Gamma-ray bursts are caused by explosive death of massive stars, new study reveals

PASADENA, Calif.-In two papers appearing in an upcoming issue of the
Astrophysical Journal, an international team of astrophysicists led
by Shri Kulkarni of the California Institute of Technology reveals
that new data show that supernovae are the source of gamma-ray bursts.

The new information was obtained from a gamma-ray burst that was
detected in November and studied by the Hubble Space Telescope, the
Australia Telescope Compact Array, the Anglo-Australian Telescope,
and optical telescopes in Chile.

For the last few years astronomers have been chasing clues linking
the mysterious gamma-ray bursts to their favored suspect: massive
stars. Previous observations hinted at debris from an exploding
star, but the observations were inconclusive.

Careful observations of gamma-ray burst GRB 011121 have uncovered
remnants of the exploded star, whose signature was buried in the
bright, fading embers. Now, for the first time, two compelling
tell-tale signatures of the massive star were observed.

As explained by Kulkarni, who is the McArthur Professor of Astronomy
and Planetary Sciences at Caltech and the head of the international
team that made this discovery, “With these observations we have tied
this gamma-ray burst to an exploding star. I am absolutely delighted
that nature provided us with such a clean answer.”

At the core of the observations, the data show that a supernova
accompanied the burst. Supernovae are a natural consequence of
exploding stars and difficult to make by other means. Joshua Bloom,
Caltech graduate student and lead author on the supernova paper,
said. “It is not often that a graduate student gets the chance to
make a major discovery. I am very fortunate to be involved in this
one.”

The astronomers were also able to deduce that the explosion took
place in a cocoon of gas fed by a “wind” of matter emanating from the
progenitor star. Paul Price, graduate student at the Australian
National University and a lead author on the second paper, was
“intensely excited. Once it became clear that we had not only seen
the supernova but also the cocoon I was very happy; I couldn’t sleep
for days.”

The gamma-ray burst in question was detected on November 11, 2001, by
the Italian-Dutch Satellite BeppoSAX in the southern-sky
constellation of Chamaeleon. The position was quickly refined by a
network of satellites. Astronomers from Poland and Chile, as well as
another U.S. team from Harvard, used optical telescopes in Chile to
rapidly identify the “afterglow,” or glowing embers, of the
gamma-ray burst and determined that the galaxy in which the burst was
located was quite near – a paltry five billion light-years from
Earth.

The sensitive optical and infrared observations were in part possible
because of the relatively small distance to the burst. Given the
proximity, the Caltech team decided to dedicate a large portion of
their allocated Hubble Space Telescope time toward observing any
possible supernova component. Kulkarni says of the decision, “We
simply went for broke because of the potential payoff.”

Kulkarni believes that this is just the beginning of a new era in our
understanding of the death of massive stars. The stars die by
collapsing, and the collapse both fuels the explosion and leaves a
stellar residue of neutron stars and black holes. Indeed, theorists
have long speculated that gamma-ray bursts are the birth cry of
spinning black holes. New facilities such as the Chandra X-Ray
Observatory, and future facilities such as gravitational-wave
observatories and neutrino telescopes, will allow astronomers to
investigate the dramatic collapse process.

Kulkarni cautions, however, that all is still not known about
gamma-ray bursts. It may be that other exotic phenomena, such as two
colliding neutron stars, or a neutron star colliding with a black
hole, produce some of the events that we see. “Despite extensive
efforts, until now we have not seen clear signatures for a cocoon in
dozens of other gamma-ray bursts, and there have been only hints of a
supernova in a few other bursts,” Bloom says.

Price adds, “It means there will be lots more to do in the future. I
have a secure thesis now!”

In addition to Kulkarni, Bloom, and Price, members of the team
reporting the results are Caltech professors S. George Djorgovski and
Fiona Harrison and postdoctoral fellows and scholars Daniel Reichart,
Derek Fox, Titus Galama, and Re’em Sari. Edo Berger and Sara Yost
are Caltech graduate students also on the team, as are Dale Frail
from the National Radio Astronomy Observatory, and many other
international collaborators. Separately, P. M. Garnavich of the
University of Notre Dame and his collaborators have reached similar
coneturions with data taken from the Magellan telescope in Chile.

Contact: Robert Tindol

tindol@caltech.edu

(626) 395-3631

Image and other information available at
http://www.astro.caltech.edu/~derekfox/grb011121pr