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Scientists Detect First Afterglow of Short Gamma-Ray Bursts

01 March 2002

In the powerful realm of gamma-ray bursts, scientists say they have detected for the first time a lingering afterglow of the shortest types of bursts, which themselves disappear within a second.

This afterglow, radiating in X rays, may provide crucial insight into what triggers the mysterious bursts, the most energetic explosions in the Universe, second only to the big bang in total power. Previously, scientists had only detected the afterglow of longer bursts, which can last from a few seconds to about a minute and which seem to be of different origin than short bursts.

The scientists -- Davide Lazzati and Enrico Ramirez-Ruiz of the Institute of Astronomy at the University of Cambridge, along with Gabriele Ghisellini of the Osservatorio Astronomico di Brera in Merate, Italy -- found evidence of the afterglows in archived data of gamma-ray bursts that occurred through the 1990s, detected by an instrument aboard NASA's Compton Gamma Ray Observatory called BATSE.

This is exciting news for scientists determined to solve the gamma-ray burst mystery. After all, most of what we know about gamma-ray bursts comes from observations of their afterglows, which kindly hang around long enough to study in at least some smidgen of detail.

"The discovery of afterglows for long bursts in 1997 was a breakthrough, allowing us to determine that these explosions originate at cosmological distances, billions of light years away," said Lazzati. "Short bursts, which sometimes last for only a few milliseconds, are naturally harder to catch. With the discovery that they too have an afterglow, we may now finally have at least a small handle to study them."

Many scientists believe that longer bursts are from the collapse of massive stars. This is the so-called collapsar model, which may entail the collapse of a theorized hyperstar, more massive than the stars that explode as supernovae. Shorter bursts, under two seconds long, may originate from the collision of two neutron stars or black holes. As exotic as they may sound, gamma-ray bursts are remarkably common, detected nearly daily by earth-orbiting satellites.

GRB
This is an animated movie of what could be a neutron star merging with a black hole and producing a gamma-ray burst. (2.8 MB)

(Credit: STScI)
(Description)

Precious little is known about short bursts. Because they fade so quickly, orbiting burst detectors have been unable to accurately determine the location of the short bursts. Thus scientists cannot study the "crime scene" to search for clues of the explosion, as they can with longer bursts. Also, short bursts might not produce bright afterglows, further complicating their study.

The afterglow of any gamma-ray burst is caused by an event different from the original explosion, likely by blast waves from the burst ramming material from its chaotic source into matter in the surrounding medium. The detection of afterglows in short bursts may allow scientists to study the critical early phases of this phenomenon, hidden under the brighter prompt emission of long bursts.

BATSE, the Burst And Transient Source Experiment, comprised eight burst detectors positioned on eight different corners on Compton in order to cover the entire sky. Although a given burst may only last a few seconds, each burst usually triggers the BATSE detectors to collect any gamma rays or higher-energy X rays (also called hard X rays or soft gamma rays) that come their way for the next 200-plus seconds.

The team studied several hundred short bursts lasting less than one second and having a high "signal-to-noise ratio," meaning that an afterglow would be less likely to be masked, or missed, by background radiation, or noise. The detection was performed on the sum of the best 76 light curves, since the afterglow of a single event is too faint to be detected against the background radiation.

"Characteristics in the light suggest that this peak in emission is produced by the deceleration of a relativistic blast wave, as predicted by the afterglow model and observed for the class of long-duration bursts," said Ramirez-Ruiz.

The similarity of afterglows does not rule out that short and long bursts are different, Ramirez-Ruiz said. Further analysis and observations, along with theoretical modeling, will be needed to determine the physics of the sources and their distance (perhaps within our Galaxy or from the distant, early Universe).

One interesting new find, Ramirez-Ruiz said, is how short gamma-ray bursts are beamed. While both types of bursts, long and short, appear to emanate from their source as beamed jets, as opposed to an expanding fireball, the BATSE data indicate that short bursts may have an opening jet angle 3-10 times larger than the long bursts. Thus, they may have a similar overall energy, only spread out over a wider distance.

The afterglow phenomenon was first detected by the Italian-led Beppo-SAX X-ray satellite, nearly five years ago. Beppo-SAX still actively detects and locates gamma-ray bursts.

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