NASA?sSwift observatory, keeping watch for the most powerful explosions in theuniverse, has hit a major milestone after spotting its 500th cosmic detonationin deep space.
Gammarays are the highest-energy form of light, and gamma-raybursts are brief but brilliant blasts that represent a colossal release ofenergy. The Swift observatory, a space-based satellite orbiting the Earth, hasbeen hunting for them since its launch in 2004 to give astronomers instantnotice of the short-lived ? but huge ? explosions in space.
??Onthe one hand, it?s just a number, but on the other it is a remarkablemilestone,? said Neil Gehrels, Swift?s lead researcher at Goddard Space FlightCenter in Greenbelt, Md. ?Each burst has turned over a new piece of the puzzleand a clearer picture is emerging.?
Swift'smain job is to quickly pinpoint each gamma-ray burst, report its position sothat others can immediately conduct follow-up observations and then study theburst using its X-ray and ultraviolet and optical telescopes. Only explosionsthat happen to be aimed in Earth's direction can be picked up by the spaceobservatory.
How gamma-ray bursts happen
Thesources of most gamma-ray bursts are dyingstars that are typically billions of light-years away from Earth, whichmeans they are not only extremely far away, but also extremely energetic andpowerful since they can be observed at such great distances.
Thebursts come in long and short varieties. The long gamma-ray bursts (thoselasting longer than two seconds) are associated with the deaths of massivestars in distant galaxies, astronomers have said.
Whensucha star runs out of fuel, its core collapses and can form a black holesurrounded by a dense, hot disk of gas called an accretion disk. These blackholes can divert part of the infalling matter into a pair of high-energy jetsthat move so fast ? upwards of 99.9 percent the speed of light ? thatcollisions within them produce gamma rays.
Thejet then continues on, later striking gas beyond the dying star that results inafterglows.
Hidden space explosion
Swift?s500th burst, officially known as GRB 100413B, exploded in constellationCassiopeia as a long burst. But it wasn?t detected in on-board analysis of datafrom the spacecraft?s Burst Alert Telescope (BAT), which was interrupted 18seconds after the burst, as the satellite slewed to a pre-planned target.
Instead,GRB 100413B came to light when David Palmer, an astrophysicist at Los AlamosNational Laboratory in New Mexico, later analyzed the data.
?TheBAT team regularly digs through the data once it comes to the ground and findsweak bursts like this one that take a bit of special care,? said Goddard?sJudith Racusin, who coordinated burst observations that day.
Swiftalso conducts ultraviolet studies of exploding stars, monitors black holes andneutron stars for surges of high-energy radiation, and carries out long-termX-ray surveys of the entire sky.
Gamma-raybursts were first discovered in 1967 by unclassified military satellitesdesigned to look for clandestine nuclear tests. The first observations requiredextensive analysis to be sure that the bursts were truly originating beyond thesolar system, and these results were not published until 1973.