Breakthrough Discovery in the Gamma-Ray Burst Mystery
Cambridge scientists, as part of an international team, have for the first time discovered the visible light counterpart of a gamma-ray burster. This discovery, made using the Isaac Newton Group of Telescopes on La Palma in the Canary Islands, is the observational breakthrough needed to solve the long-standing mystery of the origin of gamma-ray bursts. It is reported in the issue of the journal Nature published on 17 April.
Gamma-ray bursts are observed about once a day by specially designed satellites. They occur unexpectedly at random positions all over the sky. Until now, there has been no simultaneous detection of visible light and a gamma-ray burst from the same object, which would give a clue to their origin and distance. For this reason, the distance to gamma- ray bursters has remained unknown since the announcement of their discovery in 1973. Astronomers are divided as to whether the explosions responsible for gamma-ray bursts are happening relatively nearby - within our own Galaxy - or are taking place near the edge of the observable universe.
Until the Italian-Dutch X-ray satellite, BeppoSAX, was launched on 20 April 1996, the positions of most individual gamma-ray bursts were poorly known. BeppoSAX was designed to make use of the facts that X-rays are emitted during most gamma- ray bursts, and that the X-ray emission can be pinpointed more precisely on the sky than the gamma-rays.
On 28 February 1997, the Wide Field Camera on-board BeppoSAX was able to locate a bright gamma-ray burst with unprecedented accuracy. The position of the burst was narrowed down even further when other instruments on BeppoSAX looked at the fading X-ray source 8 hours after the initial burst.
The first images in visible light of this gamma-ray burster's position were made with the Isaac Newton Group of telescopes on the island of La Palma by a team headed by Jan van Paradijs (University of Amsterdam and University of Alabama). Subsequent observations by Dr Nial Tanvir (Institute of Astronomy, University of Cambridge), Dr Max Pettini (Royal Greenwich Observatory, Cambridge), and graduate student Joshua Bloom (Institute of Astronomy) revealed a fading source of visible light coincident with the position of the gamma-ray burst.
Follow-up observations by other ground based telescopes and the refurbished Hubble Space Telescope have confirmed that the visible object continued to fade for nearly a month after the burst.
Astronomers are now analysing the new data to decide whether the gamma-rays come from within our Galaxy or from near the edge of the Universe. One important clue towards solving the mystery has already emerged. "The data on the afterglow of visible light agree very well with the idea that we are seeing the blast from an explosion running into the surrounding interstellar gas at nearly the speed of light," says Ralph Wijers of the Institute of Astronomy. This so-called 'fireball model' was developed by the Astronomer Royal, Professor Sir Martin Rees, also at the Institute of Astronomy, and Professor Peter Meszaros of Pennsylvania State University.
The fireball, if it occurs very far away in the universe, would involve the collision of two neutron stars in a remote galaxy resulting in the sudden release of enormous quantities of energy. Travelling at nearly the speed of light, the gases thrown off by the explosion hit surrounding material and radiate so intensely that the gamma-ray burst can briefly become the brightest object in the sky at gamma-ray wavelengths. Joshua Bloom explains "If the gamma- ray bursters are at remote distances near the edge of the universe, then they are the most energetic phenomena known to humanity, releasing as much energy in a few seconds as the Sun does in ten billion years."
If instead the gamma-ray burst comes from an object within our own Galaxy, it could be explained by the release of much less energy from a single neutron star. In that case, however, astronomers had predicted that the bright afterglow would last only about a day.
If the theoretical predictions are correct, the fact that the afterglow of this gamma-ray burst has been going on for over a month lends weight to the argument that it is very remote, but the researchers are cautious for the time being. They warn that a single observation is not enough to distinguish finally between the two competing scenarios for gamma-ray bursters. "However," says Nial Tanvir, "It is clear that this is the breakthrough we have been waiting for. Now the first optical identification has been made, a more complete picture about the nature of gamma-ray bursts will soon emerge with optical follow-ups of future detections by BeppoSAX."
The Isaac Newton Group of Telescopes is owned by the United Kingdom and is jointly funded by the UK, Spain The Netherlands and Ireland.
An image is available via the World Wide Web at the following address:
Visible-light image of the 28 February 1997 gamma-ray burst region taken at La Palma on 9 March. The arrow points to the first fading optical source coincident with a gamma-ray burst. A nearby star and relatively bright galaxy are also indicted in the image. Credit: Nial Tanvir, Joshua Bloom (Institute of Astronomy, University of Cambridge)
At the Institute of Astronomy, Madingley Road, Cambridge CB3 0HA:
At the Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ