Three powerful recent blasts from three wholly different
regions in space have left scientists scrambling. The blasts,
which lasted only a few seconds, might be early alert systems
for star explosions called supernovae, which could start
appearing any day.
The first two blasts, called X-ray flashes, occurred on
September 12 and 16. These were followed by a more powerful
burst on September 24. The burst seems to be on the cusp
between an X-ray flash and a full-fledged gamma ray burst, a
discovery interesting in its own right. If these signals lead
to supernovae, as expected, scientists would have a tool to
predict star explosions, and researchers could watch
explosions from start to finish.
A team led by Dr. George Ricker of the Massachusetts
Institute of Technology (MIT), Cambridge, detected the
explosions with NASA’s High-Energy Transient Explorer (HETE-
2). Science teams around the world, using space- and ground-
based observatories, have joined in, torn and conflicted over
which burst region to track most closely.
“Each burst has been beautiful,” Ricker said. “Depending on
how these evolve, they could support important theories about
supernovae and gamma-ray bursts. These past two weeks have
been like ‘cock, fire, reload.’ Nature keeps on delivering,
and our HETE-2 satellite keeps on responding flawlessly,” he
said.”
Gamma ray bursts are the most powerful explosions known other
than the Big Bang. Many appear to be caused by the death of a
massive star collapsing into a black hole. Others might be
from merging black holes or neutron stars. In either case,
the event likely produces twin, narrow jets in opposite
directions, which carry off tremendous amounts of energy. If
one of jets points to Earth, we see this energy as a gamma
ray burst.
The lower-energy X-ray flashes might be gamma ray bursts
viewed slightly off angle from the jet direction, somewhat
similar to how a flashlight is less blinding when viewed at
an angle. The majority of light particles from X-ray flashes,
called photons, are X-rays, energetic, but not quite as
powerful as gamma rays. Both types of bursts last only a few
milliseconds to about a minute. HETE-2 detects the bursts,
studies their properties, and provides a location, so other
observatories can study the burst afterglow in detail.
The trio of bursts from the past few weeks has the potential
of settling two long-standing debates. Some scientists say X-
ray flashes are different beasts all together, not related to
gamma-ray bursts and massive star explosions. Detecting a
supernova in the region where the X-ray flash appeared would
refute that belief, instead confirming the connection between
the two. Follow-up observations of the September 24 burst,
named GRB040924 for the date it was observed, are already
solidifying the theory of a cosmic explosion continuum from
X-ray flashes up through gamma ray bursts.
More interesting for supernova hunters is the fact X-ray
flashes are closer to Earth than gamma ray bursts. While the
connection between gamma ray bursts and supernovae has been
made, these supernovae are too distant to study in detail. X-
ray flashes might be signals for supernovae; scientists can
actually sink their teeth into and observe in detail.
“Last year HETE-2 sealed the connection between gamma ray
bursts and massive supernovae,” said Prof. Stanford Woosley
of the University of California at Santa Cruz, who has
championed several theories concerning the physics of star
explosions. “These two September bursts may be the first time
we see an X-ray flash lead to a supernova.”
“We all expect much more of this type of exciting science to
come after the launch of Swift,” said Dr. Anne Kinney,
director of NASA’s Universe Division. The Swift spacecraft,
scheduled to launch no earlier than late October, contains
three telescopes (gamma ray, X-ray and UV/optical) for quick
burst detection and immediate follow-up observations of the
afterglow.
HETE was built by MIT as a mission of opportunity under the
NASA Explorer Program. It was built in collaboration among
U.S. universities, Los Alamos National Laboratory, N.M.,
scientists and organizations in Brazil, France, India, Italy
and Japan.
For information about HETE results and related animations on
the Internet, visit:
