Gamma-ray Bursts

GRBs are found to occur istrotropically over the sky (e.g. Briggs 1993) and the BATSE detector on board the Compton Gamma-Ray Observatory (CGRO) detects approximately one burst per day. GRBs are typically rather rapid events with log(T90), where T90 is the time within which 90% of the flux from a burst is contained, showing a bimodal distribution with peaks at about 0.3 and 20 seconds (Fishman et al. 1994). Afterglows in the X-ray, optical, and radio bands have also recently been detected (see e.g. the review by Mészáros 1998) and the optical and X-ray afterglows have been seen to decay away with power-law indices of between roughly 1 to 2.

The GRB intensity distribution shows that brighter bursts follow a peak flux distribution with a power-law index of -3/2 as expected for a uniform spatial distribution (e.g. Fenimore et al. 1993). However, fainter bursts show a flattening curve for which a simple interpretation is that we are observing the ``edge'' of the GRB distribution. This interpretation is complicated though by the unknown range of luminosities that GRBs may display.

Although GRBs were once considered to arise from phenomena arising on Galactic neutron stars, the detection within the last year or so of a number of afterglows associated with host galaxies has enabled their distances to be firmly established as ``cosmological'', i.e. at very large distances. For the first three cases where a redshift was measured, values of Z = 0.835, 3.418 and 0.966 were obtained for GRBs 970508, 971214 and 980703 respectively (Metzger et al. 1997, Kulkarni et al. 1998, Djorgovski et al. 1998). At these large distances the implied luminosities of GRBs are extremely large. For example, the implied isotropic gamma-ray energy release of GRB 971214 is $\sim$3$\times$10⁵³ ergs (Kulkarni et al. 1998). Even if the probable significant beaming is taken account of, the energy involved is at least comparable to, and likely exceeds, that associated with supernovae.

The astrophysics of gamma-ray bursts is currently very poorly understood and current explanations to account for GRBs include merging neutron stars (e.g. Mészáros 1998) and hypernovae (Paczynski 1998). However, the mechanism that causes GRBs is not important for their use in synchronizing communication across large distances.

Although the BATSE detector is more sensitive than earlier generations of instruments it has only limited precision in locating bursts and and even bright bursts are not positioned to much better than a few degrees (Pendleton et al. 1999). Progress in precisely locating GRBs has recently come from the instruments onboard the SAX satellite. In addition to a GRB detector, SAX carries X-ray instrumentation consisting of two Wide-Field Cameras (WFC) and a suite of Narrow Field Instruments. For those bursts that occur within the field of view of the WFC, a position accurate to 3 to 8 arc minutes can be obtained, follow-up observations of X-ray afterglows with the Narrow Field Instruments several hours later can then yield positions to about one arcminute (e.g. In't Zand et al. 1998). The All Sky Monitor on-board the Rossi X-ray Timing Explorer has also provided some locations for GRBs to a few arc minutes (Smith et al. 1998). It is this provision of arc minute accuracy positions that has lead to the discovery of the optical afterglows which can then make it possible to obtain positions accurate to better than an arc second.

In the near future, the HETE-II mission (Ricker 1997) should provide positions accurate to 10 arc seconds to arc minutes for about 30 bursts per year. A further possible future GRB mission is Swift (Gehrels et al. 1999). Swift is designed to produce positions accurate to better than an arc second (if optical emission is also detected) within better than 90 seconds and would view an area of 2 steradians. The sensitivity of the Swift GRB detector is such that it is expected that it will detect at least 300 bursts per year. If funded it is intended that Swift would be launched in 2003.

Systems of GRB detectors in spacecraft spread across the solar system in interplanetary networks (see e.g. Hurley et al. 1994) can also provide locations by comparing the time of arrival of a burst at the various spacecraft. Generally this type of network has not so far given very rapid determinations of source location, this can be caused by, for example, infrequent contacts with the spacecraft from ground stations or the difficulties of merging data obtained with a variety instruments in possibly differing formats.