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Gamma ray bursts tied to star collision

New Delhi, Oct. 10: The collision of dead stars shrunk into city-sized balls of neutrons triggers short gamma ray bursts, astronomers have reported, resolving a mystery that has baffled them for more than 30 years.

Gamma ray bursts are cosmic explosions that occur every day somewhere in the universe, lasting fractions of a second to a few minutes. They were first detected in 1973, but had remained a mystery for decades because of their fleeting existence.

In research papers that appeared in the journal Nature, an international team of astronomers has said that the shorter gamma ray bursts appear to be caused by the collision of closely-orbiting neutron stars.

“The mystery of short gamma ray bursts is largely solved,” said Don Lamb, professor of astronomy at the University of Chicago, and a member of the team.

Using satellites and ground-based telescopes, scientists studied the “afterglow” of a gamma ray burst that occurred on the night of July 9-10 this year in a distant galaxy. The burst lasted just 70 milliseconds.

The observations suggest that it was because of merging neutron stars, Lamb said.

Two years ago, astronomers had detected evidence that relatively long-lived gamma ray bursts that last for up to a few minutes were associated with the collapse of giant stars and the birth of black holes.

“But, all these years, we’ve had no clue about the nature of short gamma ray bursts,” says Rajaram Nityananda, the director of the National Centre for Radio Astronomy, Pune.

But scientists knew that some stars that exhaust their fuel collapse into neutron stars ' compact stars just 20 km in diameter packed with neutrons. Some neutron stars form binary pairs ' two stars locked in orbit around each other.

Theoretical studies also indicate that a gravitational disturbance might make a binary pair collide and merge into one, producing a massive explosion. “It’s been tempting to unite these ideas to explain short gamma ray bursts,” says Nityananda.

A research group in Denmark, which also observed the July 9 gamma ray burst, detected a fading source of light at the edge of the distant galaxy.

Scientists at the University of Copenhagen described it as the “optical afterglow” of the short gamma ray burst.

The observations by American and European scientists support the hypothesis that short gamma ray bursts are the consequences of the merging of two compact neutron stars.

While X-ray emissions from long gamma ray bursts provided evidence that they signalled the birth of black holes, there was nothing until now to study short bursts.

“Short duration gamma ray bursts have evaded optical detection for more than 30 years,” says Jens Hjorth, a scientist at the University of Copenhagen.

The fate of the neutron stars after collision would depend on their masses. The rules that determine the evolution of astronomical objects say that a star more than 2.5 times the solar mass would collapse into a black hole.

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