Using the powerful one-two combo of NASA's Swift
satellite and the Gemini
Observatory, astronomers from a number of institutions,
including Johns Hopkins, have
detected a mysterious type of cosmic explosion further back
in time than ever before.
The explosion, known as a short gamma-ray burst, or
GRB, took place 7.4 billion
years ago, more than halfway back to the Big Bang.
"This discovery dramatically moves back the time at
which we know short GRBs
were exploding. The short burst is almost twice as far as
the previous confirmed record
holder," said John Graham, a doctoral student in the
Henry A. Rowland
Department of
Physics and Astronomy at Johns Hopkins. Graham
presented his group's discovery at the
American Astronomical Society's 2008 winter meeting last
week in Austin, Texas.
GRBs are among the most powerful explosions in the
universe, releasing enormous
amounts of energy in the form of X-rays and gamma rays.
Most bursts fall in one of two
categories: long and short, depending on whether they last
more or less than three
seconds. Astronomers believe that long GRBs are triggered
by the collapse and explosion
of massive stars. In contrast, a variety of mechanisms have
been proposed for short
bursts. The most popular model says that most short GRBs
occur when two neutron stars
smash into each other and collapse into a black hole,
ejecting energy in two counter-
flowing beams.
The record-setting short burst is known as GRB
070714B, named because it was
the second GRB detected on July 14, 2007. NASA's Swift
Gamma-Ray Burst Mission
discovered the GRB in the constellation Taurus. Rapid
follow-up observations with the
two-meter Liverpool Telescope and the four-meter William
Herschel Telescope found an
optical afterglow in the same location as the burst, which
allowed astronomers to identify
the GRB's host galaxy.
Next, Graham and his colleagues, Andrew Fruchter of
the Space Telescope
Science
Institute and Andrew Levan of the University of Warwick
in the United Kingdom, trained
the eight-meter Gemini North Telescope in Hawaii on the
galaxy. Analysis of a spectrum
of the light from that galaxy indicated that it is 7.4
billion light-years away, meaning the
explosion occurred 7.4 billion years ago.
Swift lead scientist Neil Gehrels, of NASA's Goddard
Space Flight Center, said,
"The fact that this short burst is so far away means this
subclass has a broad range of
distances, although they still tend to be closer on average
than long GRBs."
Gehrels adds that GRB 070714B's energy was about 100
times higher than average
for short bursts, more similar to the typical energy of a
long GRB.
"It is unclear whether another mechanism is needed to
explain this explosion, such
as a neutron star-black hole merger," he said. "Or it could
be that there are a wide range
of energies for neutron star-neutron star mergers, but that
seems unlikely."
Another possibility is that GRB 070714B concentrated
its energy in two very
narrow beams and that one of the beams happened to be aimed
directly at Earth, making
the burst seem more powerful than it really was.
Researchers wonder if most short GRBs
eject their energy in wider, less concentrated beams.
"We now have a good idea of the type of star that
produces the brighter long
bursts," Fruchter said. "But how short bursts are formed
remains a mystery."
Swift is managed by NASA's Goddard Space Flight Center
and was built and is
operated in collaboration with Penn State University, the
Los Alamos National Laboratory
and General Dynamics in the United States; the University
of Leicester and Mullard
Space Sciences Laboratory in the United Kingdom; Brera
Observatory and the Italian
Space Agency in Italy; and partners in Germany and
Japan.
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