Research endeavours

I am interested generally in compact objects, stars and binaries, their structure, formation and evolution, and their use to infer fundamental physical properties. My research is based on observations, but includes interpretation, theory and numerical modelling as required. I generally try to make progress using key observations and/or physical considerations of individual, carefully selected objects. My focus over the last few years has been to try to use neutron stars to study high-density and high field-strength physics, in conditions out of reach of terrestrial experiment (and theory, as yet), and to solve associated astronomical puzzles. I have also become very interested in trying to understand what white-dwarf binaries lead to type Ia supernova explosions. I describe these two themes in turn.

Neutron stars

Much of my work focussed on detailed studies of neutron stars in and out of binaries. Outside of binaries, this concerned mostly the so-called isolated neutron stars, where (we think) we see thermal emission directly from the surface. Two very interesting clues - which we are interpreting in terms of their preceding evolution – are that the magnetic field strengths of INSs are similar despite the spectral features being rather different, and especially that one particular source, RX J0720.4-3125, changed its spectrum. Generally, I enjoy the interesting and puzzling information one gets from detailed spectra, and learning about, e.g., the very strange behaviour of matter in strong magnetic fields (leading to needle-shaped atoms and the presence of neutral hydrogen even at a million degrees!).
In binaries, I have been trying to use the companions to get a handle on the neutron star mass, with success both using the "black-widow pulsar" PSR B1957+20 and a pulsar in a short, relativistic binary with a low-mass white dwarf. I've also become very intrigued by the possibilities of pico-arcsec astrometry offered by pulsar scintellation, as uncovered by CITA colleage Ue-Li Pen. We have started to try to apply the same technique on pulsars in binaries; a prime target would be the black-widow pulsar mentioned above, for which the uncertainty in its current high mass is dominated by the uncertainty in the inclination, which we should be able to measure using scintellometry.

Exploding white dwarfs

Type Ia supernova explosions are associated with the nuclear disintegration of white dwarfs. It is not clear, however, what their progenitors are. Generally, it is thought that the explosions happen as white dwarfs are brought to approach their maximum possible mass, either by slow accretion from a normal star, or by a merger with another white dwarf. However, all known channels produce too few explosions. I am pursuing two possible resolutions. One is to look for systems in states that could hitherto not been observed, in which the accretion is so rapid that a dense wind ensues, and the other is to see whether it is possible that explosions happen even for mergers of white dwarfs with total mass below the maximum mass.


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