Gamma Ray Bursts - research summary
Gamma-ray bursts (GRBs) are the most luminous objects in the Universe, capable of outshining their host galaxy and the brightest AGNs, and are visible at the highest redshifts. These properties make them excellent tools with which to study the end products of massive stars, and stellar evolution in the early Universe as a whole. The combination of their compactness and high bulk Lorentz factor also makes them excellent laboratories for studying high energy astrophysics.
Our wide-ranging GRB research program aims to understand the origin of GRBs and their environments in distant galaxies. We utilise data from the Swift satellite and numerous ground-based telescopes. Some recent highlights are outlined below.
GRBs are usually classified by the length of time over which 90% of the gamma-ray photons were detected - the T90 parameter. Over the last decade long bursts have been shown to originate in bright, star-forming regions in distant host galaxies. Prior to Swift, no host galaxy had been clearly identified for a short burst. Indeed it was not certain that they were cosmological. The issue was solved with the Swift detection of GRB050509B. We performed the X-ray analysis for this burst which led to its localisation some 10kpc from the centre of a bright, elliptical galaxy at redshift, z=0.225. This was the first GRB located close to such a host and, along with later similar results, supports the concept that short bursts are due to compact binary mergers rather than being associated with on-going star formation as is the case for long bursts.
THE EARLY BEHAVIOUR OF GRB AFTERGLOWS
The prompt emission of GRBs and their late time afterglow behaviour were relatively well studied, but the intermediate phase, directly after the prompt emission to up to a few hours, was relatively unknown. We have studied the largest sample of GRBs with well-determined X-ray light curves. These data reveal a wealth of information and show that GRBs display remarkable behaviour in the first day after birth. The X-ray emission often declines rapidly at first, followed by a relatively slowly decaying light curve, before a faster later decay. X-ray flares are superimposed on the decay, and in extreme cases these flares can rival the initial burst. This behaviour was unexpected and has led to the concept of much longer-lasting central engine activity than previously thought. This poses a problem for all GRB progenitor models.
THE HOSTS OF GRBs AT HIGH REDSHIFT
The mean redshift of GRBs discovered by Swift is about 2.2, and several have been found at very high redshifts, including the current record holder, GRB090423, at z=8.2. Aside from proving that such objects exist, the presence of high redshift bursts which are easily detectable raises the exciting possibility of finding even higher redshift bursts during the epoch when the very first stars and galaxies were forming. GRBs may even arise from Population III stars and hence exist before the first massive black holes were formed (ie. beyond the realm of quasars). We are studying the environmental properties of such bursts which relate to the role of star formation in the early Universe and provide a probe of the reionization epoch.