DOMINIC JAMES WALTON
DOMINIC WALTON
Senior Lecturer in Astrophysics
Centre for Astrophysics Research, University of Hertfordshire
MY RESEARCH
My research primarily focuses on studying the X-ray emission from actively accreting black holes and neutron stars, particularly with the NuSTAR X-ray observatory, but also with other facilities like XMM-Newton, Chandra and Swift. I am also part of the leadership team for the High Energy X-ray Probe (HEX-P ), a potential Probe-class broadband X-ray observatory.
NuSTAR
I was fortunate enough to join the team behind the NuSTAR observatory around when it launched in 2012. NuSTAR is a NASA small explorer that carries the first ever focusing optics covering X-ray energies above 10 keV. The enhanced hard X-ray sensitivity provided by NuSTAR has been revolutionary for a number of different areas of astrophysics, but particularly for the study of accretion (my own broad area of interest).
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More information about NuSTAR can be found here.
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[Image credit: NASA/JPL-Caltech]
ULTRALUMINOUS X-RAY SOURCES
One of my main areas of research is the study of ultraluminous X-ray sources (ULXs). These are an enigmatic class of X-ray binary that appear to be more luminous than simple theory suggests should be possible for a 'normal' stellar remnant black hole (i.e. they radiate in excess of the Eddington limit for such an object). I have been leading the NuSTAR team studying ULXs, and in 2014 we made the remarkable discovery that one of these sources is actually powered by a neutron star as it exhibits X-ray pulsations (a 'ULX pulsar'), revealing a truly extreme super-Eddington accretion regime. This regime may be critical for growing supermassive black holes in the early universe, so it is vital we understand these local examples of super-Eddington accretion, and several more confirmed ULX pulsars have now been discovered. While we now know these ULX pulsars exist, we are still trying to figure out how common they are among the broader ULX population, and how exactly these neutron stars are able to reach such extreme luminosities.
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We recently published a new ULX catalogue, find it here.
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[Image credit: NASA/JPL-Caltech/SAO/NOAO]
BLACK HOLES
I also study the emission from black hole systems accreting at 'normal' rates (i.e. below the Eddington limit). Remarkably, we believe a black hole is essentially fully described by just two quantities: its mass and its spin (angular momentum). In terms of their masses, the majority of the black holes are either stellar-remnants in X-ray binary systems, or supermassive black holes powering active galactic nuclei, and I work with both types. I am particularly interested in efforts to measure black hole spin - a key quantity of interest for black hole growth/formation, relativistic jets and gravitational waves - which involves studying the extreme innermost regions of the accretion flow. In most cases we cannot take pictures of this region, but we can use X-ray spectroscopy and timing studies to identify the emission from the innermost accretion flow and measure black hole spin, particularly with the help of the sensitive broadband coverage provided by NuSTAR and e.g. XMM-Newton in combination.
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[Image credit: NASA/JPL-Caltech/R. Hurt (IPAC)]
HEX-P
I am a member of the leadership team for the High Energy X-ray Probe (HEX-P ), a potential Probe-class broadband X-ray observatory
submitted to NASA, serving as deputy chair for the science team working on accretion power (one of four main 'pillars' within the proposed science programme). HEX-P will carry focusing X-ray optics spanning the 0.3-80 keV bandpass, combining a low-energy telescope (LET; 0.3-25 keV) with two high-energy telescopes (HET; 2-80 keV), all co-aligned and operating simultaneously to provide a truly broadband view of the high-energy universe with a single facility. The improved hard X-ray imaging (>10 keV) of the HET will make HEX-P the most sensitive faciity ever flown at these energies, and its ability to cover such a broad bandpass (LET+HET) without the need to coordinate different observatories means that HEX-P will be a transformative facility for a wide variety of scientific topics.
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More information about HEX-P can be found here.
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[Image credit: NASA/JPL-Caltech]
PRESS
NuSTAR begins to provide our first hard X-ray view of both newly discovered and well-known ULXs.
NuSTAR robustly confirms our ability to view the innermost regions of active galactic nuclei, and that the active galaxy NGC1365 hosts a rapidly rotating supermassive black hole.
NuSTAR reveals coherent X-ray pulsations from the ULX M82 X-2, unambiguously confirming this to be a highly super-Eddington neutron star, and the first ULX pulsar discovered.
Chandra and XMM-Newton provide the first spin constraint for a lensed quasar, RXJ1131-1231, showing that we can use lensing to push spin measurements beyond the 'local' unvierse.
Archival Keck data reveal the redshift of the lensed quasar MG1131+0456, the first Einstein ring discovered.