Nov. 3, 2000 Exploding stars called supernovae may give rise to at least some gamma-ray bursts, the most intense blasts of energy in the universe. Two international research teams report these findings in the 3 November issue of Science.
Their discoveries may help solve a decades-old mystery over the cause of these colossal explosions, which occur daily in distant galaxies and require huge amounts of energy.
"We have wondered for a long time about the environments in which gamma-ray bursts occur, and what their progenitors are. Given that they are such energetic events, they are very strange and difficult to explain," said Science author, Filippo Frontera of the Universita' di Ferrara in Ferrara, Italy, and Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri (TESRE), CNR, in Bologna, Italy.
"We have recently realized that huge gamma-ray burst explosions do not completely erase traces of their progenitors and can actually shed light on the events that preceded them," said author Luigi Piro of the Istituto de Astrofisica Spaziale, CNR, in Rome, Italy.
Piro and Frontera are the lead authors of the two Science papers. They each participated in both research groups, which studied two recent gamma ray bursts.
The two teams found evidence that, as the bursts expanded like an inflating bubble, they moved through a nearby gas cloud enriched in iron. Iron is produced in supernova explosions, by which certain stars end their lives.
A supernova "ancestor" has been one of two favored models for the origins of gamma-ray bursts. Although what specifically initiates the burst is not yet clear, one possible scenario involves the collapse of a dense object, such as a neutron star or black hole, spawned by the supernova.
According to the other leading model, the burst is fed by energy from a merger between two dense objects that have been spiraling inwards toward each other.
The researchers studied the x-ray spectra (essentially a rainbow, at x-ray wavelengths) emitted by gamma-ray bursts. Piro's team, used the American Chandra X-Ray Observatory, and Frontera's used the BeppoSAX satellite, an Italian project with Dutch participation.
Both groups found "fingerprints" in the spectra indicating that the bursts had encountered an iron-rich gas cloud.
The cloud was probably produced during a supernova explosion, said both Frontera and Piro. 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.
Piro and his team of researchers from Italy, the United States, Japan, the Netherlands, and Russia, studied the x-ray spectra emitted by a burst on 16 December, 1999. Chandra picked up the burst 37 hours after it started, and observed the afterglow of x-rays for 3.4 hours.
The spectra showed peaks in energy at two key wavelengths. According to the authors, the first peak is due to emission from iron that has been ionized, which would happen when the burst tore through the gas cloud, stripping electrons from their atoms. The second peak shows emission from recombined iron, which has hooked up with electrons in the aftermath of the event.
"Our observations tell us that the material is moving with a velocity of 30,000 kilometers per second, which is ten percent of the speed of light, and that the iron-rich cloud is extremely dense. The large mass of ejecta tells us that the progenitor was a very massive star," Piro said.
The simplest explanation for these findings, Piro and his colleagues proposed, is that a supernova ejected the cloud shortly before the gamma ray burst.
The x-ray spectra measured by BeppoSAX, during the early moments of a burst on 5 July, 1999, also showed evidence for a swarm of iron atoms near the burst, according to Frontera and his colleagues.
Frontera's team of Italian and Dutch researchers modeled a dip found in the x-ray spectrum during the first ten seconds of the event. According to their calculations, the dip reflects ionized iron in a supernova remnant near 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 invoked 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" said Frontera.
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