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 crucial importance to derive an accurate distance to each burst. If GRBs occur near their host galaxies (≪40 kpc) then Lyman limit absorption [N(H I) ≥ 1.6 × 10¹⁷ cm^-2] should be observable in roughly half the GRB after-glow spectra. Here we outline the methodology to obtain a redshift from the GRB afterglow spectrum using the recently installed Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope. A low-resolution spectrum with the Multi-Anode Microchannel Array (MAMA) detector gives complete spectral coverage over the wavelength range 1570-3180 A (near-ultraviolet: NUV) and 1150-1740 Å (far-ultraviolet: FUV). Assuming that a Target of Opportunity observation is conducted soon (≲3 weeks) after a bright burst, a relatively small integration time (∼3 orbits) would be sufficient to detect the Lyman limit over a wide redshift range (0.3 ≲ z ≲ 2.2). Detection (or non-detection) of the Lyman limit, in concert with ground-based observations of nearby galaxies and Mg II and C IV absorption lines, should provide meaningful constraints on the relationship of GRBs to galaxies.