X-ray binaries
(Brassington, Hardcastle)
The elliptical galaxy NGC 4278: a three-colour X-ray image taken with the Chandra Observatory. Red indicates low-energy X-rays, which come from diffuse gas and supersoft sources, green indicates medium-energy emission and blue, high-energy. The objects that appear as white point sources are LMXBs, which have emission in all three bands. The large elliptical region indicates the optical extent of the galaxy.
X-ray binaries
X-ray binaries are binary star systems in which a normal star loses mass to a neutron-star or black-hole companion. These systems are classified by the mass of the star that is accreting matter onto the compact object, which can be either high-mass or low-mass (HMXBs and LMXBs). HMXBs are short-lived systems, due to the lifetime of the high-mass donor star, and they therefore provide a tracer of the star formation within the galaxy. LMXBs, on the other hand, are long-lived objects and are thought to provide a tracer of the stellar light distribution within the galaxy.
We are interested in studying the population of X-ray binaries in different galaxies to gain understanding of how these objects form. For example, one important question is whether LMXBs can be formed in normal environments as a result of standard stellar evolution processes, or whether they have to be formed in the dense environment of a globular cluster. This second scenario has received a great deal of attention over the last few years, as Chandra X-ray observations have shown that very many LMXBs reside in globular clusters. It seems plausible that all LMXBs form within globular clusters, and that those that are found elsewhere in the galaxy have either been ejected (e.g. by supernovae) or are a remnant of a globular cluster that has since evaporated.
Through Chandra and HST observations of elliptical galaxies we can test this hypothesis by identifying LMXBs and establishing which of these reside within globular clusters. We can then investigate whether the properties of LMXBs that are found within globular clusters differ from those of LMXBs found elsewhere, and so tackle the issue of LMXB formation, placing constraints on models of the formation and evolution of binaries within galaxies.
In addition, X-ray binaries of both kinds can be viewed as smaller-scale, local analogues of active galaxies and in this context we are particularly interested in their relativistic jets and the impact that these have on the external medium.