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Analysis of ETC/BATSE Overlaps

All ETC observations are correlated with BATSE bursts retrospectively. Between April 1991 and August 1995, there were seven cases in which a BATSE GRB occurred during an ETC observation and within or near an ETC field-of-view. The procedure used when searching for overlaps is described elsewhere[5]. The number of overlaps is consistent with estimates based on the sky coverage and observing efficiencies. No optical transients were detected during any of these observations. These cases are outlined in Table 1 along with the associated limits to the ratio of optical to gamma-ray fluence. The flash durations and fluences are from the BATSE 3B catalog[6]. One additional burst, GRB950805[7] was found to overlap with an ETC observation, but burst duration and fluence are not yet available.

  
Table 1: ETC-BATSE Overlaps from April 1991 - August 1995

The integrated geometric overlap for a burst is an estimate of the likelihood that each burst actually occurred within the ETC field-of-view. For each burst, the BATSE error circle is approximated by a two-dimensional Gaussian distribution based on the statistical error plus average systematic error, 1.6[] added in quadrature. This distribution is convolved with the ETC sky coverage at the time of the burst. The total integrated geometric overlap for the six bursts in Table 1 is 2.36. The relationships between BATSE error circles and ETC fields-of-view is shown graphically in Figure 1.

  
Figure 1: Views of the ETC fields-of-view during six of the ETC/BATSE overlaps. The large circles represent the BATSE 68% error circle for each burst, with a star symbol at the best fit GRB location. The ETC camera fields are arranged in overlapping numbered pairs. The small circles represent optical flashes within 40 minutes of each burst. All flashes are attributed to sunlight reflecting from earth-orbiting satellites. Many are seen to be collinear in the figure; in other cases, evidence of motion was found when specific flash images were examined.

The criterion for a flash to trigger the ETC is a 12 brightening; the corresponding minimum detectable fluence (min) for a one second flash is given in Table 1. It is calculated from , which is the magnitude of the faintest observable field stars (4) during the relevant ETC exposure.

The gamma-ray fluence is also given for each burst. The first row for each burst lists the fluence for the entire burst. This number is compared with (min) to estimate the limiting ratio / . If the entire optical fluence of a burst with (min) is produced during a single ETC exposure, it can be detected. If, however, the burst lasts longer than the exposure, as is the case for three of the six bursts studied, only that part of the burst fluence which rises during a single exposure would be detectable as a transient source. For these three bursts, the gamma-ray fluence produced during the brightest five seconds of the burst is also calculated and compared to (min) to provide a limiting ratio of / during an ETC exposure.

Next: ETC Imaging of Up: Searches for Optical Counterparts Previous: Introduction