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Thursday, 25 March, 1999, 22:46 GMT
Space blast teaser
GRB 990123: Bigger than anything previously detected
By BBC News Online Science Editor Dr David Whitehouse

Saturday, 23 January, 1999, is a day most astronomers will not forget. At 0947 GMT, a burst of gamma rays, radiation of the highest energy, flashed through our Solar System.

It had been produced by a cosmic cataclysm near the edge of the Universe and had been travelling through space for billions of years.

Some of the gamma rays hit our planet causing no harm - most passed by on their never-ending journey through the cosmos.

Some of gamma rays were picked up by two orbiting satellites with instruments set to detect them. Signals from the satellites triggered an automatic internet alert. Within 22 seconds, an optical telescope in New Mexico swung towards the position in the sky from which the burst had come.

The analysis of the data collected has now been published in a group of papers published in the leading journals Nature and Science.

Neutron stars

Astronomers detect gamma-ray bursters almost every day. They believe these flashes of intense radiation are caused when two, super-dense stars, called neutron stars, collide.

Neutron stars are small, only about 25 km across, but a teaspoonful of neutron star material would weigh a billion tonnes. Some speculate that a gamma burst signifies the violent birth of a black hole.

When the neutron stars collide there is a titanic explosion - one that literally ripples to the other end of the Universe.

This burst was named GRB 990123. It was far more powerful than anything that had ever been seen before. In fact, it was too powerful to be explained easily by current theories.

In the Science and Nature journals, astronomers have tried to come up with some explanations. One would seem to be that the burst was amplified, beamed towards us and just happened to appear brighter than it really was.

Current theories

"The burst appeared to be more luminous than the whole of the rest of the Universe, and that would be very hard to explain by most current theories," said California Institute of Technology Professor Shrinivas Kulkarni.

Caltech Professor George Djorgovski said: "It was ten times more luminous than the brightest burst seen so far, and that was quite unexpected.

"If the gamma-rays were emitted equally in all directions, their energy would correspond to ten thousand times the energy emitted by our Sun over its entire lifetime so far, which is about 5 billion years. Yet the burst lasted only a few tens of seconds."

Since that January day, astronomers all over the world have used many telescopes and satellite observatories to watch the burst's afterglow. They have also tried to work out from where the burst came.

The distance to the burst has been determined from its spectrum, and was found to be about nine billion light years from the Earth.

Host galaxy

"We were stunned," said Professor Djorgovski.

"This was much further than we expected, and together with the observed brightness of the burst it implied an incredible luminosity.

"The peak brightness of the visible light afterglow alone would be millions of times higher than a luminosity of an entire galaxy, and thousands of times brighter than the most luminous quasars known."

As the burst faded, astronomers managed to find its host galaxy. It appears different to our own galaxy, but not that unusual and contains many hot, blue, young stars.

A detailed study suggests the jet of energy came from an explosion moving close to the speed of light, and pointing nearly towards the Earth. This would have amplified its brightness and may explain its seemingly enormous brightness.

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28 Jan 99 | Sci/Tech
Scientists snap deep space blast
06 May 98 | Sci/Tech
Seeing stars after a big bang
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