Image: Exploding star
NASA/Swift/Cruz deWilde
The Swift observatory has been hunting for gamma-ray bursts since its launch in 2004 to give astronomers instant notice of the short-lived — but huge — explosions in space. In this photo, as the star explodes, the narrow beam (white) of gamma rays is emitted first, followed by the wider beam (purple).
updated 4/21/2010 10:02:57 PM ET 2010-04-22T02:02:57

NASA's Swift observatory, keeping watch for the most powerful explosions in the universe, has hit a major milestone after spotting its 500th cosmic detonation in deep space.

Gamma rays are the highest-energy form of light, and gamma-ray bursts are brief but brilliant blasts that represent a colossal release of energy. The Swift observatory, a space-based satellite orbiting the Earth, has been hunting for them since its launch in 2004 to give astronomers instant notice of the short-lived — but huge — explosions in space.

"On the one hand, it's just a number, but on the other it is a remarkable milestone," said Neil Gehrels, Swift's lead researcher at Goddard Space Flight Center in Greenbelt, Md. "Each burst has turned over a new piece of the puzzle and a clearer picture is emerging."

Swift's main job is to quickly pinpoint each gamma-ray burst, report its position so that others can immediately conduct follow-up observations and then study the burst using its X-ray and ultraviolet and optical telescopes. Only explosions that happen to be aimed in Earth's direction can be picked up by the space observatory.

How gamma-ray bursts happen
The sources of most gamma-ray bursts are dying stars that are typically billions of light-years away from Earth, which means they are not only extremely far away, but also extremely energetic and powerful since they can be observed at such great distances.

The bursts come in long and short varieties. The long gamma-ray bursts (those lasting longer than two seconds) are associated with the deaths of massive stars in distant galaxies, astronomers have said.

When such a star runs out of fuel, its core collapses and can form a black hole surrounded by a dense, hot disk of gas called an accretion disk. These black holes can divert part of the infalling matter into a pair of high-energy jets that move so fast — upwards of 99.9 percent the speed of light — that collisions within them produce gamma rays.

The jet then continues on, later striking gas beyond the dying star that results in afterglows.

Hidden space explosion
Swift's 500th burst, officially known as GRB 100413B, exploded in constellation Cassiopeia as a long burst. But it wasn't detected in on-board analysis of data from the spacecraft's Burst Alert Telescope (BAT), which was interrupted 18 seconds 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 Alamos National Laboratory in New Mexico, later analyzed the data.

"The BAT team regularly digs through the data once it comes to the ground and finds weak bursts like this one that take a bit of special care," said Goddard's Judith Racusin, who coordinated burst observations that day.

Swift also conducts ultraviolet studies of exploding stars, monitors black holes and neutron stars for surges of high-energy radiation, and carries out long-term X-ray surveys of the entire sky.

Gamma-ray bursts were first discovered in 1967 by unclassified military satellites designed to look for clandestine nuclear tests.

The first observations required extensive analysis to be sure that the bursts were truly originating beyond the solar system, and these results were not published until 1973.

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