The work of the X-ray group splits naturally into the following three topics:
An X-ray binary system (XRB) is formed after a binary star system remains gravitationally bound following the cataclysmic evolution of one of its members. The donor (or secondary) star can then feed material onto the resulting compact object (or primary) via a wind or Roche lobe overflow should the donor be suitably evolved. This material forms an accretion disc which is bright at X-ray wavelengths and can provide a picture of how material behaves under the influence of strong gravity. The Durham group has experience in analysing the following different forms of such binaries:
- White Dwarfs: In these lower mass compact object binaries, we can more readily observe the effects of strong magnetic fields on accretion - a process which is still poorly understood.
- Neutron stars: These more massive compact objects are supported against collapse by neutron degeneracy pressure. By observing the properties of the emission we can make inferences as to their complex structure.
- Black holes: The most massive stars collapse to leave black holes. These have no classical surface and so behave in different ways to the other compact objects listed above. Many observations have focused on their behaviour as this can shed light onto the nature of accretion and the curved space-time in the regions of strongest gravity.
Working in this area are Prof Chris Done, Dr Matt Middleton, Adam Ingram and Mari Kolehmainen.
Supermassive black holes with masses a million to a billion times the mass of our Sun are believed to be powering the highly luminous centres of some galaxies. The properties of these active galactic nuclei (AGN) are important as it is suggested that they can interact with the growth and development of galaxies at high redshift leading to the observational characteristics we see in the local Universe. In addition, the spectral and timing properties of emission from AGN can inform us of behaviours around the regions closer to the black hole's event horizon than we can probe in the much smaller XRB systems. This makes AGN of particular interest to both X-ray astronomers and cosmologists as they can reveal fundamental properties of accretion in regions of strong gravity and the impact this behaviour has on our view of the Universe.
The Durham High-energy astrophysics group has many members working on AGN activity including the spectral properties of the accretion flow, the complex timing behaviour and, on a larger scale, AGN sample properties in infra-red, optical and X-rays. These studies utilise a wide range of instruments (including the recently launched Herschel IR satellite) and are revealing new and exciting characteristics of these important objects.
Working in this area are Prof Martin Ward, Prof Chris Done, Dr Dave Alexander, Dr Agnese del Moro, Dr Matt Middleton, Emma Gardner and Chichuan Jin.
Ultraluminous X-ray sources (ULXs) are a mysterious class of X-ray sources associated with nearby galaxies. Their high X-ray luminosities exceed those typically seen in galactic XRBs, but their extra-nuclear locations rule out an AGN interpretation. Most evidence now points to these objects harbouring accreting black holes, but an open question remains the mass of these black holes. Do they constitute evidence for a new class of black holes, accreting in standard modes but with a mass intermediate between that found in Galactic XRBs and the supermassive black holes powering AGNs? Or are they standard, stellar-mass BHs that accrete in a rare, extremely high accretion rate mode?
Our work in Durham seeks to resolve the issue of the underlying mass of these objects, and better understand their accretion processes. We are attempting to obtain direct measurements of the black hole masses from the orbital characteristics of the optical counterparts to the X-ray sources, using optical spectroscopy on some of the world's largest telescopes (e.g. HST, VLT, Gemini). In parallel, we are investigating the X-ray emission characteristics of these objects using data from the full range of modern X-ray astronomy observatories (XMM-Newton, Chandra, Swift, Suzaku), with the aim of establishing whether the accretion is in a standard mode, as per XRBs and AGNs, or whether we are observing a new type of extreme accretion phenomenology.
Working in this area are Prof Chris Done, Dr Tim Roberts, Dr Matt Middleton and Andrew Sutton.
Contact DetailsDepartment of Physics,
Durham DH1 3LE
Tel: 44 (0)191 3343635