In the powerful, fast-fading 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.

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, published these results in a recent
issue of Astronomy & Astrophysics.

“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.

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.

Lazzati and his colleagues looked for signs of an afterglow in the archived
data of short bursts detected by the BATSE instrument aboard NASA’s Compton
Gamma-Ray Observatory, a mission that ran from 1991 to 1999. 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). While both types of bursts appear to emanate from their
source as beamed jets, as opposed to an expanding fireball, the 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. The Swift satellite — a NASA-led
international mission on schedule for a 2003 launch — will be “swift”
enough to detect and localize short gamma-ray bursts and notify other
satellites and telescopes within seconds. Swift’s X-ray and ultraviolet
telescopes will also be able to study the afterglow phenomenon in depth.

For more information about this discovery of short burst afterglows, refer
to Astronomy & Astrophysics, 379, L39-L43 (2001). For more information about
Swift, refer to http://swift.gsfc.nasa.gov. For scientific images related to
this result, refer to http://www.ast.cam.ac.uk/~lazzati/short/short.html.

Contacts for this release:

Dr. Davide Lazzati

Institute of Astronomy, University of Cambridge

Phone +44 (0)1223 766667 Fax +44 (0)1223 337501

lazzati@ast.cam.ac.uk

Mr. Enrico Ramirez-Ruiz

Institute of Astronomy, University of Cambridge

Phone +44 (0)1223 766653 Fax +44 (0)1223 337501

enrico@ast.cam.ac.uk

Dr. Gabriele Ghisellini

Osservatorio Astronomico di Brera

Phone +39 039 9991124

gabriele@merate.mi.astro.it