What are gamma-ray bursts?

Gamma-ray bursts are brief flashes of extremely high energy radiation. This flash can last anywhere between a fraction of a second and a few minutes, and for those moments is the brightest object in the sky. Gamma-ray bursts are the most powerful objects known, second only to the Big Bang itself! So where do they come from and what causes them? Chasing them can be a tricky business because they happen at unpredictable times and locations on the sky and quickly fade away. Scientists have overcome these problems with cutting-edge technology, using satellites which scan the sky and can swiftly turn around to point to any burst they detect and begin gathering information. A lot of the pioneering work was done by the BATSE satellite, and the latest to be launched (in November 2004) is the aptly named Swift satellite.

In the early 90‚s it was noticed that each gamma-ray burst fell into one of two categories: durations of over 2 seconds, or durations less than 2 seconds — the “long” and “short” bursts (see the figure on the left).

A few years later came the discovery that long gamma-ray bursts could be studied not only in high energy gamma-ray radiation, but also at all other wavelengths — including visible light, so now we could also see them with our own eyes. This lower enery radiation is called the afterglow, because it happens after the initial burst of gamma-rays, and lasts for a few hours to a few weeks. This provided plenty of time for the bursts to be studied in great detail and the breakthrough discovery that followed was to solve the mysterious origin of the long bursts.

We now know that long gamma-ray bursts are the deaths of massive stars in distant galaxies. The star collapses to form a black hole, and shock waves are sent out. Internal shocks cause the jet of gamma-ray radiation we see first. The afterglow radiation arises when the shock wave rams into the material that was surrounding the star and heats it up causing it to shine.

The connection between stars and gamma-ray bursts became apparent when supernovae, the explosions seen when massive stars die, were also seen at the locations of several gamma-ray bursts. So it was conclusive. Not only are gamma-ray bursts the most powerful explosions ever witnessed, involving the most massive stars, they are also seen out to the very edges of our Universe giving us a window back in time. For these reasons their continued study is very important so that we might understand the evolutionary behaviour of stars and galaxies throughout time.

But what of the short gamma-ray bursts? Do they have the same origins? Unfortunately the afterglows of short gamma-ray bursts remained elusive for many years, that is until the exciting developments of the past few months. Rapid progress is being made thanks to the recent launch of the agile and sensitive Swift satellite. The discovery of afterglows from the short gamma-ray bursts has revolutionised our understanding of these fleeting events. They appear to be completely different to their long-duration cousins, coming not from dying stars but from the collisions of two very compact objects - the merger of a neutron star either with another or with a black hole. The extreme physics of such an event is little known, and it seems short gamma-ray bursts will open a door to greater understanding.

The RTN works towards a full understanding of both the origin of the short bursts and the environments of the long bursts, using theoretical calculations and observational data. The latter are obtained from a wide variety of telescopes, both on the ground (eg radio, optical and even neutrino telescopes) and from space (like the Hubble Space Telescope and Swift). In the end, the combined theoretical and observational effort will tell us exactly what causes GRBs, and more, what they can tell us about the early Universe and how stars form.