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) counter-part of the γ-ray burster GRB 000301C obtained 5 days after the burst, on 2000 March 6. CCD clearaperture imaging reveals a R ≃ 21.50 ± 0.15 source with no apparent host galaxy. An 8000 s, 1150 Å < λ < 3300 Å near-ultraviolet MAMA prism spectrum shows a flat or slightly rising continuum (in f_λ) 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 log N_{H I}(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 γ-ray burster to date. Our data are compatible with an OT spectrum represented by a power law with an intrinsic index α = 1.2 (f_v ∝ v^-α) and no extinction in the host galaxy, or with α = 0.5 and extinction by SMC-like dust in the OT rest frame with A_v = 0.15. The large N_{H I} and the lack of a detected host are similar to the situation for damped Lyα absorbers at z > 2.