High mass X-ray binaries are binary star systems formed of a large companion star orbiting a companion object. Typically for high mass X-ray binaries the companion star, a Be star, has a mass around ten Solar Masses and a radius of around ten Solar Radii whilst the companion object, a neutron star or black hole, has a mass of around 1.4 Solar Masses and a radius of around 10km.
In an accretion process, matter is emitted from the larger companion star and is swept up by the companion object, as this happens the emitted matter transforms gravitational potential energy into emitted X-rays. The matter being shed by the larger companion star also forms a disc that surrounds it. Another method for accretion in this system is via this disc.
A large part of Rory's research has been on modelling the disc. One of the reasons for this is that observed Hα profiles have distinct characteristics that can be compared to the Hα profiles predicted by the model. These comparisons can be used to confirm whether the model is behaving correctly and accurately.
The Hα profile is a distinct profile in the optical spectrum that is produced by the disc. From it information can be gained about the system. For example, a greater intensity of the Hα profile implies that there is a larger disc, or the amount of redshift and blueshift gives an indication of the angle of inclination of the disc. The Hα profile has features that are not yet fully understood and leave unanswered questions that Rory hopes to gain a better understanding of, such as the peaks of the red and blueshifts not being equal, implying the number of particles coming towards us in the disc is different to the number of particles going away from us.
The modelling of the binary system is done via Smoothed Particle Hydrodynamics (SPH), a technique in which the grid coordinates move with the fluid elements rather than being fixed grid points. The code for this was originally written in Fortran 77 but has since been moved to Fortran 90 and has had work done on it so that it has OpenMP support (a previous blog involving OpenMP can be found here).
Further work is needed to be done on the simulation so that it can explain all the observable features of the disc. Once this is done it will be possible to accurately simulate accretion onto the compact object, giving us a better insight into this process. After this, work will be done to include stellar winds in the model, further increasing our understanding of high mass X-ray binaries.