Gamma-ray bursts (GRBs) are intense, short outbursts from unknown cosmic sources. They have been observed to last from milliseconds to over an hour and can outshine all other objects in the gamma-ray sky by several orders of magnitude. Although the radiation from these events between 10 keV and 1 GeV can be quite intense, emission at other wavelengths has rarely been observed.
GRBs were discovered in the late 1960s by U.S. Defense Department satellites though it wasn't until the late 1970s that missions devoted to the study of these events were launched. A gradual consensus was formed that the sources of GRBs were nearby (~100pc) neutron stars based on energies assumed for the burst and the observation of absorption features in GRB spectra implying absorption of gamma-rays by electrons trapped in 10^12 Gauss magnetic fields. A clear consequence of the local neutron star hypothesis is that there should be a concentration of weak GRBs toward the Galactic plane. However, missions launched before 1990 lacked the sensitivity to detect such a concentration.
In 1991, the most ambitious burst experiment to that time was released from the space shuttle Atlantis. The Burst and Transient Source Experiment (BATSE) was designed to be sensitive to weak GRBs and prove the local neutron star hypothesis. BATSE does see weak bursts though they are not concentrated in the Galactic plane as was expected. The BATSE results show that we are apparently at the center of a spherical but bounded distribution of GRB sources. If GRBs come from a single population of sources, then they must lie in an extremely large (~150 kpc) Galactic halo or reside at the edge of the Universe.
Many attempts were made to definitively prove that the source population is Galactic or cosmological using BATSE data. These efforts tended to point toward a cosmological origin for bursts but some studies were either inconclusive or more consistent with a Galactic population. The situation was enormously improved when the BeppoSAX satellite was able to image one grb in late February and another in early May of 1997. These images provided precise locations for the GRBs which enabled the first successful optical and radio counterpart searches to take place. These counterparts appear to be associated with distant galaxies (though some questions remain) and the behavior of the counterpart emission is consistent with the most popular cosmological GRB models. These observations appear place GRBs sources at the edge of the universe.
While the distance scale problem appears to be solved, another important mystery regarding GRBs remains; the absence of a definitive detection of an absorption line in the BATSE spectra. Although most of the detections from previous missions can be regarded as problematic, at least one detection (from the Japanese satellite GINGA) appears solid. While there is not at present a contradiction between GINGA detection and BATSE non-detection, the situation is disturbing and the BATSE team devotes a great deal of energy toward determining whether or not any line features exist in the data. The answer is vital because not only do such features provide important physical information about the source, but their very existence can place powerful constraints on many GRB models.
GRBs are perhaps the most mysterious objects in modern astrophysics. Observations continue to be carried out in the hopes that some definitive characteristic will be found which will, if not solve the problem, at least greatly reduce the ambiguity of it. Until then, we can only speculate about whether or not we are seeing previously unknown objects or evidence of physical processes that cannot be accommodated with accepted theories of nature.Back to Lyle Ford
Updated August 21, 1997.