Fermilab scientists find the glow without the burst -- a first
instruments with the
techniques of patient
attention to detail,
scientists at the
University of Chicago
and other institutions believe they have made the first optical
observation of a gamma ray burst afterglow unprompted by prior
observation of the gamma ray burst itself-a so-called "orphan
This unprompted observation has significance for astrophysicists
because it helps them distinguish among competing models for the
mechanism of these phenomenally powerful cosmic explosions.
"A gamma ray burst lasts for just seconds," said Fermilab
astrophysicist and David N. Schramm Fellow John Beacom, a
collaborator in the research. "But it produces an afterglow that lasts
for a week or so, and that astronomers can see as a bright object in
optical telescopes. The trick in seeing an afterglow comes in
knowing where to look. All previously observed GRB afterglows
have been found as follow-ups to observations from satellite-borne
gamma ray detectors. Finding the glow without the burst is a first,
and it's an important clue to how gamma ray bursts work."
Astrophysicists believe that gamma rays are emitted in two narrowly
focused jets in opposite directions from the site of the GRB. But
there are competing views on the directionality and extent of the
afterglow. If the GRB jet were not pointing right at you, would you
see its afterglow? Some models predict that the afterglow takes the
same focused direction as the burst itself; others predict it might be
isotropic, emitting light in all directions. The observation of an orphan
afterglow supports the isotropic model, because now observers
have seen the glow without first seeing the gamma rays themselves,
meaning the gamma ray jets likely emerged in a different direction.
In a meticulous examination of
data taken in 1999 and 2000 by
the Sloan Digital Sky Survey, a
project to create a
three-dimensional map of the
universe, the researchers located
an object about 100 times
brighter than the brightest known
supernova. The object was
associated with an otherwise
normal galaxy about six billion
light-years away. Based on its
colors, the astronomers thought
the bright object might be a
quasar. But when they looked at data taken about a year later, they
found that the brightness had faded by a factor of at least 10. Since
quasars don't vary that much in brightness, the observers knew they
had found something unusual, neither supernova nor quasar but a
"highly luminous optical transient."
"When we saw that it had faded so much, we knew it couldn't be a
quasar," said Fermilab astrophysicist Dan Vanden Berk. "Another
class of very bright objects whose luminosity varies is a gamma ray
burst afterglow. When we calculated the object's luminosity from our
knowledge of its distance, that was our first hint that we might be
looking at a GRB afterglow."
When the observers found that the pattern of intensity in the object's
colors closely matched the typical pattern for a GRB afterglow, their
conviction grew that they had indeed found an orphan afterglow.
"All of these pieces-brightness, transience and characteristic
colors-came together to spell 'afterglow,'" said Fermilab
astrophysicist Kev Abazajian, a collaborator. "Other celestial objects
have some of these characteristics, but a GRB afterglow combines
Their observation was a marked departure from usual afterglow
sightings. Although gamma ray bursts have been detected for more
than 30 years, all the GRB afterglows on record have been
prompted by gamma ray detection by satellites. When they detect a
gamma ray burst, the satellites pass on the alert to ground-based
astronomers, telling them when and where they should begin
searching for the burst's optical afterglow. Even with the satellite
prompting, afterglows are very hard to spot. Although astronomers
have detected thousands of GRB's, only about 20 afterglows have
been observed so far. Finding an orphan afterglow, one without a
previously observed GRB, is much more difficult.
"Astronomers have searched for orphan afterglows for years," said
Fermilab astrophysicist Brian Lee. "It took the capability of the Sloan
Digital Sky Survey to give us a realistic chance of seeing one."
The SDSS is designed to
peer deeply into wide
swaths of the sky,
compiling a definitive map
of more than 100 million
celestial objects, including
galaxies and quasars. SDSS can gather images in five wavebands,
analogous to photographic filters, to select interesting objects (such
as quasars) for spectroscopic follow-up. The spectra reveal the
identities and redshifts of celestial objects, the key to determining
their distance from earth, and hence their brightness. The SDSS
telescope's unique combination of features—its wide field of view, its
reach in seeing faint objects, and its simultaneous images in five
wavebands—enables it to discern luminosities in different colors and
effectively screen out background images.
Even using the Sloan Data, finding the afterglow was a painstaking
process. Vanden Berk, Lee, and astrophysicists James Annis of
Fermilab and Brian Wilhite of the University of Chicago sifted through
thousands of digital images taken in the course of more than a year
of observations with the 2.5-meter SDSS telescope at Apache Point
Observatory in New Mexico. They used a technique developed by
Vanden Berk to winnow the data to manageable size by selecting for
color and then looking for fading brightness.
University of Chicago astrophysicist Don Lamb, a collaborator,
pointed out that SDSS has so far collected only a small fraction of
the data it will ultimately amass, opening the possibility for identifying
more orphan afterglow candidates, and thus shedding more light on
gamma ray bursts.
"Gamma ray bursts are like bright beacons," Lamb said, "telling us
that if we look in their direction we will learn something very
interesting and important about cosmology and the universe."
The researchers have submitted their results for publication to The
Astrophysical Journal. They also announced their results
Wednesday, Nov. 7 at the Woods Hole 2001/Gamma Ray Burst and
Afterglow Astronomy workshop in Woods Hole, Mass.
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