Accreting Compact Objects

My main research area focuses on accreting compact objects - binary systems consisting of a stellar remnant of some decription (either a white dwarf, neutron star or black hole) accreting matter from a normal (usually main sequence) star. My research can be subdivided into studies of low-mass X-ray binaries (LMXBs; containing a neutron star or black hole), and accreting white dwarfs (AWDs; often known as cataclysmic variables).

To accomplish my research, I employ a variety of analysis techniques, ranging from X-ray spectroscopy and imaging, to photometry and spectroscopy in the optical and infrared bands. I am also interested in timing analysis and owe a great debt to open source software such as Stingray that make learning such techniques easy and accessible. Astropy has also been a huge help throughout my career.

Low-Mass X-ray Binaries

During my PhD I spent the majority of my time studying LMXBs, focusing in particular on such systems as Swift J1753.5-0127 and V404 Cygni. These exhibit transient outbursts, characterised by a huge increase in flux, followed by a gradual decay on varying timescales of days to months.

Artist's impression of an X-ray binary: Material from the companion star forms an accretion disk around the compact object which emits across the electromagnetic spectrum. We also see collimated jets emanating from close to the compact object. (Image Credit: R. Hynes)

After finishing my PhD, my research started to branch out and I became more involved with studies of poorly understood subsets of accreting binaries. Most X-ray transients (such as the two mentioned above) exhibit peak X-ray luminosities of ≳1036 erg s-1. However, a fainter class of X-ray sources known as Very Faint X-ray Transients (VFXTs) display lower peak luminosities of ∼1034-36 erg s-1 exists, which are relatively understudied compared to their more luminous counterparts.

A collaboration which I am part of initiated the Swift Bulge Survey, a biweekly X-ray survey of the Galactic bulge with the Neil Gehrels Swift Observatory, in order to detect new and known VFXTs, characterize their variability, and derive their natures. Near infrared follow-up (led by myself) of sources detected in the survey revealed that binaries containing (sub)giant stars may make a substantial contribution to the VFXT population. See my recent paper on some of our results from the survey.

Accreting White Dwarfs

Aside from neutron stars and black holes, I also have a keen interest in AWDs, in particular, a subclass of them called magnetic cataclysmic variables. These systems consist of white dwarfs accreting material onto their poles from their companions along magnetic field lines. The infalling plasma forms a standing shock above the surface of the white dwarf, with a temperature directly related to its mass.

Artist’s impression of a magnetic cataclysmic variable: Material from the companion star flows towards the hot white dwarf and forms an accretion disk. The magnetic field of the white dwarf is strong enough to disrupt the innermost regions of the disk, forcing the material to flow along the magnetic field lines on to the poles of the white dwarf. (Image Credit: Julie Bauschardt)

Modelling of the X-ray spectrum enables us to derive the mass of the white dwarf. I was the PI of a ~1 Ms Legacy Survey of magnetic CVs with the NuSTAR hard X-ray telescope. This survey allowed us to derive the mass distribution of magnetic white dwarfs and draw comparisons to single white dwarfs and non-magnetic CVs, which has important consequences for theories of accretion-nova cycles as well as binary evolution. See our latest paper on the survey here, and the related NuSTAR news item.