Short-Duration Gamma-Ray Bursts
Short-duration gamma-ray bursts are generally defined as those lasting
less than 2 seconds. These bursts have a duration between a few
milliseconds and 2 seconds with an average duration of 300
milliseconds (0.3 seconds). Short-duration GRBs are also
significantly dimmer than long-duration ones, by a factor of
10. In addition, short duration bursts have more higher energy gamma
rays do than longer bursts. Finally, there is also evidence that in
long duration bursts energy is converted into gamma rays at a steady
rate. In short bursts, the energy conversion rate appears to decrease
as the burst progresses.
Although the HETE-2 satellite has begun
to detect afterglows from them, they are only beginning to emerge from
the category of "dark bursts" -- those GRBs with no afterglow. (At one time, all GRBs were "dark bursts.").
Astronomers think short-duration GRBs are not related to supernova, meaning the collapsar/hypernova model is not applicable to them. Rather, astronomers think some other phenomenon involving the collision and coalescence of compact objects such as neutron stars, although other possibilities exist. These include some variation of the core-collapse of a dying star, which would mean short duration GRBs were in fact related to the collapsar/hypernova model. Or it may be related to the viewing geometry. Since GRBs occur along an axis, as opposed to an expanding spherically, their lower power and lack of afterglow could simply be the observational "selection effect" whereby the Earth is at the very edge of the GRB "cone" and so astronomers only weakly detect the burst. In this way, short duration GRBs and their less energetic X-ray counterpart, the X-ray flash (XRF) -- are both "really" both regular (long-duration) GRBs, as seen from a glancing angle. (The demarcation between GRB and XRF depends on where one draws the line between X-rays and gamma-rays, a somewhat arbitrary distinction, since "hard" X-rays are also called "soft" gamma-rays.)
To date, all of the ideas involving short-duration GRBs remain
strictly theoretical. Much more observational work is needed -- a
detailed study of an afterglow will enable astronomers to pinpoint how
far away short-duration GRBs are. Only then will astronomers be able to glean important clues from explosions that are thought to be the biggest in the observable Universe since the "explosion" of the Big Bang itself 13.7 billion years ago.
 Return to "GRBs: Lingering Mysteries".
Additional Links
 Short and Long GRBs Different to the Core (http://universe.gsfc.nasa.gov/press/2003/030220a.html)
Dark GRBs More Flighty Than Shy (http://universe.gsfc.nasa.gov/press/2003/030911b.html)
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