Gamma burst gang of four

A remarkable cosmic phenomenon that has baffled astronomers may provide fresh evidence of the universe's most violent accident - the collision of two neutron stars.

Scientists from the Institute for Nuclear Research in the Russian Academy of Sciences in Moscow have devised an explanation for the mysterious events of October 1996, when no fewer than four gamma ray bursters (GRBs) were detected coming from the same point in the heavens in the space of two days.

GRBs are the biggest explosions yet witnessed by mankind, events of almost unimaginable violence that appear in our skies as sudden flashes of gamma rays travelling from far beyond the Earth. Approximately one is detected every day by Earth-orbiting satellites.

Only two years ago, scientists were able to show that GRBs originated in distant galaxies and were immensely violent. That prompted myriad explanations as to what was being witnessed, such as the birth pangs of a new black hole.

Astronomers agree, however, that such an event is remarkably rare and should only happen about once every million years in a galaxy.

Vyacheslav Dokuchaev and Eugeny Babichev have calculated that one way to explain the so-called "gang of four" event of October 1996 is if each of the GRBs is a time-distorted image of the same colossal explosion.

In this model, each image would be twisted in space by the gravitational field of an unseen galaxy sitting between the Earth and the explosion and due to the immense distances involved, would arrive days apart, a little like echoes of a single gunshot except on a truly cosmic scale.

The only requirement for this calculation to work, the scientists argued, is that the fireball of the explosion must be travelling close to the speed of light, which happens to be in line with what should theoretically happen when immensely dense neutron stars collide.

"Our model, if correct, would be the first observation of gravitational lensing of a relativistic fireball and, surprisingly, additional justification of the standard neutron star model of the origin of GRBs," said Dr Dokuchaev.

The research is published in the latest edition of Phys Lett A.