TITLE: GCN CIRCULAR NUMBER: 11164 SUBJECT: GRB 100901A: Gemini-N Redshift DATE: 10/09/01 17:02:09 GMT FROM: Ryan Chornock at Harvard R. Chornock, E. Berger (Harvard), D. Fox (Penn State), A. J. Levan (U. Warwick), N. R. Tanvir, and K. Wiersema (U. Leicester) report on behalf of a larger collaboration: We obtained spectroscopy of the optical afterglow (Immler et al., GCN 11159; Guidorzi et al., GCN 11160; Ivanov et al., GCN 11161; Klunko et al., GCN 11162) of GRB 100901A (Immler et al., GCN 11159) using GMOS on the Gemini-North 8-m telescope starting on 2010 September 1.61 UT. A set of four 400s observations were obtained covering the wavelength range 3825-6675 Angstroms. Numerous absorption lines are present. The highest redshift system is at z=1.408, with many absorptions from Fe II, Al II, and Si II. The presence of several Fe II* absorptions establishes this as the redshift of the host galaxy of GRB 100901A. Two intervening systems at z=1.315 and 1.318 also have absorptions from Mg II and Fe II. We thank A. Stephens and the Gemini staff for their help in obtaining these observations.