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Although gamma-ray bursts (GRBs) were discovered more than thirty years ago the physical mechanism which produces these brief flashes of high-energy radiation remains a mystery. GRBs generally have durations between 0.1 s and 100 s and appear at random times from unpredictable positions on the sky.

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Since gamma-rays cannot penetrate the Earths atmosphere, GRB detectors must be placed on board high-altitude balloons or Earth orbiting satellites. The longest running and perhaps most successful GRB detection instrument is BATSE (Burst and Transient Source Experiment) on-board CGRO (Compton Gamma Ray Observatory). During almost seven years of operation the BATSE detectors have measured the timing, spectra, and approximate location of over 2000 GRBs at a rate of roughly one per day. The two most significant BATSE observations are that the GRB locations are isotropic over the sky and that their brightness distribution is inhomogeneous. These measurements provide a strong argument against models of local GRB sources within the plane of our galaxy. Recent detections of optical and radio afterglows support the theory that bursts originate in other galaxies at cosmological distances.

Current BATSE GRB Sky Map
Current BATSE GRB Sky Map
This map shows the postions in galactic coordinates of all the GRBs detected by BATSE to date. In this coordinate system the visible stars in the plane of the Milky Way lie along the major axis of the ellipse. The center of the plot corresponds to the center of the galaxy while both the left and right sides represent the anti-center. If GRBs occurred in the plane of the galaxy their distribution would be clustered toward the major axis.

Although BATSE has collected a wealth of GRB data the unique properties (i.e. light curves and spectra) of each individual burst have made it difficult for astrophyscists to agree upon a model of their origin or their "central engine". Since high-energy data has failed to pinpoint the origin of GRBs, information provided at other wavelengths, either optical or radio, is likely to play a major role.

Because high-energy photons such as gamma-rays are difficult to focus the precise locations of a GRB cannot easily be determined. The error boxes provided by the present burst detectors are so large that there are literally thousands of possible sources which include external galaxies containing billions of stars. Due to this imprecise localization, simultaneous or follow-up observations with large optical telescopes which have small fields-of-view are nearly impossible. In addition since the physical mechanism which causes a burst is unknown it is unclear when optical photons would be emitted, if at all, and how bright they might be. The best scenerio for an optical counterpart search experiment is a simultaneous observation of the entire GRB error box with a large field-of-view telescope followed by continuous observations long after the gamma-ray emmission has dissipated. Because of the short durations and random positions of GRBs these observations are best done using a triggered automated telescope.

The original purpose of simultaneous optical counterpart search experiments using small wide field-of-view telescopes was to provide more precise GRB locations to allow for follow-up observations with larger telescopes. These follow-up observations might associate a GRB with an external galaxy thus suggesting they occur at large cosmological distances. If GRBs originate in external galaxies theories of the central engine would be constrained based on energy considerations. This strategy has changed as a result of recent observations of GRB counterparts at X-ray, optical and radio wavelengths. These observations have provided the precise locations sought however they have failed to definitively reveal the underlying source. Although redshift measurements of one counterpart have revealed that at least some GRBs occur at cosmological distances, only one of the observations reveal a fuzzy background which could be the light from the source galaxy.

Only three optical counterparts have been observed thus far with the earliest observations occuring several hours after the gamma-ray emission ceased. The emmission detected as optical counterparts is probably from an "afterglow" resulting from a relativisticly expanding electron-positron "fireball" interacting with the surrounding interstellar medium. However this fireball acts to conceal the nature of the very cataclysmic event which caused it thus restricting further observations of the central engine. If the mechanism which produces the gamma-ray emmission in a GRB is different than the mechanism which produces the afterglow multiwavelength observations simultaneous with the gamma-ray emmission provide the only method of further probing the central engine.

Simultaneous optical counterpart search experiments now have a new primary purpose; to detect or place constraints on optical emmission during the GRB thus extending the spectrum into the lower energy regime. This broad band spectrum could further constrain central engine models leading to a better understanding of the GRB phenomena.




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