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Research

These are brief descriptions of only a few of the things I am currently working on... this page will, by definition, always be a work in progress! ;)

Double degenerate binaries in SDSS

Observations of Type Ia supernovae are extremely important to the cosmic distance ladder. They are very bright, so can be seen over very large distances, and they are consistent enough to track the expansion history of the universe. But there are still many questions regarding the type of system that lead to such an explosion, One possible progenitor population is double white dwarfs - they explode when the two stars of the binary merge to form a super-Chandrasekhar mass white dwarf. In this project we use SDSS to determine if there are enough such double white dwarf binaries in the Galaxy to account for the rate of Type Ia supernovae we see.

Population studies of Cataclysmic Variables and AMCVn stars

Even though accretion and binary star interaction is a well-studied topic, we understand preciously little of the physics of the interaction and the mechanism through which angular momentum is transferred in the system. CVs are the ideal test population to study these effects, as they are nearby, bright, and plentiful enough to do population studies on. I have worked on a large project to characterise the faint CVs found in the Catalina Real-time Transient Survey (CRTS) and am extending this work to detemine a reliable space density estimate of these accreting binaries that can be used as direct input to theoretical models of binary evolution. A particular interest at the moment are the AMCVn stars - they are rare, ultra-compact helium accreting binaries of which only ~50 are known so far. They have orbital periods that are typically shorter than an hour!

Ultra-fast multi-colour photometry with ULTRACAM

Eclipses, stellar pulsations, spinning neutron stars and exploding events are just a few examples of astrophysicsal processes that cause variability on second- or sub-second timescales. Observing these phenomena simultaneously in three colours can tell us a lot about the physics and/or geometry of these systems. I am part of the team in Warwick and Sheffield who operate and run ULTRACAM as a visitor instrument on a variety of telescopes (WHT, NTT, VLT).

Individual object parameter studies, such as the unusual CV V455 Andromedae

Known as "the CV that has it all", this system displays a host of unusual timing properties. Notably, permanent superhumps during quiescence and a ~3.5h modulation of its Balmer emission lines -- apparently unrelated to the orbital motion! (Porb = 81min) In addition, it also has a non-radially pulsating white dwarf, displays grazing eclipses, a possibly has a brown dwarf donor star. I keep a small list of important parameters values on a separate page for easy reference.

Multiwavelength variability of AGN

The relationship between the X-ray and optical variability can give us important clues about the interaction and relationship between physically separated part of the system.
Our long term optical monitoring programme of a sample of AGN have revealed a correlation between the short timescale (~days) X-ray and optical variations in Markarian 79, suggesting that the optical emission results from X-ray reprocessing by the accretion disc.

However, in addition, there are long-term (several years) variations in the optical light curve, which does not occur in the X-ray lightcurve. These long timescale variations may be accounted for by allowing the disc accretion rate or the geometry of the system to vary over timescales of months - years. (Breedt et al, 2008)

Ongoing work (in collaboration with the University of Southampton) explores how the strength of the X-ray/optical correlation varies with black hole mass (Uttley 2003).