A team of Italian and Dutch astronomers have discovered in the prompt emission of a gamma-ray burst detected on July 5, 1999 a 'fingerprint' which might be the key to unraveling the origin of these mysterious events. The discovery is reported in the 3 November issue of Science (paper by Lorenzo Amati, Filippo Frontera, Mario Vietri, Luigi Piro and other collaborators) and was obtained with the BeppoSAX satellite, a large astronomy project of the Italian Space Agency (ASI) with Dutch participation.

Gamma-ray bursts (GRBs) are intense flashes of energy that occur a few times per day throughout the Universe. "Thanks to the prompt and accurate localization provided by BeppoSAX, the discovery of X-ray and optical afterglows from these events has been made possible and the mystery of the GRB distance scale has been cleared up", says Luigi Piro, BeppoSAX mission scientist. We know now that GRBs occur at cosmological distances, billions of light-years from the Earth, and involve huge amounts of energy, such as to outshine the whole Universe.

While much progress has been achieved in the last three years, the origin of these events remains a mystery. Many theoretical models have been put forth. According to one favorite model, the GRBs originate in the supernova explosion of a very massive star, that leads to the formation of a dense object, such as a neutron star or a black hole. According to another leading model, the bursts are fed from the merger between two dense objects that have been spiraling inwards towards each other. In the case of supernova explosions, the GRBs are expected to occur in regions of star formation with dense interstellar environments.

"We have wondered for a long time about the environments in which gamma-ray bursts occur, and what their progenitors are" says the team leader Filippo Frontera, of the Physics Department of the University of Ferrara in Ferrara, Italy, and Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri (TESRE), CNR, in Bologna, Italy.

The researchers studied the X- and gamma-ray spectrum (essentially a rainbow, at X-ray wavelengths) emitted by the gamma-ray burst of July 5, 1999 during the earliest phase of its outburst. The X-ray spectra measured during the early moments of this burst showed evidence of a dip that is typically found when the X-rays from a celestial source cross a cloud of material along their path to us.

"We tested different types of absorbing material to model the dip, but only iron was found to give the most reasonable results" says Lorenzo Amati of TESRE. Assuming that the material was iron, the distance to the GRB could also be derived, which gave a result consistent with a cosmological distance, says Amati.

The amount of iron inferred from our results is that typical of a cloud produced during a supernova explosion, says Frontera. As stars evolve and become hotter, nuclear reactions in their core produce increasingly heavy elements. By the time a star gets to the supernova stage, it's producing the heaviest element possible, iron. The simplest explanation for these findings is that a supernova ejected the cloud before the gamma ray burst. When the researchers calculated the density of their cloud, however, they came up with a relatively low figure. They concluded, therefore, that the supernova explosion happened about 10 years before the burst.

To explain their findings, the authors invoke what they call the "supranova" model, in which the gamma-ray burst arises from the delayed collapse of a neutron star formed by the supernova, rather than directly from the supernova explosion.

"We cannot rule out other scenarios yet, but this one is the simplest, and the most consistent with our results" says Frontera.