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Gamma-Ray Bursts (CGRO Special Exhibit)

Gamma-Ray Bursts

The Biggest Explosions in the Universe

In Compton's album of greatest hits, gamma-ray bursts (GRBs) would be the torch song -- the tune that gets everyone off their seat with lighters flashing high in the air screaming for more. Although these bursts were known about since 1967, Compton was the first satellite that truly enabled an in-depth study of the phenomenon.

GRBs are the most energetic events known in the Universe, second only to the Big Bang in power. During this fantastic flash -- ranging from as short as a few milliseconds to as long as a minute or more -- the burst can outshine the rest of the Universe. Then it disappears forever. The nature and origin of the bursters remains unknown.

Animation of a gamma-ray
The time history of a gamma ray burst. Left hand side shows the gamma ray sky as the burst brightens and fades. Right hand side shows how the gamma ray intensity rises and falls through the course of the burst.

Prior to Compton, the 300-some known GRBs were thought to be associated with Galactic neutron stars, with a distribution that would trace the Galactic plane. BATSE, many assumed, would simply confirm this scenario, after which GRBs would be relegated to an insignificant place in high-energy astrophysics research.

Instead, BATSE showed the GRB distribution to be isotropic, or spread out uniformly across the sky, and to be spatially limited (i.e. the distribution has an edge). This rules out the hypothesis that GRBs are objects in our own Galaxy, and favors the notion that the 2,500 GRBs from BATSE originated at cosmological distances, well beyond our Galaxy.

BATSE distribution of GRBs, with Fluence
BATSE Distribution of GRBs. Fluence is a measure of the total energy emitted in the burst.

BATSE provided gross localizations of bursts, relayed to the astronomical community in near real-time. The idea is that someone somewhere with something will record the burst before it fades from view. On January 23, 1999, BATSE aided ROTSE-I in catching the first optical counterpart of a GRB while it was bursting. The burst had reached ninth magnitude and would have been visible with a good pair of binoculars. The Beppo-SAX satellite detected an afterglow that allowed measurements proving that the gamma ray bursts are at cosmological distances.

Sequence of 6 ROTSE-I images of the
fading optical counterpart to GRB 990123
Sequence of 6 ROTSE-I images of the fading optical counterpart to GRB 990123

BATSE may be in retirement, but there may be gold in its treasure chest of data. Characteristics in the bursts' light curves appear to be a promising way to determine distance, allowing them to be used as cosmological probes.


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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