Two Triggers for Celestial Explosions

The most powerful known explosions in the universe since the big bang, gamma ray bursts, go boom in two ways: either in short bursts that last less than 2 seconds or longer bursts that can last more than a minute. Now astrophysicists report unexpected findings that support speculation that longer and shorter bursts have completely different origins.

Gamma ray bursts can loose as much energy in seconds as our sun will release in its entire 10-billion-year lifetime. Little is known about these celestial detonations, in part because virtually all of them emerge from the oldest, farthest corners of space. However, scientists have been better able to study the longer variety, which comprise about two-thirds of the bursts. These bursts are thought to arise from the rapid collapse of massive stars. The cause of short bursts remains debated, and little is known about whether they result from the same sources as their longer-lived counterparts.

To investigate, theoretical astrophysicist Peter Mészáros of Pennsylvania State University, University Park, and colleagues in Hungary and the Czech Republic analyzed 1972 bursts spotted by the NASA Compton Gamma Ray Observatory. The team found a fundamental difference in light patterns from short and long bursts. Whereas long bursts throw out gamma rays at more or less constant rates, the short bursts peter out. That suggests they come from entirely different sources, the team argues in a paper to be published in an upcoming issue of Astronomy & Astrophysics. "They could be like Jekyll and Hyde, the same person operating with completely different personalities, [but] it does look like they are different animals," Mészáros says. No one had ever proposed such a profound difference.

"Whatever the mystery of long gamma ray bursts, there could be an equally fascinating mystery behind short gamma ray bursts," says theoretical astrophysicist Chuck Dermer of the U.S. Naval Research Laboratory in Washington, D.C. Mészáros says the next step is to check whether various models of gamma ray burst origins--such as neutron star mergers, which perhaps occur rapidly enough to explain short bursts--can reproduce the findings. He adds that the Swift Gamma Ray Burst Explorer, scheduled to launch in December, promises to pin down the location of short bursts with greater accuracy, which will help astronomers "find out what the heck they are."

Related sites
A copy of the paper in press
Peter Mészáros's site
The Burst and Transient Source Experiment that spotted the bursts examined in the report
Gamma Ray Burst FAQ