few are identified to young OB star/supergiants that are presumably the secondaries in the ULX systems. (3) The X-ray timing properties of ULXs, in comparison to Galactic X-ray binaries. I have found for a ULX in M51 a possible two hour period, and for a ULX in M74 a possible two hour quasi-period which may suggest a black hole of 2,000-20,000 solar masses based on the scaling relation between the QPO break frequency and the black hole mass.

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