PASADENA, Calif.-For the past several decades, astrophysicists have
been puzzling over the origin of powerful but seemingly different
explosions that light up the cosmos several times a day. A new study
this week demonstrates that all three flavors of these cosmic
explosions–gamma-ray bursts, X-ray flashes, and certain supernovae
of type Ic–are in fact connected by their common explosive energy,
suggesting that a single type of phenomenon, the explosion of a
massive star, is the culprit. The main difference between them is
the “escape route” used by the energy as it flees from the dying star
and its newly born black hole.
In the November 13 issue of the journal Nature, Caltech graduate
student Edo Berger and an international group of colleagues report
that cosmic explosions have pretty much the same total energy, but
this energy is divided up differently between fast and slow jets in
each explosion. This insight was made possible by radio
observations, carried out at the National Radio Astronomy
Observatory’s Very Large Array (VLA), and Caltech’s Owens Valley
Radio Observatory, of a gamma-ray burst that was localized by NASA’s
High Energy Transient Explorer (HETE) satellite on March 29 of this
year.
- A common origin for cosmic explosions inferred from calorimetry of GRB030329, Nature
- Evolution of the polarization of the optical afterglow of the gamma-ray burst GRB030329, Nature
The burst, which at 2.6 billion light-years is the closest classical
gamma-ray burst ever detected, allowed Berger and the other team
members to obtain unprecedented detail about the jets shooting out
from the dying star. The burst was in the constellation Leo.
“By monitoring all the escape routes, we realized that the gamma rays
were just a small part of the story for this burst,” Berger says,
referring to the nested jet of the burst of March 29, which had a
thin core of weak gamma rays surrounded by a slow and massive
envelope that produced copious radio waves.
“This stumped me,” Berger adds, “because gamma-ray bursts are
supposed to produce mainly gamma rays, not radio waves!”
Gamma-ray bursts, first detected accidentally decades ago by military
satellites watching for nuclear tests on Earth and in space, occur
about once a day. Until now it was generally assumed that the
explosions are so titanic that the accelerated particles rushing out
in antipodal jets always give off prodigious amounts of gamma
radiation, sometimes for hundreds of seconds. On the other hand, the
more numerous supernovae of type Ic in our local part of the universe
seem to be weaker explosions that produce only slow particles. X-ray
flashes were thought to occupy the middle ground.
“The insight gained from the burst of March 29 prompted us to examine
previously studied cosmic explosions,” says Berger. “In all cases we
found that the total energy of the explosion is the same. This means
that cosmic explosions are beasts with different faces but the same
body.”
According to Shri Kulkarni, MacArthur Professor of Astronomy and
Planetary Science at Caltech and Berger’s thesis supervisor, these
findings are significant because they suggest that many more
explosions may go undetected. “By relying on gamma rays or X rays to
tell us when an explosion is taking place, we may be exposing only
the tip of the cosmic explosion iceberg.”
The mystery we need to confront at this point, Kulkarni adds, is why
the energy in some explosions chooses a different escape route than
in others.
At any rate, adds Dale Frail, an astronomer at the VLA and coauthor
of the Nature manuscript, astrophysicists will almost certainly make
progress in the near future. In a few months NASA will launch a
gamma-ray detecting satellite known as Swift, which is expected to
localize about 100 gamma-ray bursts each year. Even more
importantly, the new satellite will relay very accurate positions of
the bursts within one or two minutes of initial detection.
The article appearing in Nature is titled “A Common Origin for Cosmic
Explosions Inferred from Calorimetry of GRB 030329.” In addition to
Berger, the lead author, and Kulkarni and Frail, the other authors
are Guy Pooley, of Cambridge University’s Mullard Radio Astronomy
Observatory; Vince McIntyre and Robin Wark, both of the Australia
Telescope National Facility; Re’em Sari, associate professor of
astrophysics and planetary science at Caltech; Derek Fox, a
postdoctoral scholar in astronomy at Caltech; Alicia Soderberg, a
graduate student in astrophysics at Caltech; Sarah Yost, a
postdoctoral scholar in physics at Caltech; and Paul Price, a
postdoctoral scholar at the University of Hawaii’s Institute for
Astronomy.
