Massive X-ray Binary Systems in External Galaxies

Malcolm Coe works on investigating X-ray Binary systems (XRBs) using a variety of different waveband techniques. In particular his group is studying High Mass X-ray Binaries (HMXRBs) in the Magellanic Clouds - companion galaxies to the Milky Way.

These High Mass X-ray Binaries systems consist of a neutron star exhibiting X-ray pulsations, and a massive early-type star such as a Be star. Our recent work has revealed several new features of these Be/X-ray binary systems, demonstrating complex interactions between the neutron star and the massive envelope around the Be star.

To fully understand the astrophysics of these systems it is essential that observations are carried out at as many wavelengths as possible, and as close together in time as possible. Consequently this project relies heavily on extensive space-based and ground-based observations, primarily in the Southern Hemisphere. In the near future we hope to also develop simulations of these systems.

A specific project that a new PG student starting in Oct 2009 could work on is:

PhD in modelling the circumstellar disk structures in X-ray Binary systems

As part of an intensive campaign over the last few years it has been shown that an exceptionally large population of massive X-ray binaries exist in the Small Magellanic Cloud (SMC) – see, for example Coe et al, 2008 Multi waveband observations show correlations between the properties of the circumstellar disk and the X-ray activity in Be X-ray binaries (eg Coe et al 2006 MNRAS 368, 447). Long-term hard X-ray lightcurves, coupled with long-term optical photometric lightcurves from the OGLE project and the Liverpool and Faulkes telescopes, together with H alpha monitoring with the VLT, will provide valuable insights into the size and behaviour of the circumstellar disks. Recent identification of the optical counterparts to the BeX systems in the SMC have enabled multiwaveband data to be included in the interpretation of their behaviour. To date, more than a dozen of these systems have revealed optical outbursts, or binary modulation linked to X-ray phenomena. One excellent example is that of SXP327 (Coe et al 2008 MNRAS 387, 724) which reveals multiple optical brightenings each binary cycle.

Detailed modelling of the dynamics of the disc are required to interpret observations and the best approach to use is believed to be Smoothed Particle Hydrodynamics (SPH). Work by Okazaki & Negueruela (eg 2001 A&A 377, 161 – see movie on this page) has shown the power of SPH simulations of these systems. In collaboration with Atsuo Okazaki (Tokyo) (harmas.arc.hokkai-s-u.ac.jp/~okazaki/BeX/sim/index.html) and Kenji Takeda (Joint Director of the Microsoft Institute for High Performance Computing here at Southampton University - www.soton.ac.uk/ses/research/mshpci/index.html) we will establish their SPH code running on a dedicated machines and explore the disk-neutron star interactions in BeX systems in the Milky Way and SMC. Needless to say, a strong interest in computational simulations of physical systems is an essential ingredient for this PhD programme.

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Malcolm Coe's main home page may be found here.