Exploding giant star sent longest gamma-ray blast, scientists say

The most powerful stellar explosion since the big bang has been linked to a newly discovered class of ultra-long gamma ray burst, astronomers said Tuesday at the 2013 Huntsville Gamma-Ray Burst Symposium in Nashville, Tenn.

“We really think we’ve found this new class of gamma-ray bursts and a natural explanation for creating them in a type of star collapse that we haven’t previously talked about,” said Andrew Levan, an astronomer at the University of Warwick in Coventry, England, who led a study on the phenomena.

Gamma-ray bursts are the brightest explosions in the universe, shooting out powerful blasts of high-energy light, and they typically signal the death of a massive star collapsing to form a black hole.

Up until now, scientists have classified gamma-ray bursts as short (typically less than two seconds) and long (typically 20 to 50 seconds, with some lasting up to a few minutes). NASA’s Swift satellite, launched in 2004, swivels around when it detects some high-energy activity to catch these fleeting events before their light disappears.

But these strange ultra-long bursts that Swift has documented in the last few years can last hours -- 100 times longer than those stellar flashes in the pan, said Bruce Gendre of the French National Center for Scientific Research.

Swift picked up the first such burst, GRB 101225A, on Christmas Day 2010. It baffled astronomers who guessed that the strange, long-lasting signal was either coming from a comet or asteroid falling onto a neutron star, or a neutron star falling onto a larger companion star.

But because the scientists didn’t know the distance to the event, either of those explanations could be true, Levan said.

“We didn’t actually know the distance, and when you don’t know the distance you don’t know the energy, and so everything is up in the air,” he said.

And as it turns out, both explanations were probably wrong. Using the Gemini telescope in Hawaii, the Very Large Telescope in Chile and the Hubble Space Telescope, researchers found that the burst occurred at a whopping 7 billion light-years away. This meant it was coming from a very distant object, one that must be very powerful to shine so brightly from so far.

Two other strange, extended bursts have cropped up as well. One discovered in 2011, called GRB 111209A, clocked in at a full seven hours, making it the longest gamma-ray burst ever recorded, said Gendre, who led a recent study in the Astrophysical Journal on the standout event.

Researchers realized that these beams of gamma rays must be coming from massive blue supergiant stars, which are 100 to 1,000 bigger than our sun and can stretch a hundred million to a billion miles in diameter, Levan said.

(They’re only about 20 times as massive, however — the extra girth comes from a hydrogen atmosphere that extends far out into space.)

“If you stuck a supergiant in the position of the sun in the solar system, it would extend almost all the way to the orbit of Jupiter,” Levan said. “So these are really very large stars.”

Such an explosion in range of Earth could cause major damage, but fear not: According to John Graham, an astronomer at the Space Telescope Science Institute in Baltimore, these ultra-long gamma-ray bursts tend to occur primarily in low-metallicity stars -- that is, stars with relatively low abundances of any elements heavier than hydrogen and helium.

Since stars in the Milky Way tend to be relatively rich in heavier elements, such an explosion is very unlikely to occur in our galactic neighborhood, scientists said.

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