Gamma-Ray Burst research at Leicester
In Leicester, both the X-ray and Observation Astronomy (XROA) and Theoretical Astronomy groups study GRBS. XROA use a variety of satellite and ground-based observatories to study the multi-wavelength nature of the bursts, while the Theory group are involved in modelling the origin and evolution of GRBs.
GRB 090423: the most distant object in the Universe
GRB 090423 was found to lie at a redshift of z = 8.2, meaning the Universe was only 640 million years old when the GRB occurred. See this press release for more details. The plot below shows the light-curve containing the Photon Counting mode data from the Swift X-ray telescope.
GRB 080307: the onset of the afterglow?
GRB 080307 showed an unusual smooth rise in its X-ray afterglow, peaking a few hundred seconds after the trigger, at the start of which the emission briefly softened. This `hump' is unlike typical flares and can actually be modelled as the onset of the afterglow, something which is rarely seen in Swift data. The BAT-XRT is shown below, modelled with two components (shown as dashed lines): one for the prompt emission, one for the afterglow.
See Page et al. 2009 for more details.
GRB 070616: Spectrally evolving prompt emission
GRB 070616 showed a complex multi-peaked prompt emission structure, with a very long duration of more than 400 seconds. Spectral and temporal evolution was followed in detail, and the plot below (Figure 3 from Starling et al. 2008) shows this clearly, with the X-ray data plotted in red and the gamma-rays in grey.
In this case the light-curve cannot be easily modelled with a two-component fit (as detailed by O'Brien et al. 2006 and Willingale et al. 2007), indicating the requirement for an additional component.
GRB 061121: A very bright burst with a precursor
Some GRBs show what is known as a precursor: that is, there is an increase in gamma-rays before the main explosion. Swift triggered on such an event for GRB 061121, allowing both the XRT and UVOT to observe the main prompt emission as shown in the figure below.
It appears that the afterglow component starts early on, before or during the main burst peak. See Page et al. 2007 for more details
At the time of discovery, GRB 061121 was the instantaneously brightest long Swift burst, though this record has since been beaten by GRB 080319B (Racusin et al. 2008), which displayed both the brightest optical and X-ray fluxes ever measured for a GRB - and would even have been visible to the naked eye!
Although GRBs were discovered in the 1960s, it was not known what caused the events. There were 2 main theories: GRBs are formed either when a supermassive star collapses at the end of its life, to form a black hole, or orbiting neutron stars spiral together and coalesce, again forming a black hole. Both of these events could lead to an outburst of gamma-rays, followed by an X-ray afterglow, as is often seen.
An XMM-Newton observation of GRB 011211, lead by Dr James Reeves, found strong emission lines in the X-ray spectrum, due to light elements (magnesium, silicon, sulphur, argon and calcium); previously only iron lines had been observed. The emission lines were found to be blue-shifted with respect to the host galaxy, meaning that the hot gas emitting the lines was moving outwards at a tenth of the speed of light. This strongly implied that the GRB was preceded a few days earlier by a supernova explosion.
Below, to the left, is shown the XMM-Newton PN image of the afterglow, 11 hours after the burst went off. The plot to the right shows the X-ray spectrum with the blue-shifted emission lines.
More details can be found on the Leicester XROA news page. The paper published in Nature can be found here. Following the discovery of these lines, other GRB spectra were found to show similar emission, as described here.
A (12 MB) movie of the strengths of the emission lines changing over time is available here, courtesy of Dr James Reeves.
GRB 030329 was perhaps the most important burst in the last few years. It
was very bright and so became the most intensely followed GRB of all
time. An optical spectrum showed that GRB 030329 had a redshift of 0.169;
it was thus the closest apparently normal GRB ever seen (GRB 980425 was
much closer, at z=0.0085, but up until then had been as highly atypical in
its very low luminosity). After about 10 days, optical spectra showed the
features of a supernova, thus firmly associating a long GRB with a prompt
supernova for the first time.
Dr Richard Willingale, with a team from Leicester, performed an extensive
analysis of all of the multi-wavelength data available up to about 70
days after the burst, fitting the X-ray, optical and radio evolution with
a standard fireball model. Such time-dependant full-spectrum analyses
have only rarely been performed. This work showed how well the model
performed, strongly constraining the model parameters. It also revealed
pulses of extra radio and optical flux, the origin of which is still
under debate.
The diagram above (Fig. 1 in the paper
by Willingale et al. 2004) shows the afterglow flux of GRB 030329 in X-rays (blue), optical (V, B, R and I shown as blue, green, magenta and yellow dots) and radio (7.7 GHz diamond, 8.5 GHz square, 15.2 GHz star, plotted in red).
In December 2003, XMM-Newton observed a set of rings surrounding the area in
the sky where GRB 031203 had exploded earlier in the month. This amazing phenomenon has been explained, by Dr Simon Vaughan and co-workers, by the radiation from the GRB being scattered by 2 distinct areas of dust in our Galaxy. A schematic diagram can be found here.
The diagrams below show the images of the expanding halo and a simulation of the event. At the centre of each, the X-ray afterglow of the GRB can be seen. Click on the images to see larger versions.
More details can be found on Simon Vaughan's webpage and on the Leicester XROA news page.
 
With thanks to Drs Simon Vaughan and Richard Willingale for the image and simulation, respectively.
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UK Swift Science Data Centre
Last updated 2010 July 30
Web page maintained by Kim Page ()
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