Hubble Space Telescope STIS Observations of GRB 000301C: CCD Imaging and Near-Ultraviolet MAMA Spectroscopy
Fruchter, Andrew S.; Hogg, David W.; Livio, Mario; Ferguson, Henry; Gull, Theodore R.; Hurley, Kevin; Hjorth, Jens; Kouveliotou, Chryssa; Fynbo, Johan P. U.; Lindler, Don J.; Sahu, Kailash C.; Rhoads, James; Pian, Elena; Smette, Alain; Pedersen, Holger; Cline, Tom; Petro, Larry; Macchetto, Duccio; Thorsett, Stephen E.; Wijers, Ralph A. M. J.; Levine, Alan; Gorosabel, Javier; Gibbons, Rachel; Metzger, Mark R.; Jensen, Brian L.; Smith, Donald A.; Trombka, Jack
United States, Belgium, Australia, Netherlands, Denmark, Italy, Germany
Abstract
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 fλ) 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 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 (fν~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 AV=0.15. The large NHI and the lack of a detected host are similar to the situation for damped Lyα 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.