Hubble Space Telescope STIS Observations of GRB 000301C: CCD Imaging and Near-Ultraviolet MAMA Spectroscopy

We present Space Telescope Imaging Spectrograph observations of the optical transient (OT) counterpart of the gamma-ray burster GRB 000301C obtained 5 days after the burst, on 2000 March 6. CCD clear-aperture imaging reveals a R~=21.50+/-0.15 source with no apparent host galaxy. An 8000 s, 1150 Å<lambda<3300 Å near-ultraviolet MAMA prism spectrum shows a flat or slightly rising continuum (in flambda) between 2800 and 3300 Å, with a mean flux of (8.7+0.8-1.6+/-2.6)×10-18 ergs s-1 cm-2 Å-1, and a sharp break centered at 2797+/-25 Å. We interpret this as the H I Lyman break at z=2.067+/-0.025, indicating the presence of a cloud with an H I column density logNHI(cm2)>18 on the line of sight to the OT. This measured redshift is conservatively a lower limit to the GRB redshift. However, as all other GRBs that have deep Hubble Space Telescope images appear to lie on the stellar field of a host galaxy, and as the large H I column density measured here and in later ground-based observations is unlikely on a random line of sight, we believe we are probably seeing absorption from H I in the host galaxy. In any case, this represents the largest direct redshift determination of a gamma-ray burster to date. Our data are compatible with an OT spectrum represented by a power law with an intrinsic index alpha=1.2 (fnu~nu-alpha) and no extinction in the host galaxy, or with alpha=0.5 and extinction by SMC-like dust in the OT rest frame with AV=0.15. The large NHI and the lack of a detected host are similar to the situation for damped Lyalpha absorbers at z>2. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.