The optical emission from ultraluminous X-ray sources

• Main Author: Copperwheat, C. M.
• Published: University College London (University of London) 2007
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.689445

Ultraluminous X-ray sources (ULXs) are point-like, non-nuclear sources which exceed the Eddington luminosity for a stellar mass black hole (BH). The emission from these sources might be beamed or super-Eddington, but it has also been pro posed that the compact object in these sources are intermediate mass BHs (IMBHs), which fit in the mass range between the two known populations of BH in the galaxy. The existence of IMBHs is under intense debate, and study of the X-ray data has been unable to resolve this issue. This thesis describes a model I have constructed in order to examine the optical/IR emission from these sources: an alternative channel by which their nature may be understood. I assume a binary model with a black hole accreting matter from a Roche lobe filling companion star. I consider the effects of radiative transport and radiative equilibrium in the irradiated surfaces of both the star and a thin accretion disc. I use current stellar evolutionary models as an input component in this model, and hence determine the mass, radius and age of the donor stars in a range of ULX systems, and in some cases provide limits on the BH mass. In addition I determine the mass transfer rate in these systems from the X-ray luminosity and compare this to transfer rate calculations based on the stellar evolutionary models. Since this method is independent of the optical data it is a powerful additional constraint on the parameter space. For systems where optical observations are available at multiple epochs, I make further determinations of the binary parameters based on the optical variability. Where it is possible to constrain the masses of the BHs, I find them to be consistent with BHs of up to 100A . I find that in general the donor stars are older and less massive than previously thought, and are consistent with being of spectral type B. I discuss how these results affect our understanding of the evolution and history of ULXs. I discuss how future studies of ULX optical counterparts will be even more revealing, and I make predictions for these optical campaigns, estimating binary periods, variability and the results of IR observational campaigns, which my results suggest will be a important tool in future studies of the nature of this class of sources.