Research

My research activities have covered a number of related topics in the general field of high energy astrophysics, including: 

Ultraluminous X-ray Sources and Intermediate Mass Black Holes
Studies of Slow X-ray Pulsars
Long-term Variability in X-ray Binaries
Click Here for a List of My Telescope Proposals

  • Ultraluminous X-ray Sources and Intermediate Mass Black Holes

    The bulk of my current research is centred on the study of the class of objects known as ultraluminous X-ray sources (ULXs). ULXs are extragalactic objects that are located outside the nuclei of their host galaxies. The characteristics of their X-ray emission are consistent with accreting black holes, but their X-ray luminosities exceed the maximum theoretical Eddington limit for a stellar mass black hole (i.e. black holes with masses ~3 - 80 Solar masses, formed through the collapse of massive stars). These luminosities have been interpreted as evidence of a new class of intermediate mass black holes with masses between ~100 - 100,000 Solar masses. The brightest of these objects, the hyperluminous X-ray sources, have luminosities above 1E41 erg/s that cannot be easily explained without intermediate mass black holes. However, the luminosities of the bulk of ULXs could be explained through a combination of mild super-Eddington accretion and beaming. The brightest ULX dubbed HLX-1 was discovered in 2009 in the galaxy ESO 243-49 (Farrell et al. 2009), with a record-breaking luminosity of ~1E42 erg/s that is extremely difficult to explain through hyper-accretion and/or beaming. Since its discovery I have taken a leading role in the intense mult-wavelength observing campaign targeting HLX-1, using world-class observatories including the Hubble, Chandra, XMM-Newton, Swift, VLT, Gemini, EVLA and ATCA telescopes. This campaign has resulted in 10 articles published in leading peer-reviewed astronomy journals (including papers in Nature and Science). Our latest results continue to indicate that HLX-1 is a ~10,000 Msun black hole, possibly the remnant of a low-mass dwarf galaxy that has been accreted by ESO 243-49 and stripped of most of its mass. In addition to my work on HLX-1, I have been actively seeking additional candidate intermediate mass black holes. These ongoing activities fall into two separate projects: (1) the search for HLX-1 like objects in archival X-ray data and through targeted observing campaigns, and (2) hunting for evidence of nuclear intermediate mass black holes in dwarf galaxies.

    While it is widely accepted that most (perhaps all) galaxies host a supermassive black hole (with masses of millions to billions of times the mass of the Sun) in their centres, how these black holes form is not understood. It is possible that they are formed from the mergers of intermediate mass black holes. Intermediate mass black holes could thus constitute a crucial missing link between stellar mass and supermassive black holes, with substantial ramifications for our understanding of galaxy formation and evolution. The existence of intermediate mass black holes also has important implications for other areas of astrophysics including the search for dark matter annihilation signals (predicted to be produced through the compression of dark matter by wandering intermediate mass black holes in galaxy halos), the reionisation of the Universe (micro-quasars containing intermediate mass black holes are predicted to have contributed between ~25% - 50% of cosmic reionisation), and the detection of gravitational wave radiation (from in-spiralling intermediate mass black holes). A relationship has been observed between the bulge mass of galaxies and the mass of the nuclear black hole, and if this relationship continues to low mass (i.e. dwarf) galaxies then we would expect them to contain nuclear intermediate mass black holes. However, evidence for nuclear black holes (of any mass) in dwarf galaxies is very sparse, with only a handful of candidates so far detected (mostly through the detection of bright nuclear X-ray emission, a singature of an accreting nuclear black hole). I am currently supervising an honours project at The University of Sydney that is focused on the search for new active galactic nuclei (AGN) in dwarf galaxies (i.e. dwarf AGN). This project is ongoing, but has already uncovered 5 new candidate dwarf AGN.