4.2.2 GAMMA-RAY BURSTS

Twenty years of intensive effort have passed with no concrete identification of a quiescent counterpart to a gamma-ray burst. Rather than solving the puzzle, BATSE on CGRO has eliminated the previously favored model of galactic disk neutron stars. The current highest priority in understanding GRBs is obtaining the distance scale for the population of sources. However, once that is done, additional study will be required to determine the physical nature of the sources. Many approaches to the determination of the distance scale have been proposed, including (see section 2.3): 1) identification of counterparts in other wavebands 2) detection of an extended halo distribution in the nearby Andromeda Galaxy, 3) search for lines and photoelectric absorption in the spectra of gamma-ray bursts in the X-ray, and 4) search for TeV emission from bursters. Of the four, results from the first two would be less ambiguous. The search for counterparts requires few arcsec error boxes for burst directions and/or rapid notification of burst occurrence to ground observers. GRB positions accurate to 5 arcsec are required to identify spiral and dwarf galaxies detected to redshift z ~ 1. Detection of the anisotropy in the angular distribution of GRBs caused by a putative extended halo around Andromeda requires an instrument approximately 10 times more sensitive than BATSE. A number of GRB missions have been proposed and studied. As New Mission Concepts, ETA, BASIS, and EXIST are being studied, and, as MIDEX Missions, BLAST and EXIST, are contenders. Of these concepts, BLAST, BASIS, and ETA provide arcsec burst positions - ETA by burst arrival timing with an array of satellites, BLAST and BASIS by accurate positioning of the incident flux. They all provide capabilities to measure, to varying degrees, a possible anisotropy in the direction of Andromeda. With the exception of ETA, all provide hard X-ray survey capabilities to address the objectives outlined in section 4.2.1. The characteristics of a GRB mission which addresses both accurate positions and the Andromeda anisotropy measurement are summarized in Table 4.4. Interplanetary networks of gamma-ray burst detectors are a means of providing many small error boxes, in principle down to several arcsecond size. The miniature, low-cost instruments required for planetary missions are fully developed, and require only to be included in future missions.