X-ray studies of ultraluminous X-ray sources

Abstract

Ultraluminous X-ray sources (ULXs) are extra-galactic, non-nuclear point sources, with X-ray luminosities brighter than 10^39 erg s^-1, in excess of the Eddington limit for 10 M_sun black holes. Recent results indicate that the majority of ULXs are stellar remnant black holes accreting material at or above the Eddington rate, rather than sub-Eddington accretion onto intermediate mass black holes. However, precisely how these ULXs accrete material at a super-Eddington rate remains an open question. This thesis focuses on the nature of these system as well as their environments, and attempts to explain physically how the sources operate in this super-critical accretion regime.

This work begins with a study of the X-ray spectra of ULXs in very nearby galaxies (D < 5 Mpc). A range of physical models is used to explain the ULX spectra and to interpret the results physically. The outcomes consistently suggest that ULXs are stellar remnant black holes accreting material at or above the Eddington rate. It is demonstrated that the hard spectral component is consistent with emission from the inner radius of an advection-dominated slim accretion disc; the mass of black holes powering ULXs can be constrained from this hard emission, falling in the regime of stellar-mass black hole (~3 - 30 M_sun). Assuming that the soft spectral component represents soft thermal emission from an optically-thick outflowing wind, the size of the wind is constrained to be between ~10^4 – 10^6 R_g.

We further explore the nature of ULXs by studying the X-ray spectral evolution of the individual source Holmberg IX X-1 with observed source luminosity. We find that the spectra tend to evolve from relatively flat or two-component spectra in the medium energy band, at lower luminosities, to a spectrum that is distinctly curved and disc-like at the highest luminosities. This spectral variability is consistent with the prediction of super-Eddington accretion models, in which the outflowing wind is expected to be launched from within the photospheric radius; the increase in accretion rate causes the more powerful wind to scatter a higher fraction of hard photons into the line of sight, while those that survive the passage through the wind will be Compton down-scattered to lower energies; these increase and soften the hard spectral component, resulting in a disc-like spectrum peaking at lower energy than the hard component seen at lower luminosity. Furthermore, we find observational evidence that the ULX might precess around its rotational axis, implied by a degree of degeneracy between different spectra observed at the same luminosity.

Finally, we study the population of ULXs present in a sample of 17 nearby luminous infrared galaxies (LIRGs). It is found that the LIRGs possess significantly fewer ULXs per unit star formation rate than nearby normal galaxies, by a factor of about 10. We argue that part of the deficit could be due to the high metallicity environment of the host galaxies suppressing the formation of ULXs, and the lag between star formation starting and the appearance of ULXs; however, the majority of the deficit of ULXs is likely to be due to the high amount of gas and dust in the LIRGs obscuring a large fraction of ULXs.