XMM-Newton observations of ultraluminous X-ray sources and their host galaxies
The work begins with an investigation into the properties of a luminous extra-nuclear X-ray source in the Magellanic-type galaxy NGC 55. The observed X-ray luminosity of this source (> 1039 erg s -1) places it in the ultraluminous X-ray source (ULX) regime, a class of sources that may host a new, intermediate-mass class of black-holes, though this interpretation is currently being hotly debated. This particular source exhibits complex and rapid variability including a gradual increase in flux and pronounced dips in its X-ray light curve. In addition, the spectral analysis revealed an unusual X-ray spectra shape with curvature at high energies (> 2 keV). Detailed studies of the X-ray properties of NGC 55 as well as the nearby spiral galaxy NGC 4945 are then presented. The X-ray emission from NGC 55 is dominated by its discrete X-ray source population which, on the basis of an X-ray colour classification scheme, largely consists of accreting X-ray binaries. The galaxy also displays evidence of soft diffuse X-ray emission in and around its disc. NGC 4945 is a much more energetic galaxy, hosting a heavily obscured active nucleus as well as a nuclear starburst. The galaxy has a more luminous discrete X-ray source population than NGC 55 and shows clear evidence for diffuse X-ray emission along its disc as well as a possible outflow perpendicular to a galactic plane. The observed luminosities of these galaxies are consistent with the luminosities predicted on the basis of their star-formation rates. Finally, a detailed analysis of the deepest XMM-Newton observations of a sample of ULXs is then presented with the aim of addressing their true underlying nature. Interestingly, this work provides new evidence that the high energy spectral curvature seen first in the NGC 55 ULX may be prevalent throughout this class. Such curvature may be evidence for a cool optically thick Comptonising corona in these systems which is unlike most Galactic binaries, and poses new challenges to the intermediate-mass black-hole interpretation for these systems.