Bill Steigerwald June 18, 2003<br />NASA Goddard Space Flight Center<br />(301) 286-5017<br /><br /><br />Release: 03-67<br /><br /><br />ROSETTA STONE DECODES GAMMA-RAY BURST MYSTERY<br /><br /><br /> Scientists have pieced together the key elements of a <br />gamma-ray burst, from star death to dramatic black hole <br />birth, thanks to a "Rosetta stone" found on March 29, 2003.<br /><br /><br />This telling March 29 burst in the constellation Leo, one of <br />the brightest and closest on record, reveals for the first <br />time that a gamma-ray burst and a supernova -- the two most <br />energetic explosions known in the Universe -- occur <br />simultaneously, a quick and powerful one-two punch.<br /><br /><br />The results appear in the June 19 issue of Nature. The burst <br />was detected by NASA's High-Energy Transient Explorer (HETE) <br />and observed in detail with the European Southern <br />Observatory's Very Large Telescope (VLT) at the Paranal <br />Observatory in Chile.<br /><br /><br />"We've been waiting for this one for a long, long time," said <br />Dr. Jens Hjorth, University of Copenhagen, lead author on one <br />of three Nature letters. "The March 29 burst contains all the <br />missing information. It was created through the core collapse <br />of a massive star."<br /><br /><br />The team said that the "Rosetta stone" burst also provides a <br />lower limit on how energetic gamma-ray bursts truly are and <br />rules out most theories concerning the origin of "long <br />bursts," lasting longer than two seconds.<br /><br /><br />Gamma-ray bursts temporarily outshine the entire Universe in <br />gamma-ray light, packing the energy of over a million billion <br />suns. Yet these explosions are fleeting -- lasting only <br />seconds to minutes -- and occur randomly from all directions <br />on the sky, making them difficult to study.<br /><br /><br />A supernova is associated with the death of a star about <br />eight times as massive as the Sun or more. Its core <br />implodes, forming either a neutron star or (if there is <br />enough mass) a black hole. The star's surface layers blast <br />outward, forming the colorful patterns typical of supernova <br />remnants. Scientists have suspected gamma-ray bursts and <br />supernovae were related, but they have had little <br />observational evidence, until March 29.<br /><br /><br />"The March 29 burst changes everything," said co-author Dr. <br />Stan Woosley, University of California, Santa Cruz. "With <br />this missing link established, we know for certain that at <br />least some gamma-ray bursts are produced when black holes, or <br />perhaps very unusual neutron stars, are born inside massive <br />stars. We can apply this knowledge to other burst <br />observations."<br /><br /><br />GRB 030329, named after its detection date, occurred <br />relatively close, approximately 2 billion light years away <br />(at redshift 0.1685). The burst lasted over 30 seconds. <br />("Short bursts" are less than 2 seconds long.) GRB 030329 is <br />among the 0.2% brightest bursts ever recorded. Its afterglow <br />lingered for weeks in lower-energy X-ray and visible light.<br /><br /><br />With the VLT, Hjorth and his colleagues uncovered evidence in <br />the afterglow of a massive, rapidly expanding supernova <br />shell, called a hypernova, at the same position and created <br />at the same time as the afterglow. The following scenario <br />emerged:<br /><br /><br />A bluish Wolf-Rayet star -- containing about 10 solar masses <br />worth of helium, oxygen and heavier elements -- rapidly <br />depleted its fuel, triggering the Type Ic supernova / gamma-<br />ray burst event. The core collapsed, without the star's <br />outer part knowing. A black hole formed inside surrounded by <br />a disk of accreting matter, and, within a few seconds, <br />launched a jet of matter away from the black hole that <br />ultimately made the gamma-ray burst.<br /><br /><br />The jet passed through the outer shell of the star and, in <br />conjunction with vigorous winds of newly forged radioactive <br />nickel-56 blowing off the disk inside, shattered the star. <br />Meanwhile, collisions among pieces of the jet moving at <br />different velocities, all very close to light speed, created <br />the gamma-ray burst. This "collapsar" model, introduced by <br />Woosley in 1993, best explains the observation of GRB 030329, <br />as opposed to the "supranova" and "merging neutron star" <br />models.<br /><br /><br />"This does not mean that the gamma-ray burst mystery is <br />solved," Woosley said. "We are confident that long bursts <br />involve a core collapse, probably creating a black hole. We <br />have convinced most skeptics. We cannot reach any conclusion <br />yet, however, on what causes short gamma-ray bursts."<br /><br /><br />Short bursts might be caused by neutron star mergers. A <br />NASA-led international satellite named Swift, to be launched <br />in January 2004, will "swiftly" locate gamma-ray bursts and <br />may capture short-burst afterglows, which have yet to be <br />detected.<br /><br /><br />The VLT is the world's most advanced optical telescope, <br />comprising four 8.2-meter reflecting Unit Telescopes and, in <br />the future, four moving 1.8-meter Auxiliary Telescopes for <br />interferometry. HETE was built by MIT as a mission of <br />opportunity under the NASA Explorer Program, with <br />collaboration among U.S. universities, Los Alamos National <br />Laboratory, and scientists and organizations in Brazil, <br />France, India, Italy and Japan. For new gamma-ray burst <br />animation, refer to:<br /><br /><br />http://www.gsfc.nasa.gov/topstory/2003/0618rosettaburst.html