Ultraluminous X-ray sources and their environment

Details Ultraluminous X-ray sources and their environment Ultraluminous X-ray sources and their environment

Jun 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009phdt.........8b&link_type=abstract

Proquest Dissertations And Theses 2009. Section 0043, Part 0606 181 pages; [Ph.D. dissertation].United States -- District of Co

Astronomy and Astrophysics

Astrophysics

1

Ultraluminous X-Ray Sources, Intermediate-Mass Black Holes, Accretion Discs, Black Holes

Scientific paper

Ultraluminous X-ray sources (ULXs) continue to draw a lot of interest, because of the possibility that they are powered by accretion unto intermediate-mass black holes (IMBHs). The existence of IMBHs has far-reaching implications for astrophysics. Alternatively, ULXs may be compact objects undergoing super- Eddington accretion, or they may be beamed sources. One of the main goals of this thesis is to test the paradigm that ULXs contain IMBHs. This can be done by probing directly the X-ray emission from the accretion disk, but also by exploring the connection with the stellar population and finally by the impact ULXs have on the surrounding interstellar medium. We performed detailed spectral analysis of the best ULX data in the Chandra archive and a similar quality sample of lower-luminosity sources. We found that cool accretion disk components are not found preferentially in ULX spectra, and their strength is not correlated with luminosity. This damages the argument that cool disks are a signature of IMBHs, for which strong statistical support was never found. Moreover, we establish a general property that the most luminous ULXs posses the flattest X-ray spectra, which is best explained with super-Eddington models. To study the stellar population where ULXs are located, we used ultraviolet (UV) images taken with the Optical Monitor aboard XMM-Newton . By comparing the UV colors with synthetic models of stellar populations, we find that most ULXs are located near young star-forming regions. This spatial coincidence provides support for the super-Eddington scenario, with stellar- mass BHs accreting from their massive companions. Finally, using Spitzer observations of the ULX in the Holmberg II galaxy, we opened a new direction of study: photoionization modeling of nebular infrared lines produced by ULXs. We show that the emission of this ULX is not highly beamed, and its luminosity is indeed super-Eddington for a stellar-mass BH.

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