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.

Schematic view of the likely evolution of a massive star progenitor for a long duration gamma-ray burst. The star gradually fuses its initial material creating heavy elements in the core. But once iron has been produced the star cannot produce heavier elements via fusion and the core collapses to produce a supernova and a central compact object - either a neutron star or a black hole. Only a small fraction of massive stars, possibly the most rapidly rotating, produce a GRB powered by accretion onto the compact object. [picture credit: NASA]

THE ORIGIN OF SHORT BURSTS

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 field around GRB050509B. The larger image shows the Swift gamma-ray error circle superimposed on the digital sky survey. The inset shows the X-ray error circle which overlaps with a bright elliptical galaxy.

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.

A schematic view of the X-ray light curve of GRBs. After the initial prompt phase the GRB can either decay gradually or displays a steep decay followed by a shallower phase (the late emission hump). Around half of all GRBs also have significant X-ray flares. The complex behaviour implies multiple emission components are contributing to the observed light curve. Spectral variability is also observed during each emission phase.

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.

Schematic view of the evolution of the Universe. Current cosmological models suggest that the first stars end the so-called dark ages at less than a billion years after the Big Bang. The first stars to form, which are Population III objects made of hydrogen and helium, are likely to be massive and hence good candidates for GRB progenitors which will be visible before the growth of quasars and massive galaxies. As the first stars form they will star to reionize the Universe, reducing the absorbing column of gas. This effect can be studied using the far-UV continuum of GRBs.