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Astronomers have taken a big leap towards solving a long-standing mystery – the source of the titanic cosmic explosions called short gamma-ray bursts.
The breakthrough came when, for the first time, astronomers were able to accurately pinpoint the locations of two recent short GRBs. They traced the bursts to relatively distant galaxies, bolstering the prevailing theory that the GRBs arise from collisions involving dense stellar corpses, called neutron stars.
GRBs are volleys of very high energy photons that can originate from any direction in the sky and come in two classes. "Long" bursts last from seconds to minutes and have been found to coincide with powerful supernovae, suggesting they form when massive stars explode and their cores collapse into black holes.
But until recently "short" bursts - lasting just a split-second - have proved frustratingly elusive, disappearing without a trace before researchers could pinpoint or study them. That began to change on 9 May 2005, when NASA's Swift space telescope detected a short burst and spun around in less than a minute to catch its brief X-ray afterglow. That allowed astronomers to identify a general location for the burst, near a galaxy full of ancient stars.
Now, researchers have observed afterglows at longer wavelengths for two bursts that detonated in July, homing in on their locations with 10 times the accuracy of the May burst.
Telling location
NASA’s High Energy Transient Explorer gamma-ray satellite (HETE-2) detected the first burst on 9 July. It lasted just one-tenth of a second but, two and a half days later, the Chandra space telescope captured its X-ray afterglow, caused by matter from the burst ramming into surrounding gas.
Ground-based telescopes in Chile and Hawaii then scanned the same region and found an afterglow radiating at optical wavelengths. That afterglow lay about 10,000 light years away from a galaxy that itself lies 1.8 billion light years from Earth.
Then, Swift detected a 0.25-second burst on 24 July. The next day, the Very Large Array - a collection of 27 radio dishes in New Mexico, US - found an afterglow at radio wavelengths.
The afterglow came from within a galaxy full of old stars that lies about 2.8 billion light years from Earth. This burst was also found to have an optical afterglow, making it the first short GRB measured to radiate at X-ray, optical, and radio wavelengths.
The location of these bursts hints at their source. Astronomers had suggested flare-ups in highly magnetised neutron stars, called magnetars, might produce short bursts. But these flare-ups can only get so strong before they destroy the stars, putting an upper limit on how far away a magnetar-based GRB can be observed. These bursts occurred about 10 times farther away than this limit, ruling out a magnetar source.
Ripples in space-time
The observations instead favour the popular "coalescence model". This suggests that short bursts occur during mergers between two neutron stars, or between a neutron star and a black hole.
The two objects, unable to escape each other’s immense gravity, can nevertheless take billions of years to finally collide and merge. During this time, the pair may travel out of the galaxy in which they formed. This may explain why the short GRB seen on 9 July occurred outside a nearby galaxy and why the second burst occurred inside a galaxy full of old, red stars.
"The coalescence model is the model to beat," says Dale Frail, a Swift team member at the National Radio Astronomy Observatory in New Mexico, US.
Frail says such mergers should produce ripples in space-time in addition to explosive flashes of gamma rays. And if the mergers happened within a few hundred million light years of Earth, the ripples could be measured by existing gravitational wave detectors such as LIGO.
"Gravitational waves would be a whole new window on the universe, and these short bursts might be our best opportunity to detect them," says Frail.
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