In A Flash NASA Helps Solve 35-Year-Old Cosmic Mystery

George Deutsch/Erica Hupp
Headquarters, Washington Oct. 5, 2005
(Phone: 202/358-1324/1237)

Susan Hendrix
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-7745)

RELEASE: 05-334

IN A FLASH NASA HELPS SOLVE 35-YEAR-OLD COSMIC MYSTERY

Scientists have solved the 35-year-old mystery of the origin
of powerful, split-second flashes of light known as short gamma-ray
bursts. These flashes, brighter than a billion suns, yet lasting
only a few milliseconds, have been simply too fast to catch
-- until now.

Through the unprecedented coordination of observations from
several ground-based telescopes and NASA satellites, scientists
determined the flashes arise from violent collisions in space.
The clashes are either between a black hole and a neutron star
or between two neutron stars. In either scenario, the impact
creates a new black hole.

In at least one burst, scientists saw tantalizing, first-time
evidence of a black hole eating a neutron star. The neutron
star was first stretched into a crescent, then swallowed by
the black hole.

Two recently detected bursts are featured in four papers in
this week's Nature magazine. These observations could enable
direct detection of exotic gravitational waves that have never
before been seen.

"Gamma-ray bursts in general are notoriously difficult to study,
but the shortest ones have been next to impossible to pin down,"
said Dr. Neil Gehrels, principal investigator for the Swift
satellite at NASA's Goddard Space Flight Center, Greenbelt,
Md. "All that has changed. We now have the tools in place to
study these events," he said.

Gamma-ray bursts, first detected in the 1960s, are the most
powerful explosions known. They are random, fleeting and can
occur from any region of the sky. Two years ago, scientists
discovered longer bursts, lasting more than two seconds, arise
from the explosion of very massive stars. About 30 percent of
bursts are short and under two seconds.

The Swift satellite detected a short burst on May 9, and NASA's
High-Energy Transient Explorer (HETE) detected another on July
9. The May 9 event marked the first time scientists identified
an afterglow for a short gamma-ray burst, something commonly
seen after long bursts.

"We had a hunch that short gamma-ray bursts came from a neutron
star crashing into a black hole or another neutron star, but
these new detections leave no doubt," said Dr. Derek Fox, assistant
professor of Astronomy & Astrophysics at Penn State University,
State College, Pa. Fox is lead author of one Nature report detailing
a multi-wavelength observation.

Fox's team discovered the X-ray afterglow of the July 9 burst
with NASA's Chandra X-ray Observatory. A team led by Jens Hjorth,
a professor at the University of Copenhagen identified the optical
afterglow using the Danish 1.5-meter telescope at the La Silla
Observatory in Chile.

Fox's team continued studying the afterglow with NASA's Hubble
Space Telescope and ground-based telescopes of the Carnegie
Institution, the National Astronomical Observatory of Japan,
and the National Radio Astronomy Observatory.

"The July 9 burst was like the dog that didn't bark," said Dr.
George Ricker, HETE principal investigator at the Massachusetts
Institute of Technology, Cambridge, and co-author of another
Nature article. "Powerful telescopes detected no supernova as
the gamma-ray burst faded, arguing against the explosion of
a massive star. Also, the July 9 burst, and probably the May
9 burst, are located in the outskirts of their host galaxies,
just where old merging binaries are expected," he added.

Mergers create gravitational waves, ripples in space-time predicted
by Einstein but never directly detected. The July 9 burst was
about 2 billion light-years away. A big merger closer to the
Earth could be detected by the National Science Foundation's
Laser Interferometer Gravitational-Wave Observatory (LIGO).
If Swift detects a nearby short burst, scientists could go back
and check the data with a precise time and location.

"This is good news for LIGO," said Dr. Albert Lazzarini, Data
& Computing group leader at the California Institute of Technology
LIGO Lab, Pasadena. "The connection between short bursts and
mergers firms up projected rates for LIGO, and they appear to
be at the high end of previous estimates. Also, observations
provide tantalizing hints of black hole-neutron star mergers,
which have not been detected before," he said.

For information about this discovery, visit:

http://www.nasa.gov/mission_pages/sw...urst_oct5.html

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