Scientists have pieced together the key elements of a gamma-ray
burst, from star death to dramatic black hole birth, thanks to a
“Rosetta stone” found on March 29, 2003.

This telling March 29 burst in the constellation Leo, one of the
brightest and closest on record, reveals for the first time that a
gamma-ray burst and a supernova — the two most energetic explosions
known in the Universe — occur simultaneously, a quick and powerful
one-two punch.

The results appear in the June 19 issue of Nature. The burst was
detected by NASA’s High-Energy Transient Explorer (HETE) and observed
in detail with the European Southern Observatory’s Very Large
Telescope (VLT) at the Paranal Observatory in Chile.

“We’ve been waiting for this one for a long, long time,” said Dr.
Jens Hjorth, University of Copenhagen, lead author on one of three
Nature letters. “The March 29 burst contains all the missing
information. It was created through the core collapse of a massive
star.”

The team said that the “Rosetta stone” burst also provides a lower
limit on how energetic gamma-ray bursts truly are and rules out most
theories concerning the origin of “long bursts,” lasting longer than
two seconds.

Gamma-ray bursts temporarily outshine the entire Universe in
gamma-ray light, packing the energy of over a million billion suns.
Yet these explosions are fleeting — lasting only seconds to minutes
— and occur randomly from all directions on the sky, making them
difficult to study.

A supernova is associated with the death of a star about eight times
as massive as the Sun or more. Its core implodes, forming either a
neutron star or (if there is enough mass) a black hole. The star’s
surface layers blast outward, forming the colorful patterns typical
of supernova remnants. Scientists have suspected gamma-ray bursts
and supernovae were related, but they have had little observational
evidence, until March 29.

“The March 29 burst changes everything,” said co-author Dr. Stan
Woosley, University of California, Santa Cruz. “With this missing
link established, we know for certain that at least some gamma-ray
bursts are produced when black holes, or perhaps very unusual neutron
stars, are born inside massive stars. We can apply this knowledge to
other burst observations.”

GRB 030329, named after its detection date, occurred relatively
close, approximately 2 billion light years away (at redshift 0.1685).
The burst lasted over 30 seconds. (“Short bursts” are less than 2
seconds long.) GRB 030329 is among the 0.2 percent brightest bursts
ever recorded. Its afterglow lingered for weeks in lower-energy
X-ray and visible light.

With the VLT, Hjorth and his colleagues uncovered evidence in the
afterglow of a massive, rapidly expanding supernova shell, called a
hypernova, at the same position and created at the same time as the
afterglow. The following scenario emerged:

A bluish Wolf-Rayet star — containing about 10 solar masses worth of
helium, oxygen and heavier elements — rapidly depleted its fuel,
triggering the Type Ic supernova / gamma-ray burst event. The core
collapsed, without the star’s outer part knowing. A black hole
formed inside surrounded by a disk of accreting matter, and, within a
few seconds, launched a jet of matter away from the black hole that
ultimately made the gamma-ray burst.

The jet passed through the outer shell of the star and, in
conjunction with vigorous winds of newly forged radioactive nickel-56
blowing off the disk inside, shattered the star. Meanwhile,
collisions among pieces of the jet moving at different velocities,
all very close to light speed, created the gamma-ray burst. This
“collapsar” model, introduced by Woosley in 1993, best explains the
observation of GRB 030329, as opposed to the “supranova” and “merging
neutron star” models.

“This does not mean that the gamma-ray burst mystery is solved,”
Woosley said. “We are confident that long bursts involve a core
collapse, probably creating a black hole. We have convinced most
skeptics. We cannot reach any conclusion yet, however, on what
causes short gamma-ray bursts.”

Short bursts might be caused by neutron star mergers. A NASA-led
international satellite named Swift, to be launched in January 2004,
will “swiftly” locate gamma-ray bursts and may capture short-burst
afterglows, which have yet to be detected.

The VLT is the world’s most advanced optical telescope, comprising
four 8.2-meter reflecting Unit Telescopes and, in the future, four
moving 1.8-meter Auxiliary Telescopes for interferometry. HETE was
built by MIT as a mission of opportunity under the NASA Explorer
Program, with collaboration among U.S. universities, Los Alamos
National Laboratory, and scientists and organizations in Brazil,
France, India, Italy and Japan. For new gamma-ray burst animation,
refer to:

http://www.gsfc.nasa.gov/topstory/2003/0618rosettaburst.html