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Gregory Brown

I am an Early Career Fellow with the Institute of Advanced Study, working in the Department of Physics with Andrew Levan. My research has primarily concerned extrgalactic transients with a focus on tidal disruption flares (TDFs).

Research Topics

The concept of the tidal disruption and subsequent accretion of a star by the central supermassive black hole (SMBH) of its host was suggested as a potential method for the feeding of the accretion powered active galactic nuclei throughout the 1980's, until it was realised that the required stellar number densities to produce the required repeated events were well beyond what was deemed likely to exist in most galaxies. Subsequently it was realised that single tidal disruption events, occuring only once every hundred thousand years or so for each galaxy, would provide excellent diagnostics of the central regions of distant galaxies. The confirmation of a detected TDF unambiguously represents the presence of an SMBH, even in quiescent dwarf or distant galaxies, where other methods, namely the identification of ongoing nuclear activity or the analysis of spatially resolved velocity dispersion measurements across the face of the host, would fail. Detecting large numbers of these events would allow for the analysis the SMBH demographic, both in the local Universe particularly at the low mass end where current sample sizes from other methods tend to be small, and as a measure of SMBH growth as TDFs probe higher and higher redshifts.

TDFs have to date been discovered mostly in the UV, soft X-rays and occasionally in the optical. This is a natural consequence of the flare which tends to have a high temperature (105K) thermal spectrum that peaks at UV and soft-X-ray wavelengths. Some examples may have been found through the excitation and ionisation of their nuclear regions by the strong UV flux associated with the flare which presents itself as line variability within the spectrum of the host.

However, to date, only 2 examples have been found through their high-energy emission, Swift J1644+57 and Swift J2058+05. These flares have distinct properties that set them apart from the others, namely long-lived (days) gamma ray emission, extremely bright and variable X-ray emission coupled with only modest optical emission, and a rising radio flare with evidence of relativistic expansion. Observations of these flares have led to the favored explanation being a TDF that launches a moderately relativistic jet, enhancing the observed high-energy emission through collimation and relativistic beaming. These flares are observable to much greater distances than their thermal cousins and may be a source of ultra-high energy cosmic rays.

As part of my research I have worked on the late time analysis of Swift J1644+57, the high-energy component of which shut off shortly after the beginning of my PhD. I have worked on the analysis of other candidate rTDFs, including GRB130925A, one of the recently discovered class of ultra-lomng GRBs, and recently I have been working on the characterisation of Swift J1112-8238, which shares a number of the key observational characteristics of the two previous candidates.

Publications

'Swift J1112.2-8238: A Candidate Relativistic Tidal Disruption Flare' - Brown et al., MNRAS, 452, 4297-4306 (2015)

Brown et al., 2017, MNRAS, under review: (PDF Document)

'Late time multi wavelength observations of Swift J1644+5734: A luminous optical/IR bump and quiescent X-ray emission' - Levan, Tanvir, Brown et al., ApJ, 819, 51 (2016)

'A New Population of Ultra-long Duration Gamma-Ray Bursts' - Levan et al. (including Brown, G.C.), 2014, ApJ, 781, 13

'Flows of X-ray gas reveal the disruption of a star by a massive black hole' - Miller, J. et al., (including Brown, G.C.), Nature, 526, 542 (2015)

'A Multiwavelength study of the relativistic tidal disruption candidate Sw J2058+05 at late times' - Pasham, D.R. et al. (including Brown, G.C.), ApJ, 805, 68 (2015)

"Placing limits on the transit timing variations of circumbinary exoplanets" - Armstrong et al, 2013, MNRAS, 434, 3047

Greg Brown

Write to:

Gregory Brown,
Department of Physics,
University of Warwick,
Coventry CV4 7AL
UK
 

Contact details:

E-Mail: G.C.Brown AT warwick.ac.uk