Press Release

Afterglow the hard way: Fermilab Scientists FInd the Glow Without the Burst – A First

By SpaceRef Editor
November 9, 2001
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Batavia, Ill.-Combining the newest of astronomical instruments with the most
venerable techniques of patient attention to detail, scientists at the
Department of Energy’s Fermi National Accelerator Laboratory, 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 afterglow.”

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 all three.”

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 telling us that if we look in
their direction we will learn something very interesting and important about
cosmology and the universe,” Lamb said.

The researchers have submitted their results for publication to The
Astrophysical Journal. They also announced their results Wednesday, November
7 at the Woods Hole 2001/Gamma Ray Burst and Afterglow Astronomy workshop in
Woods Hole, Mass. To see the abstract, preprint and accompanying images, go
to www.arxiv.org/abs/astro-ph/0111054.

The SDSS (www.sdss.org) is a joint project of The University of Chicago,
Fermilab, the Institute for Advanced Study, the Japan Participation Group,
The Johns Hopkins University, the Max-Planck-Institute for Astronomy (MPIA),
the Max-Planck-Institute for Astrophysics (MPA), New Mexico State
University, Princeton University, the United States Naval Observatory, and
the University of Washington. Funding for the Sloan Digital Sky Survey
(SDSS) has been provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Aeronautics and Space
Administration, the National Science Foundation, the U.S. Department of
Energy, the Japanese Monbukagakusho, and the Max Planck Society.

Fermilab(www.fnal.gov) is operated by Universities Research
Association, Inc., under contract with the U.S. Department of Energy.

SpaceRef staff editor.