A giant stellar explosion has supplied indisputable proof that gamma ray bursts (GRBs) are triggered by supernovae--the explosive collapse of very massive stars into black holes. Although this theory has been favored for 5 years, scientists have had trouble confirming it with experimental evidence. The new find should help astronomers better understand the most violent bangs in the universe.
The first hint of a supernova-GRB link came in April of 1998, when a faint burst occurred almost simultaneously and in the same region of the sky as an unusual supernova explosion. This suggested that the brief bursts of energetic gamma rays are produced when some massive stars explode at the end of their lives, leaving a dense core to collapse into a black hole. Ever since, astronomers have detected subtle brightness and color changes of the slowly fading afterglows of some GRBs that hint at an additional source of energy in the region, but firm evidence was lacking. "It has always been possible to come up with other explanations," says Paul Vreeswijk of the European Southern Observatory, co-discoverer of the 1998 burst.
But now, a team led by Thomas Matheson and Krzysztof Stanek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Peter Garnavich of the University of Notre Dame, Indiana, have made the definitive link. In observations with two large telescopes in Arizona and Chile, they detected the telltale spectroscopic fingerprints of a supernova, very much like the one in 1998, in the afterglow of GRB 030329, a nearby gamma ray burst discovered 29 March by NASA's High-Energy Transient Explorer satellite (HETE-2).
According to Vreeswijk, proving the supernova connection has been the Holy Grail of GRB physics in recent years. Competing theories, like the one in which a GRB occurs a couple of months after a supernova explosion, are now far less likely with the new discovery, he says. But Stanek warns he can't be sure that all GRBs are associated with supernovas. In particular, short bursts, lasting less than 2 seconds or so, could be produced by another mechanism.