Gamma-Ray Bursts Light the Way to the Early Universe

Donald Savage

Headquarters, Washington, DC

(Phone: 202/358-1547)

Bill Steigerwald

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-5017)

RELEASE: 99-139

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

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