EXIST\u27S Gamma-Ray Burst Sensitivity

We use semianalytic techniques to evaluate the burst sensitivity of designs for the EXIST hard X-ray survey mis-sion. Applying these techniques to the mission design proposed for the Beyond Einstein program,we find that with its very large field of view and faint gamma-ray burst detection threshold, EXIST will detect and localize approximately two bursts per day, a large fraction of which may be at high redshift. We estimate that EXIST’s maximum sensitivity will be ~4 times greater than that of Swift’s Burst Alert Telescope. Bursts will be localized to better than 40 at thresh-old, with a burst position as good asa few arcsecondsfor strong bursts. EXIST’s combination of three different detector systems will provide spectra from 3 keV to more than 10 MeV. Thus, EXIST will enable a major leap in the understand-ing of bursts, their evolution, environment, and utility as cosmological probes

New Constraints on Simultaneous Optical Emission from Gamma-Ray Bursts Measured by the Livermore Optical Transient Imaging System Experiment

LOTIS is a gamma-ray burst optical counterpart search experiment located near Lawrence Livermore National Laboratory in California. Since operations began in 1996 October, LOTIS has responded to five triggers as of 1997 July 30, which occurred during good weather conditions. GRB 970223 (BATSE trigger 6100) was an exceptionally strong burst, lasting »30 s with a peak at »8 s. LOTIS began imaging the error box »11 s after the burst began and achieved simultaneous optical coverage of 100% of the region enclosed by the BATSE 3 j error circle and the interplanetary network annulus. No optical transients were observed brighter than the m » 11 completeness limit of the resulting images, providing a new V upper limit on the ratio of simultaneous optical to gamma-ray fluence of R \u3c 1024 and on the ratio of L simultaneous optical (at 700 nm) to gamma-ray (at 100 keV) flux density of R ! 305 for a B-type spectrum and R \u3c 475 for an M-type spectrum