The supernova connection.

he fog surrounding the identity of the progenitors of ȍ-ray bursts (GRBs) is beginning to lift, at least for the class of GRBs known as 'long' bursts. This is thanks to a series of observations of a burst that began on 29 March 2003, very close to our Galaxy. On pages 843, 844 and 847 of this issue, Uemura et al. 1 , Price et al. 2 and Hjorth et al. 3 reveal the evolution of this burst in unprecedented detail — and show that behind the GRB is the unmistakable signature of a supernova. The GRB population divides neatly into long ones and short ones, depending on whether the burst of ȍ-rays lasts more or less than a few seconds 4. About two-thirds of all observed bursts are long, and these are the only ones for which longer-lasting 'afterglows' at X-ray, optical and radio wavelengths have also been found. These afterglows may last up to several months, and from them the distance to the GRB and the identity of its host galaxy can be determined. There is good evidence that long bursts are largely associated with active, star-forming regions in small, blue galaxies. And, in at least three cases, there has been tantalizing evidence that GRBs are associated with a particular type of supernova 4 — although that interpretation has so far been fraught with uncertainty. A 'usual' supernova arises when the core of a massive star collapses, ejecting the stellar outer envelope. The majority of such super-novae result from parent stars that are less than about 30 times heavier than the Sun, and the core collapse produces a neutron star. These supernovae are normally detected weeks after the collapse, because the ejected envelope only brightens sufficiently to be detected at optical wavelengths some weeks later. The only signals of the collapse that are expected to reach the Earth promptly are a flux of tiny particles called neutrinos (which was picked up for the supernova SN1987a by the Japanese neutrino detector Kamio-kande) and gravitational waves (which have yet to be detected). For heavier stars, however, the core is thought to collapse into a black hole, and the resulting brief episode of mass accretion has been proposed as the central engine driving GRBs 5. This kind of collapse was initially referred to as a 'failed' supernova, as it was thought that the stellar envelope would not be ejected. A GRB …