Christmas gamma ray burst explained?

Posted by Kate Taylor

Astronomers have come up with two competing theories to explain a mysterious cosmic explosion first detected by NASA's Swift observatory on Christmas Day 2010.

It was caused either by a novel type of supernova located billions of light-years away or a crash between a comet and a neutron star within our own galaxy, say researchers in two different papers published in Nature.

"What the Christmas burst seems to be telling us is that the family of gamma-ray bursts is more diverse than we fully appreciate," says Christina Thoene of the Institute of Astrophysics of Andalusia, the supernova study's lead author.

"It's only by rapidly detecting hundreds of them, as Swift is doing, that we can catch some of the more eccentric siblings."

The Christmas burst, also known as GRB 101225A, was discovered in the constellation Andromeda by Swift's Burst Alert Telescope at 1:38 pm EST on Dec. 25, 2010. Follow-up observations by the Hubble Space Telescope and ground-based observatories failed to determine its distance.

The gamma-ray emission lasted at least 28 minutes, which is unusually long.

Thoene's team suggests that the burst occurred in an exotic binary system where a neutron star orbited a normal star that had just entered its red giant phase, enormously expanding its outer atmosphere. This expansion engulfed the neutron star, they say, causing the giant's atmosphere to be ejected and the neutron star's orbit tightened.

The neutron star may then have merged with the giant's core after just five orbits, or about 18 months, creating a black hole and oppositely directed jets of particles, producing gamma rays, followed by a weak supernova.

The team says this event would have taken place about 5.5 billion light-years away - and they've found what could be a faint galaxy at the right location.

However, Sergio Campana, who led the collision study at Brera Observatory in Merate, Italy, suggests an alternative model. This involves the tidal disruption of a large comet-like object and the ensuing crash of debris onto a neutron star located only about 10,000 light-years away.

Gamma-ray emission would have occurred when debris fell onto the neutron star. X-ray variations lasting several hours, detected by Swift's X-Ray Telescope, may have been caused by late-arriving clumps that struck the neutron star as the disk formed.