Accretion Disks Around Black Holes in Compact Binaries


Principal Investigator

Dayal Wickramasinghe

Department of Mathematics

The Faculties


James Murray

Lilia Ferrario

Walid El-Khoury

Department of Mathematics

The Faculties


w56 - PC

In the last few years, there has been a rising interest in
close binaries, such as cataclysmic variables (CVs),
low-mass X-ray binaries (LMXBs) and binary radio pulsars (BRPs).

LMXBs and CVs, known as semi-detached close binaries, are believed to have a compact object - a neutron star or a black hole in LMXBs, and a white dwarf in CVs, which accretes matter from a low mass companion star. However, BRPs are detached binary systems which consist of an accreting neutron star and a white dwarf.

The aim of the project is to model two major aspects of close binaries. The thermal instabilities in LMXBs and the spectra of AM CVn stars, which are CVs in a very late stage of evolution.



What are the results to date and the future of this work?

We have been able to show the effect of X-ray irradiation on the structure of LMXB accretion discs where the inner disc regions are responsible for X-ray irradiation of the outer disc regions. Unlike what had been assumed previously, the disc is not necessarily compact enough to absorb all or any of the incident X-rays. The results of this work are published in the Monthly Notices of the Royal Astronomical Society (in print).

The objective of the second project is to attempt to fit theoretical optical spectra with observed spectra from AM CVn stars. This will enable us to put constraints on the physical parameters of AM CVn systems, such as the mass of the accreting star, the accretion rate, the disc's chemical composition and inclination angle. From the six known AM CVn stars, we have available three observed optical spectra, two of which, that of AM CVn and CR Bootis, were obtained from the literature and their analysis constitutes the main topic of our second paper in preparation. This is anticipated to take about four months before it will be ready for publication. The third spectrum, that of EC15330-1403, was collected by us in conjunction with Dr Stephane Vennes of the ANUATC. The analysis of this spectrum will be published in a paper where we may be able to include phase analysis of the star which was not possible for AM CVn and CR Bootis. It is anticipated that this work will take between three and four months to complete.

- Appendix A



What computational techniques are used?

The modelling aspects of these projects have been more complicated than anticipated. For instance, the modelling of X-ray heated accretion discs, the subject of the first paper, involved more complex equations than expected and, as a result, their numerical solution was very lengthy. As for the next two projects, there seem to be no foreseen difficulties if enough compute time is available.














Appendix A -