USING STIS TO FIND GAMMA-RAY BURST REDSHIFTS

A recent spectrum of the optical afterglow of GRB 970508 suggests that gamma-ray bursts (GRBs) are cosmological in origin, and it is of cruc ial importance to derive an accurate distance to each burst. If GRBs o ccur near their host galaxies (<<40 kpc) then Lyman limit absorption [ N(H I) greater than or equal to 1.6 x 10(17) cm(-2)] should be observa ble in roughly half the GRB afterglow spectra. Here we outline the met hodology to obtain a redshift from the GRB afterglow spectrum using th e recently installed Space Telescope Imaging Spectrograph (STIS) instr ument on board the Hubble Space Telescope. A low-resolution spectrum w ith the Multi-Anode Microchannel Array (MAMA) detector gives complete spectral coverage over the wavelength range 1570-3180 Angstrom (near-u ltraviolet: NW) and 1150-1740 Angstrom (far-ultraviolet: FUV). Assumin g that a Target of Opportunity observation is conducted soon (less tha n or similar to 3 weeks) after a bright burst, a relatively small inte gration time (similar to 3 orbits) would be sufficient to detect the L yman limit over a wide redshift range (0.3 less than or similar to z l ess than or similar to 2.2). Detection (or non-detection) of the Lyman limit, in concert with ground-based observations of nearby galaxies a nd Mg II and C IV absorption lines, should provide meaningful constrai nts on the relationship of GRBs to galaxies.