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Overview

Two views of the Southern Pinwheel galaxy, M83. Left: VLT optical image (Credit: ESO). Right: Chandra X-ray image (Credit: NASA)

My research interests lie in the X-ray astrophysics of nearby galaxies. Galaxies are gravitationally-bound systems of many billions of stars, sat alongside interstellar gas and dust and a mysterious "dark matter" component. However, in X-rays their emission is generally dominated by a small number of individual objects with exotic properties - usually black holes or neutron stars, illuminated in X-rays by matter falling into their deep gravitational potential wells - and a hot interstellar gas component energised by supernova explosions. As a simple illustration of this contrast, the picture to the right shows images of the Southern Pinwheel Galaxy (M83) in both the optical and X-ray regimes.

As a result of this unique view, the study of X-ray emission from galaxies is of great importance to many fields, addressing issues such as the life-cycle of matter, the end points of stellar evolution and the physics of extreme environments. An understanding of the high energy phenomena revealed by X-ray studies can therefore add crucial elements to our picture of the Universe, and its evolution over cosmic time.

Research interests

Major current activities

• The nature of ultraluminous X-ray sources (ULXs). My current main area of interest is in the nature of the brightest point X-ray sources found outside the nuclei of nearby galaxies - the ULXs. Their extreme luminosities exceed the theoretical `Eddington' limit for accretion onto the ~ 5 - 15 solar mass stellar-mass black holes that we see in our own Galaxy. Explanations for their nature initially focussed on two main solutions to this issue: larger intermediate-mass black holes (IMBHs; ~ 100 - 10000 times solar mass) that accrete below this limit, or stellar-mass black holes that somehow exceed it. However, in recent years the detection of pulsations from a handful of ULXs has revealed that neutron stars are present in these objects, and perhaps many more. My research focusses on analysing the data from ULXs, taken from the current fleet of international X-ray observatories (XMM-Newton, Chandra, Swift, NuSTAR), and using it to distinguish between these scenarios. Our current results suggest most ULXs are consistent with stellar remnants (black holes or neutrons stars) in a new, super-Eddington accretion regime, although there may be a minority population of IMBHs also present.
• The multi-wavelength counterparts of ULXs. ULXs also have various associated phenomena that are evident when their locations are observed outside the X-ray regime. Most notably these include point-like counterparts, observed in visible, UV and IR light, that are either the companion star that the compact object feeds from, or light emitted by the disc of material falling onto the black hole. However, larger structures are also visible around some ULXs that are likely to have been excited and inflated by processes emanating in the ULX (the radiation field of the ULX and/or a jet/outflow of material from the system). Such objects can be hundreds of light years in extent, and are visible in both optical line emission and radio continuum. In some ULXs we are also now detecting radio emission that might be associated with a jet close to the black hole itself, that may allow us to constrain the black hole mass.

Other interests

• The X-ray emission of normal late-type galaxies.
• The X-ray phenomenology of starburst galaxies.
• The presence and properties of LLAGN in nearby galactic nuclei.
• 0.25-keV shadowing experiments and the soft extragalactic X-ray background.

Some research CV details

• 124 refereed journal articles (17 as first author)
• 5444 citations in the literature, H-index of 42
• Recent grants held: STFC Standard Grant holder (Super-Eddington accretion and Ultraluminous X-ray sources); STFC consolidated grant case (Super-Eddington accretion onto black holes); Royal Society international exchange scheme (Resolving the nature of ultraluminous X-ray sources with Chandra)
• PI on 1.23 Ms of observing time on XMM-Newton and Chandra
• PI on optical/UV follow-up programmes using HST, Gemini, VLT and WHT
• Galaxies TAC member for both XMM-Newton and Chandra (twice apiece), accreting binaries TAC member for XMM-Newton and Chandra (also twice each), accreting binaries TAC chair for Chandra (twice)
• Panel member and subsequently chair for Liverpool Telescope TAC
• Referee for all the major astronomical journals
• Science team member for the XEUS/IXO/Athena, NHXM and XIPE missions
• "Burst advocate" for Swift GRB mission

This page written and maintained by Tim Roberts. Last updated 08/09/21

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