Gamma-ray Bursts Pinpointed to Star-Forming Regions
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 4, 2001
Baltimore, Md. -- While much still remains a mystery about gamma-ray bursts, the most energetic events known in the Universe outside of the Big Bang, astronomers have uncovered the strongest evidence to date that some bursts signal the explosive death of very massive stars and emanate from the same regions where stars are born.
Dr. Luigi Piro of the Consiglio Nazionale delle Ricerche (CNR) in Rome, Italy, leading an international team of astronomers, has used a combination of data from BeppoSAX, an Italian X-ray observatory with Dutch participation, and NASA's Chandra X-ray Observatory to pinpoint several bursts to distant stellar nurseries. He presents his findings today at the Gamma Ray 2001 conference in Baltimore, Maryland.
"We know that when a gamma-ray burst explodes, it produces a blast of material, called a fireball, which expands at relativistic speed like a rapidly inflating bubble," said Piro, who works within CNR's Istituto di Astrofisica Spaziale. "We have found evidence in two recent bursts that the blast wave caused by the fireball brakes against a wall of very dense gas totally surrounding the burst. This level of density can only be found in very crowded regions where stars are formed."
Several theories exist about what causes gamma-ray bursts, and not all bursts need to originate from the same phenomenon. Among more popular theories are that gamma-ray bursts come from various combinations of merging neutron stars and black holes, or from the explosion of massive stars, called hypernovae.
Piro's observations not only support the hypernova model but also show that these largest of stars may meet their demise along side the youngest of stars. Such a massive star evolves rapidly, on the order of one million years. Thus the explosion can occur in the same environment that produced the massive star, a stellar nursery. The massive explosion may even trigger star formation.
Piro has gathered convincing evidence of hypernovae exploding in dense stellar nurseries from observations of several gamma-ray burst afterglows.
"Because gamma-ray bursts are going off in extremely distant galaxies, it is difficult to 'see' the regions that harbor them," said Piro. "We can only gather circumstantial evidence as to where and how they form."
First, in the afterglow of a burst on Sept. 26, 2000, Prof. Gordon Garmire of Pennsylvania State University found X-ray emission in excess to that expected by the standard scenario of a fireball in a low-density medium. Also, in the afterglow of a burst on Feb. 22, 2001 -- one of the brightest bursts ever observed by BeppoSAX -- there was evidence of a fireball expanding in a very dense gas, as reported by Jean in 't Zand of the Space Research Organization of the Netherlands, one of the scientists working in the BeppoSAX gamma-ray burst team.
In four other bursts observed with BeppoSAX and Chandra (GRB970508, GRB990705, GRB991216, GRB000214), Piro and his team found "fingerprints" indicating that the burst had encountered an even denser gas, enriched of iron. The spectra of these bursts show, in fact, the presence of lines produced from iron that has been ionized, which would happen when the burst tears through the gas cloud, stripping electrons from their atoms.
These observations suggest that this gas was ejected by a very massive star before the gamma-ray burst explosion. Piro reported on the detection of this phenomenon, from the burst GRB991216, at a scientific meeting on November 3, 2000.
A key element in the success of these observations has been the perfect timing and liaison between the two satellites, Chandra and BeppoSAX, Piro said. Piro is the Mission Scientist for BeppoSAX, the instrument that first detected X-ray afterglows from gamma-ray bursts.
Currently, astronomers usually are not notified about gamma-ray bursts until an hour or so after they occur. These bursts last only for a few milliseconds to about a minute, although their afterglow can linger in X-ray and optical light for days or weeks. The HETE-2 satellite, launched in October 2000, and Swift, scheduled for a 2003 launch, will provide nearly instant notification of bursts in action, providing satellites such as Chandra and BeppoSAX a better opportunity to study the afterglow phenomenon in depth.
"It is important to be fast, since these sources change rapidly and we want to track their intensity and spectral properties as they expand into the surrounding medium and eventually fade," said Dr. Harvey Tananbaum, Director of the Chandra X-Ray Observatory. "That is not easy, because one has to reprogram in few hours the observation, but we have been able to observe two of these GRB with both satellites very quickly."
Because gamma-ray bursts are the most distant and powerful sources of the Universe, scientists can hope to use them to probe the formation of stars and galaxies at they first moment they began to form, Piro said.
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