For Release: June 13, 1995

Steve Roy
Office of Media Services
(205) 544-0034
steve.roy@msfc.nasa.gov

RELEASE: 95-37

MYSTERIOUS GAMMA-RAY BURSTS RANDOM, UNPREDICTABLE

Despite four years of data from an experiment on NASA's Compton Gamma Ray Observatory, the behavior of mysterious gamma-ray bursts in the universe remains unpredictable. Not only do the bursts originate from all directions throughout the universe, they also vary greatly in duration and energy output, according to NASA astrophysicist Dr. Gerald Fishman.

"Gamma-ray bursts were discovered some 27 years ago, but were not investigated thoroughly until the launch four years ago of the Burst and Transient Source Experiment, an instrument on the Compton Observatory," said Fishman, NASA's principal investigator for the experiment. "Inspite of intensive study, the origin of the gamma-ray energy bursts remains unknown."

Fishman and colleague Dr. Geoffrey Pendleton of the University of Alabama in Huntsville summarized findings from the experiment at a meeting of the American Astronomical Society in Pittsburgh, Pa., today.

"Gamma-ray bursts observed by the Burst and Transient Source Experiment continue to be perhaps the least understood of all observed astronomical phenomena and one of the major mysteries in astronomy this century," explained Fishman of Marshall Space Flight Center in Huntsville, Ala.

"The bursts are seen to occur at random in the sky daily from all directions. Even the distance to the gamma-ray sources is being debated by astronomers."

'We are observing gamma-ray bursts at the rate of about one each day," said Fishman, "and during the four-year-mission of the instrument, Marshall Center's Burst and Transient Source Experiment team has observed some 1,200 gamma-ray bursts."

"The experiment has observed single gamma-ray bursts lasting a fraction of a second to bursts which last up to 10 minutes," said Pendleton. "Within single bursts the average energy of gamma-rays varies remarkably. They often produce much higher energy radiation than other known sources of gamma-ray energy such as pulsars, and then evolve to much lower energy states that are more like other known gamma-ray sources such as the Cygnus X-1 binary star system. Gamma-ray emissions produce some of the highest energy radiation phenomena recorded by scientists and they are characterized by dramatic changes in intensity and duration," said Pendleton.

Scientists at Marshall Center also are observing rare gamma-ray flashes above Earth-bound atmospheric thunderstorms at a rate of one per week. It is suspected the gamma-ray flashes come from a rare type of powerful electrical discharge, similar to lightning, above large thunderstorm regions.

The Burst and Transient Source Experiment, launched in 1991 on the Compton Gamma Ray Observatory is providing scientific data to hundreds of astronomers worldwide. The experiment was developed and managed by Marshall Center, while the Compton Gamma Ray Observatory is managed by NASA's Goddard Space Flight Center in Greenbelt, Md.

A time-compressed video animation of the Burst and Transient Source Experiment showing color-coded spectral properties of the gamma-ray bursts over the course of several months may be obtained by calling Steve Roy at (205) 544-6535. The video animation, B-roll and sound bites with Fishman and Pendleton also will be broadcast on the NASA Television program Video File on June 13 at noon., 3 p.m., 6 p.m., and 9 p.m. EDT.