RELEASE:  H99-139

GAMMA-RAY BURSTS LIGHT THE WAY TO THE EARLY UNIVERSE

NASA astronomers say they have uncovered a specific property of gamma-ray bursts that will enable them to gauge the distances to thousands of these powerful explosions, many perhaps beyond the reach of all existing telescopes.

This finding, experts say, may allow scientists to determine the geometry of the Universe throughout its various epochs, as well as when and where massive stars formed in the very early Universe.

A team led by Dr. Jay Norris, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., performed the new analysis using data from NASA's Compton Gamma Ray Observatory and several optical telescopes.

"If our finding holds up, this could be a new window on the distant Universe," said Norris. "Many gamma-ray bursts can be detected beyond the farthest supernovae and quasars we can now see."

Gamma-ray bursts occur randomly several times a day without warning, typically last only a few seconds to a minute, and apparently release more energy than any explosions in the Universe other than the Big Bang itself.

Norris found that, in a single burst, gamma rays of different energies reached the Earth-orbiting detectors at slightly different times, with the higher-energy gamma rays arriving before the lower-energy gamma rays. The amount of lag time between the two corresponded to the burst's estimated peak luminosity and distance. The lag was shorter for the more luminous bursts.

The new work was reported at the Fifth Huntsville Gamma-Ray Burst Symposium in Huntsville, Alabama, on October 19, and has been accepted for publication to The Astrophysical Journal. Related findings, derived independently by Dr. Edward Fenimore of Los Alamos National Laboratory and also reported to the Huntsville conference, lend confidence to the new result, astronomers say.

Gamma-ray bursts were discovered in the late 1960s, but only recently have most astronomers agreed that a large fraction of the bursts originate in the very distant, early Universe. The bursts fade quickly at gamma-ray energies and are hard to pinpoint, making it difficult to observe a burst's optical afterglow and determine a distance, or redshift.

Redshift is a common measurement of astronomical distances.  The more distant an object is from Earth, the faster it is receding due to the expansion of the Universe, and the greater its light is stretched or redshifted. This is similar to the way a siren on an ambulance appears to drop in pitch as the ambulance speeds away. Objects at high redshifts serve as probes to the early Universe, for their light has taken billions of years to reach Earth.

Yet of the thousands of gamma-ray bursts detected, fewer than ten have had an afterglow or host galaxy whose redshift could be determined with optical telescopes. This new finding by Goddard scientists has the potential of gauging the distances of many bursts from gamma-ray data alone.

Comparing the intrinsic burst luminosity (the actual brightness regardless of distance, as measured by redshifts and now, perhaps, by photon lag times) with the measured luminosity (how bright the burst appears to Earth-orbiting gamma-ray detectors) yields a distance to the source.

Images and more information on gamma-ray bursts can be found on the Internet at:

http://pao.gsfc.nasa.gov/gsfc/SpaceSci/gamma/gamma.htm