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Strange gamma ray source a dying super star?

ultra long Gamma Ray Burst

The discovery of ultra-long gamma-ray bursts indicate a new source for these most powerful cosmic blasts (Source: Mark A. Garlick/University of Warwick)

Cosmic blast Astronomers have found a mysterious new type of the most powerful explosions in the universe since the Big Bang.

These explosions create powerful blasts of high energy gamma rays, but while most bursts are over in minutes, this new type can last for hours.

The discovery, presented at the Gamma-Ray Burst (GRB) 2013 Symposium in Nashville Tennessee, may be caused by the cataclysmic death of a supergiant star, say researchers.

Scientists including lead author Dr Andrew Levan at the University of Warwick, examined an unusually long duration gamma-ray burst originally spotted by NASA's Swift satellite on Christmas Day 2010.

"It was remarkable in that its burst was still going strong an hour and a half later, so it was immediately apparent that this was something different," Levan.

Using data from the Gemini Telescope in Hawaii, Levan and colleagues calculated this ultra-long gamma-ray burst was seven billion light-years way.

"We found this was a cosmological event occurring about half way to the edge of the observable universe," says Levan.

"That single measurement of distance allowed us to pin-point it's energy and understand what we're looking for, opening up a new route to understanding what that event might be."

Supergiant stars

Prior to this discovery there were two other known types of gamma-ray bursts.

Short duration gamma-ray bursts of under two seconds are thought to be caused when two neutron stars collide, producing a black hole.

Long duration gamma-ray bursts which usually last about a minute are caused when a star up to 20 times the Sun's mass known as a Wolf-Rayet explodes as a supernova at the end of its life, producing a black hole at its core.

"We think these ultra-long gamma ray bursts are caused by the same process but generated by a much larger star called a supergiant," says Levan.

Supergiants are among the biggest and brightest stars in the universe, up to 3.2 billion kilometres wide, a thousand times that of the Sun.

"The matter falling into the black hole has a lot further to fall, and the jet coming out, has a lot longer to travel, so it's taking longer for the energy jet to burn through the stellar material, which is why the burst lasts longer," says Levan.

The ultra-long duration of the Christmas Day gamma-ray burst, and two similar bursts examined by Levan and colleagues, are simply down to the sheer size of the supergiants exploding as supernovae.

The hypothesis is further supported by these bursts being similar in power to long duration gamma-ray bursts.

"It was only slightly more powerful than your average long duration gamma-ray bursts, just that the energy was spread over a much longer time," says Levan.

"And that makes sense, these supergiant stars aren't necessarily more massive than Wolf-Rayet stars, they're just less dense because they're so much larger."

"So it's giving out the same amount of total energy, just over a longer period of time."

According to Levan, the power generated by a gamma-ray burst is phenomenal.

Gamma-ray bursts in other galaxies have been known to ionise Earth's upper atmosphere.

"That's why they're discussed as origins of things like mass extinction events over the history of the Earth," says Levan.

"The Ordovician extinction [440 to 450 million years ago] looks very much like what you might expect from a gamma-ray burst mass extinction event."

The team's research is reported in full on the pre press website

Tags: blackholes, physics, stars, galaxies