2001ApJ...556...70S

We present Space Telescope Imaging Spectrograph observations of the optical transient (OT) counterpart of the γ-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<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±2.6x10^–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 logN_HI_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_ν∝ν^–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_HI_ and the lack of a detected host are similar to the situation for damped Lyα absorbers at z>2.