4 Gamma-ray burst localization studies

Rapid and accurate localization of bursts is a very important requirement for a wide field coded mask imager, and these capabilities were studied in detailed model calculations for cases near the detection threshold. These simulations addressed burst detection, identification, and localization for an imager with a $60^\circ\times 60^\circ$ FCFOV, $129\times 127$ URA, 369 cm² detector, and 150 c/s background from 50-200 keV (Slavis et al. 1998, give results on balloon flight background tests.) Two variables were considered: the ratio of burst count rate to background rate, and integration time. The three ratio values of 1:2, 2:1, and 10:1 were used and the integration time was inferred from the number of background counts.

The results show that for all ratios, when the expected detection significance is 5$\sigma$, the fraction of bursts correctly localized is >80% and the RMS position uncertainty is 4.3$^{\prime}$. Some of these cases have as few as 70 counts in the burst. One thousand burst counts, and negligible background, corresponds to a 22$\sigma$ detection and $\sim$1$^{\prime}$ localization accuracy, and ten thousand counts yields an accuracy of $\sim$0.4$^{\prime}$.