Scientific Motivation and Background

Gamma-Ray Bursts (GRB) have been studied for more than 25 years with more than two thousand detected since their discovery (Klebesadel et al., 1973). Yet none have identified source objects. Since GRBs occur at random times from unpredictable directions, GRB instruments must be responsive to large portions of the celestial sky to get a significant enough detection rate. The very nature of the way gamma-ray photons interact with matter makes it extremely difficult to design an instrument which is able to determine precisely the direction of the arriving photons. With the discovery of optical transients on old archival astrographic plates in the error boxes of modern-day GRBs (Schaefer, 1981), the possibility of detecting GRBs in other wavebands was born. New strategies, therefore, have been initiated to search for emission in other energy regimes.

Two different strategies have been employed to make follow-up observations to identify the GRB source object in the optical and other band passes. The first method is to "stare" at the sky and wait for a GRB to occur in the FOV. Because it is impossible to predict when and where a GRB will occur, very large fields-of-view (FOV) are required to get a reasonable probability of "observing" a GRB. Because the large FOVs result in poor spatial resolution and crowded fields at the detector (Greiner et al., 1992), these methods have poor sensitivity -- 4th magnitude at best. The second method improves the sensitivity by limiting the FOV at the expense of making follow-up observations with some time delay after the GRB. By waiting till after the GRB, position information from instruments in orbit (e.g. GRO-BATSE /COMPTEL, IPN, WATCH-GRANAT/ EURECA) can be obtained to direct narrower FOV instruments to the GRB error box. While the time delays between the GRBs and the observations have been steadily improving over the years -- currently, they are down to 5-to-36-hours range (Barthelmy et al., 1993; Schaefer et al., 1993; Boer, 1993) -- still, no positive identification of a GRB source object has been made. The general conclusion of these observations is that the optical emission from the source objects must have faded below ~20th magnitude within several hours.

Since detector technology is unlikely to develop in the next few years to the point where the first method (stare mode) is likely to yield any new or different results, attempts are being made to improve the second method (follow-up). Clearly, the desired goal is to obtain positional information on the GRB while the burst is still ongoing and get that information to instruments capable of making rapid follow-up observations.

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This file was last modified on 09-Jan-03.