TITLE: GCN CIRCULAR NUMBER: 9873 SUBJECT: GRB 090902B: Gemini-N absorption redshift DATE: 09/09/03 08:23:17 GMT FROM: Antonino Cucchiara at PSU A. Cucchiara, D. B. Fox (PSU), N. Tanvir (U. Leicester), E. Berger (Harvard U.) report on behalf of a large collaboration. "On 2009 September 3.27 UT we observed the Fermi/LAT GRB 090902B (Bissaldi et al., GCN 9866, de Palma 9867) with the Gemini-N telescope equipped with the GMOS spectrograph. A single object has been found at the position of the Swift/XRT afterglow (Kennea et al. GCN 9868, Perley et al., GCN 9870) in our acquisition image. A pair of 900 sec exposures spectra were obtained with a wavelength coverage of 4500-7500A under good conditions. The spectrum reveals a series of metal absorption features, MgII[2796,2803], MgI[2853],MnII[2606], FeII[2600], MnII[2594], FeII[2586], FeII[2260], SiII[1808], SiII*[1816]. All these features are consistent with a redshift of z = 1.822, which, therefore, we conclude is the redshift of GRB 090902B. We thank the Gemini-N staff for performing this observation, in particular Ricardo Schiavon."