Gamma-Ray Bursts: their Nature and use as Cosmological Probes

Gamma-ray bursts, usually known as GRBs, were originally discovered in the 1960s by orbiting military satellites, but only in 1997 was the distance to one first measured. To many astronomers' surprise it turned out to be about half way across the observable universe, which meant that GRBs are by far the most luminous objects known to science. This breakthrough was made possible by the detection of the faint, fading remnant of the GRB, a so-called afterglow, in optical light. GRBs themselves are characterised by the intense bursts of high-energy gamma-rays they produce, which only last typically a few seconds or minutes. By contrast, the afterglows, fade away over a period of days and weeks. Optical detection turned out to be critical because it provided a very accurate position for the GRB, and ultimately a spectroscopic redshift, which astronomers can easily translate into a measurement of distance. Since 1997, further research has shown that GRBs are produced when certain rare kinds of star, much more massive than the Sun, collapse at the end of their lives to form black-holes. In the process, by means we still don't understand, jets of material are ejected at velocities very close to the speed of light. These jets are the source of the flashes of gamma-rays we see, and when they crash into the tenuous gas surrounding the star, the afterglow light is produced. My research is aimed at better understanding these astonishing events, and using these ultimate cosmic light-houses to probe distant regions of the universe. The next few years promise to be a very exciting time in the GRB field thanks to the new US/UK/Italian satellite called Swift. This satellite detects around 100 GRBs per year, and rapidly transmits their positions to the ground. My own programs are largely concerned with following up these positions, to find and monitor afterglows, and to search for rare exciting types of GRBs. To do this I use the UK's RoboNet network of 3 large robotic telescopes sited at various locations around the world, along with many other powerful facilities, including the VLT and Gemini 8 m telescopes, the Hubble Space Telescope. A particularly exciting possibility is that with Swift we will discover GRBs at distances greater than any previous galaxy or quasar has been found. Since when we look across the universe we are looking backward in time, such a discovery would open a new window on the very earliest times, shortly after the Big Bang. At these distances galaxies and even quasars are expected to be rare and faint. GRBs by contrast should be bright enough to detect, and providing we can observe them quickly enough, should provide a great deal of information about the regions they occur in, and the state of matter in the universe at that time. My programmes have already contributed to discovering the two most distant GRBs to date, and have followed up one that has been tentatively claimed may be the most distant of them all. Another prime goal is to investigate the nature of so-called 'short duration' bursts. These events are very similar to the standard GRBs, but their gamma-ray flashes are briefer, often much less than a second. Only in the last year have afterglows been found for these bursts, and they seem to be a surpringly diverse population. A favourite idea is that they might be produced when two neutron-stars, extremely dense objects with masses similar to that of the Sun, but sizes only a few miles across, collide and merge with each other releasing enormous reservoirs of energy. However, it is increasingly clear that some GRBs must come from different sources, in particular giant flares from highly magnetic neutron stars in nearby galaxies. Observing GRBs is intensive and time-critical, and requires observers to be available 24 hours a day, 365 days a year. This grant would pay for a dedicated PDRA to share the burden of this effort and ensure the PI is able to effectively coordinate the followup.