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Conclusions

To date, the ETC has detected no optical counterparts to BATSE GRBs, despite a sensitivity such that a counterpart to the 19 Nov. 1978 GRB for which Schaefer (1981) reported an optical counterpart would be detected with > 100,000. Limits for are established for six of the seven BATSE coincidences listed.

When interpreting these limits, it is important to consider the effects of incomplete geometric overlap between the ETC and BATSE. While it is certainly possible that one or more bursts were not detected because they occurred outside the ETC field-of-view, the probability that all of the bursts were missed for such reasons is small (approximately 4%).

The limits reported here compare favorably to previously published results. Hudec (1993) compiled a list of eight reports of simultaneous optical observations of GRB error boxes between 1970 and 1983. The largest value of that Hudec reported is 560 for a burst on 1.11.1972 with 50% geometric overlap, quite comparable to our result for GRB940616. Limits ranging between were found for the six overlaps with 100% coverage. Hudec (1995) has also reported some 50 overlaps, mainly from the Ondrejov Observatory meteor sky patrol, with more limited sensitivity (

The only other reports of simultaneous optical observations are from Castro-Tirado (1993) who found three archival Schmidt plates coincident with GRBs detected by the WATCH experiment. Extent of geometric overlap was not reported, and limiting magnitudes were slightly smaller (brighter) than in the ETC events. The GROCSE collaboration (Akerlof, 1995) listed eight observations of BATSE error circles made 15-82 sec after the bursts, but and geometric overlap were not reported.

In addition to optical flash data, the ETC stores several full field images each night. Images taken before and after a burst event in the field-of-view are compared to search for transients rising too slowly to be detected during standard operations. A search carried out for the seven burst overlaps gave negative results.

The ETC team is grateful for the assistance of C. Kouveliotou (USRA), G.J. Fishman (NASA/MSFC) and the BATSE science team as well as Scott Barthelmy and the BACODINE team. This work was supported by a NASA Compton GRO Guest Investigation under Grant NAGW-2089.

Next: References Up: Searches for optical counterparts Previous: Analysis of ETC/BATSE