Searches for optical counterparts of BATSE gamma-ray bursts

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Abstract
The Explosive Transient Camera (ETC) is a wide‐field CCD camera system capable of detecting short (1–10 s) celestial optical flashes as faint as m∼10 over a field‐of‐view of 0.75 steradians between −15° and +45° declination. This sensitivity would allow observations down to Lγ/Lopt≳105 of the optical counterpart to a gamma‐ray burst as intense as that of November 19, 1978, for which a counterpart was reported by Shaefer. The ETC has been operating automatically under computer control since January 1991. Since the launch of the Compton Gamma Ray Observatory, it has been capable of observing an optical flash coincident with a gamma‐ray burst (GRB) detected by the Burst and Transient Spectroscopy Experiment (BATSE). All GRBs which occur during an ETC observation are examined for spatial overlap between the BATSE 68% confidence positional error box and the ETC field of view. Between April 1991 and May 1993, there were four cases of at least partial BATSE/ETC spatial overlap during an ETC observation. In each case upper limits are placed on the optical‐to‐gamma‐ray flux ratio. The expected rate of future BATSE/ETC coincident events is also discussed.

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