Faint echo. The European x-ray satellite XMM-Newton observed circular light echoes from the afterglow of gamma ray burst GRB031203.

Gamma Ray Bursts Without the Calories!

For the past 7 years, gamma ray bursts have been ranked as the most powerful explosions in the universe, surpassed only by the big bang itself. But new observations of a burst that occurred on 3 December 2003 suggest that the vast majority of gamma ray bursts are actually weak hiccups compared with the titanic blasts studied so far.

Astronomers first spotted brief bursts of high-energy gamma rays from random directions in the sky in the early 1960s. Only in 1997 did they learn just how much energy was being blasted out. The bursts are probably triggered when massive, rapidly spinning stars collapse into black holes. As the holes spew jets of matter at almost the speed of light, they release a brief pulse of gamma rays. This is followed by a more symmetric and slowly fading afterglow from the expanding fireball.

For the most part, the 3 December explosion, first seen by the European gamma ray observatory INTEGRAL, has all the hallmarks of a “classical” gamma ray burst. But follow-up observations from space and Earth revealed that GRB031203 was much closer than any other burst; it appeared in the southern constellation Puppis, the Stern of the Argo, just 1.6 billion light-years away. It was also much weaker than other gamma ray bursts, releasing just 1% of their typical energy, a team of astronomers led by Sergey Sazonov of the Space Research Institute in Moscow concludes in a paper in this week's issue of Nature.

GRB031203 was more similar to another nearby and dim explosion seen on 25 April 1998, a team led by Alicia Soderberg of the California Institute of Technology in Pasadena and her colleagues writes in a second Nature paper--but the new results are much more convincing. Because these weaker bursts can only be detected when they happen relatively close to Earth, the vast majority must go undetected, and their true numbers must be staggering, the authors conclude.

The observations suggest a new class of gamma ray bursts, says Donald Lamb of the University of Chicago. "It's pretty tantalizing," he says. It's not entirely clear whether they are also caused by the births of black holes or by a different mechanism, Lamb says: "I'm hesitant to rule out anything." The puzzle may be solved by NASA's Swift satellite, slated for launch on 7 October, whose sensitive suite of gamma- and x-ray telescopes will probably observe many more of these less dramatic gamma ray bursts, Lamb predicts.

Related sites
Abstract of the paper by Sergey Sazonov and colleagues
Abstract of the paper by Alicia Soderberg and colleagues
ESA's INTEGRAL gamma ray observatory
NASA's Swift gamma ray burst mission

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