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News

The intense cosmic outbursts of high-energy radiation known as gamma-ray bursts (GRBs) - among the most energetic astrophysical processes known - are thought to arise from the collapse of massive stars in a supernova. But not all supernovae generate GRBs, and it remains unclear what causes some but not others to produce these bursts. Four international teams of astronomers, including several from the Carnegie Observatories, recently reported in Nature different views of a burst event that occurred in February 2006, which promises to yield new insights into the process.



New research looks at an X-ray flash that appeared on February 18, 2006, about 440 million light years away in the constellation Aries. The "before" image on the left is from the Sloan Digital Sky Survey. The "after" image on the right is from NASA Swift's Ultraviolet/Optical Telescope. The pinpoint of light from this star explosion outshines the entire host galaxy. Most other sources are foreground stars. Credit: SDSS (left), NASA/Swift/UVOT (right)


This burst, GRB060218, which occurred 440 million light years away in the constellation of Aries, was a mild type of GRB known as an X-ray flash. These are not as bright as normal GRBs, and don't produce as many gamma rays. It was previously unclear whether X-ray flashes were generated by the same basic process of stellar collapse during a supernova. But the new results seem to make that link secure.

A team led by Elena Pian, and another headed by Sergio Campana, show that GRB060218 was associated with a supernova called SN2006aj. The researchers think that this supernova was less energetic than those that produce fully fledged GRBs, but brighter than supernovae that don't generate such bursts at all. Alicia Soderberg and her colleagues, including Carnegie's Edo Berger, Eric Persson, and Pat McCarthy, have used observations of GRB060218 at radio and X-ray wavelengths to deduce that it was around a hundred times less energetic than ordinary GRBs - but they think such events should be ten times more common. The burst was unusual in that it went on producing X-rays for several weeks, which the researchers say was the result of high-energy debris thrown out by the explosion.

But what kind of collapsing star created the event? Campana and colleagues calculate that it was probably smaller than the stars of normal GRB-producing supernovae. And Paolo Mazzali and his colleagues use the observations of Pian's team to deduce that the progenitor star had a mass only about 20 times that of our Sun. Whereas it has been suggested that stars that generate full GRBs as they collapse end up as black holes, the progenitor of SN2006aj and GRB060128 is thus too small to have done so, and instead probably collapsed to form a neutron star. So black-hole formation does not seem to be an essential component of GRBs.