Gamma-ray bursts are the most violent explosions in the universe, producing more energy in a few seconds than the Sun will generate in its whole life. The powerful blasts are a daily occurrence in remote galaxies but scientists are still unsure what causes them. Now two international teams of astronomers have found evidence that supernovae may play a role in some gamma-ray bursts (2000 Science 290 L Amati et al 953; L Piro et al 955).
A supernova explosion marks the death of a certain type of star. During the evolution of these stars, nuclear reactions take place at their cores, creating light elements like hydrogen and helium. Heavier elements are produced as the chain reaction proceeds, but iron is the heaviest element that it is energetically favourable for the star to make. Such stars therefore contain a high proportion of iron, which is ejected into space when the supernova explodes.
Filippo Frontera and co-workers used the BeppoSAX satellite to scrutinize the x-ray emission from a gamma-ray burst – known as GRB-990705 – that took place on 5 July 1999. The group found that the spectrum contained the signature of iron, and concluded that the x-rays travelled through a dense cloud of ionized iron. Frontera and colleagues believe that the cloud is the aftermath of a supernova that erupted just ten years earlier, and that the explosion triggered the gamma-ray burst.
Luigi Piro and co-workers used the Chandra X-ray Observatory to examine the x-ray spectrum of GRB-991216, a burst that took place on 16 December 1999. The team found the same tell-tale signs that the x-rays had encountered clouds of iron ions. “Our observations tell us that the material in the clouds is moving at ten percent of the speed of light and that the iron-rich cloud is extremely dense”, says Piro. “The large mass of ejected material tells us that the progenitor [supernova] was a very massive star”. Piro and colleagues believe that the gamma-ray burst occurred soon after the supernova ejected the cloud.
Gamma-ray bursts are so intense that only an extremely energetic event like a supernova could give rise to them. “We cannot rule out other scenarios yet”, says Frontera, “but this one is the simplest and the most consistent with our results”. An alternative to the supernova hypothesis is the theory that the collision of two extremely dense objects, for example black holes or neutron stars, could lead to a gamma-ray burst.
