Afterglows, the hard way

Fermilab scientists find the glow without the burst -- a first

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," 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.

On the web:

Abstract, preprint and accompanying images http://www.arxiv.org/abs/astro-ph/0111054

Sloan Digital Sky Survey http://www.sdss.org