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Imagine an explosion that produces as much energy in a single second as ten suns do in their entire ten-billion-year lifetimes. Imagine we could 'hear' the birth cry of a new black hole, created by the death-throes of a star more than 20 times the size of our own sun. Welcome to the mysterious and astounding world of the Gamma Ray Burst (GRB).
In December 2004, NASA's SWIFT satellite made its first observation of a GRB, marking a new era for astronomers worldwide who are studying the strange phenomenon. For the first time, they were able to make almost instantaneous observations of the most powerful, and yet one of the most elusive, events in the cosmos.
GRBs were first discovered in the 1960s by accident, by American military satellites watching for violations of the Atmospheric Nuclear Test Ban Treaty, but their origins remained a mystery for over 20 years. It wasn't until the 1990s, with the launch of NASA's Burst and Transient Source Experiment (BATSE) satellite, that it was even determined with certainty that GRBs occur outside of our galaxy, at distances of billions of light years1.
A Whole New World
A GRB is detected from somewhere in the universe about once a day on average. With such an abundance of GRBs, it is perhaps surprising that so little is known about their origins. The problems that have faced astronomers over the past few decades are the suddenness of these bursts, and their seemingly random distribution: by the time powerful ground-based telescopes manage to turn in the direction of a burst, little is left to observe.
There are two types of GRB: the short-duration, which last for less than two seconds, and the long-duration, which can last for up to several minutes. The two are thought to be caused by two different phenomena, and have very different observed properties.
The long-duration GRBs are believed to be the result of a particularly energetic supernova explosion called a hypernova. Supernovae are caused when a very large star runs out of fuel, which results in the star's core collapsing, and its outer layers being blown off in a cataclysmic explosion. The death of super-massive stars, such as the Wolf-Rayet stars — which are greater than 20 times the size of our sun and have a surface temperature at least five times hotter — can produce hypernovae. It is believed that gamma rays are produced inside the star by shockwaves triggered by the collapsing core, which travel close to the speed of light. These shockwaves then travel out of the star and collide with the surrounding medium, producing an afterglow. Initially observed in the X-ray region of the spectrum, this then fades as time goes on, into the less energetic visible light region, and eventually down right through to the radio region.
This model, known as the 'Collapsar', or 'Fireball', model, is believed to be the explanation for the long-duration GRBs. In March 2003, GRB0303292 exploded relatively close to Earth (within half-a-billion light years), allowing detailed analysis of its afterglow. The properties of its afterglow were almost identical to that of a previously-recorded supernova (SN1998bw), suggesting that the link between GRBs and supernovae outlined by the Collapsar model is indeed possible.
This produces a wonderful array of possibilities. The stars involved in hypernova explosions are almost certainly massive enough for the core to continue collapsing. Maybe what we are witnessing is the loud piercing scream of a black hole taking its first breath, announcing its creation and transmitting its existence to the entire visible universe.
An explanation for the short-duration GRBs, on the other hand, remains elusive. There are many theories, such as the merger of two neutron stars3, the collapse of the core of a less massive star or even just a long-duration GRB observed at a glancing angle. The fact is that nobody really knows.
Because of their briefness and random positioning, it has been extremely difficult to get the early observations required in order to study the GRBs in depth. Within the last decade, the establishment of the Gamma Ray Burst Coordinates Network, which locates and then notifies astronomers around the world of the locations of the sudden bursts, has helped, but it still takes many minutes to realign large, cumbersome terrestrial telescopes and specialised satellites into position, by which time the much sought-after afterglow has faded dramatically. SWIFT, as the name suggests, aims to combat these problems. It has built-in multi-wavelength detectors that can detect bursts and turn to their precise location in about 15-25 seconds. Connected to the Gamma Ray Burst Coordinates Network, this will allow astronomers worldwide to observe the bursts in much more detail than ever before, and maybe once and for all solve the mystery of one of the most powerful events in the known universe.