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‘Naked-Eye’ Gamma-Ray Burst Was Aimed Squarely at Earth

Written by thomas · Filed Under Aeronautics News 

September 10, 2008

thomas

WASHINGTON, Sept. 10 /PRNewswire-USNewswire/ — Data from satellites
and observatories around the globe show a jet from a powerful stellar
explosion witnessed March 19 was aimed almost directly at Earth.

NASA’s Swift satellite detected the explosion — formally named GRB
080319B — at 2:13 a.m. EDT that morning and pinpointed its position in the
constellation Bootes. The event, called a gamma-ray burst, became bright
enough for human eyes to see. Observations of the event are giving
astronomers the most detailed portrait of a burst ever recorded.

“Swift was designed to find unusual bursts,” said Swift principal
investigator Neil Gehrels at NASA’s Goddard Space Flight Center in
Greenbelt, Md. “We really hit the jackpot with this one.”

In a paper to appear in Thursday’s issue of Nature, Judith Racusin of
Penn State University and a team of 92 coauthors report on observations
across the spectrum that began 30 minutes before the explosion and followed
its afterglow for months. The team concludes the burst’s extraordinary
brightness arose from a jet that shot material directly toward Earth at
99.99995 percent the speed of light.

At the same moment Swift saw the burst, the Russian KONUS instrument on
NASA’s Wind satellite also sensed the gamma rays and provided a wide view
of their spectral structure. A robotic wide-field optical camera called “Pi
of the Sky” in Chile simultaneously captured the burst’s first visible
light. The system is operated by institutions from Poland.

Within the next 15 seconds, the burst brightened enough to be visible
in a dark sky to human eyes. It briefly crested at a magnitude of 5.3 on
the astronomical brightness scale. Incredibly, the dying star was 7.5
billion light-years away.

Telescopes around the world already were studying the afterglow of
another burst when GRB 080319B exploded just 10 degrees away. TORTORA, a
robotic wide-field optical camera operated in Chile with Russian-Italian
collaboration, also caught the early light. TORTORA’s rapid imaging
provided the most detailed look yet at visible light associated with a
burst’s initial gamma-ray blast.

Immediately after the blast, Swift’s UltraViolet and Optical Telescope
and X-Ray Telescope indicated they were effectively blinded. Racusin
initially thought something was wrong. Within minutes, however, as reports
from other observers arrived, it was clear this was a special event.

Gamma-ray bursts are the universe’s most luminous explosions. Most
occur when massive stars run out of nuclear fuel. As a star’s core
collapses, it creates a black hole or neutron star that, through processes
not fully understood, drive powerful gas jets outward. These jets punch
through the collapsing star. As the jets shoot into space, they strike gas
previously shed by the star and heat it. That generates bright afterglows.

The team believes the jet directed toward Earth contained an ultra-fast
component just 0.4 of a degree across. This core resided within a slightly
less energetic jet about 20 times wider.

“It’s this wide jet that Swift usually sees from other bursts,” Racusin
explained. “Maybe every gamma-ray burst contains a narrow jet, too, but
astronomers miss them because we don’t see them head-on.”

Such an alignment occurs by chance only about once a decade, so a GRB
080319B is a rare catch.

Swift is managed by Goddard. It was built and is being operated in
collaboration with Penn State, the Los Alamos National Laboratory, and
General Dynamics in the U.S.; the University of Leicester and Mullard Space
Sciences Laboratory in the United Kingdom; Brera Observatory and the
Italian Space Agency in Italy; plus additional partners in Germany and
Japan.

SOURCE NASA

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