Gamma-ray bursts may originate in star-forming regions

New findings from two X-ray satellites suggest that gamma-ray bursts, some of
the most intense blasts in the Universe, may be created in the same area
where stars are born.

Dr. Luigi Piro of the Consiglio Nazionale delle Ricerche (CNR) in
Rome, Italy, presented data from NASA’s Chandra X-ray
Observatory and the Italian-Dutch ASI BeppoSAX observatory
today at the Gamma Ray 2001 conference in Baltimore, Md.

“We know that when a gamma-ray burst explodes, it produces a
blast of material called a fireball, which expands at relativistic
speeds like a rapidly inflating bubble,” said Piro, who works within
CNR’s Istituto di Astrofisica Spaziale. “Our team found evidence
that the blast wave caused by the fireball brakes against a wall of
very dense gas, which we believe is the crowded region where
stars form.”

Several theories exist about what causes gamma-ray bursts.
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.

“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.”

Piro’s observations support the hypernova model. Scientists
believe that within dense star-forming regions, the massive star
required for a hypernova explosion evolves extremely rapidly. On
astronomical time scales, the supermassive star would evolve
over the course of only about one million years. Thus, the
hypernova explosion may occur in the same stellar environment
that originally produced the massive star itself, and perhaps may
trigger even more star formation.

The hint that gamma-ray bursts can occur in dense media came
during a Chandra observation of an afterglow that occurred on
September 26, 2000. Prof. Gordon Garmire of Pennsylvania
State University, University Park, Pa., found X-ray emission to be
greater than that expected by the standard scenario of a fireball in
a low-density medium – an important clue that the explosion
occurred in a dense region. Next, on February 22, 2001, Piro
said that Chandra observations of the burst’s afterglow, one of
the brightest bursts ever observed by BeppoSAX, provided
evidence of a fireball expanding in a very dense gas.

These recent results supported data from four other gamma-ray
bursts observed by BeppoSAX and Chandra (GRB970508,
GRB990705, GRB991216, and GRB000214). In these bursts,
Piro and his team found evidence indicating that the burst had
encountered an extremely dense gas. The properties of this gas
suggest that it originated from a very massive progenitor before it
exploded as a gamma-ray burst.

A key element in the success of these observations has been the
perfect timing and liaison between the two satellites, Chandra
and BeppoSAX, according to Piro. Piro is the Mission Scientist
for BeppoSAX, the instrument that first detected X-ray afterglows
from gamma-ray bursts.

Currently, astronomers are not usually 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 a better opportunity to study the afterglow phenomenon
in depth.

The ACIS X-ray camera was developed for NASA by Penn State
and the Massachusetts Institute of Technology. The High Energy
Transmission Grating Spectrometer was built by MIT. NASA’s
Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program. TRW, Inc., Redondo Beach, Calif., is the
prime contractor for the spacecraft. The Smithsonian’s Chandra
X-ray Center controls science and flight operations from
Cambridge, Mass. Images associated with this release are
available on the World Wide Web at: