TITLE: GCN CIRCULAR NUMBER: 21177 SUBJECT: GRB 170531B: Redshift from GTC/OSIRIS DATE: 17/06/01 06:34:45 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC), D.A. Kann (IAA-CSIC), C.C. Thoene (IAA-CSIC), Z. Cano (IAA-CSIC), J.P.U. Fynbo (DARK/NBI), D. Garcia Alvarez (GTC, IAC), D. Perez Valladares (GTC), A. Nuñez (GTC), M. Huertas (GTC) report on behalf of a larger collaboration: We observed the afterglow of GRB 170531B (Evans et al. GCN 21171, Lipunov et al. GCN 21173, Pozanenko et al. GCN 21174) using OSIRIS at the 10.4 m GTC telescope in La Palma (Spain). Observation consisted in 3x900s exposures using the R1000B grism, covering the range between 3700 and 7880 AA. The mean epoch of the combined spectrum is 03:09:29 UT (5.12 hr after the GRB onset). Additionally, g, r, i, and z-band observations were obtained. We note that the coordinates of the afterglow are 19:07:32.09, -16:25:05.8 (J2000 +/-0.5”), which places the GRB approximately 5” away from those of the source identified by Lipunov et al. (GCN 21173) and Pozanenko et al. (GCN 21174). The afterglow can be also detected in the early Swift/UVOT images. In the acquisition image we measure a magnitude of r_AB = 21.9 for the afterglow, as compared to PANSTARRS catalogue field stars. The afterglow spectrum shows continuum throughout the wavelength range and several strong absorption features corresponding to Lyman-alpha, SiII, OI, CII, SiIV, CIV, AlII and AlII at a common redshift of 2.366, which we identify as the redshift of the GRB. [GCN OPS NOTE(01jun17): Per author's request, authors DPV, AN, & MH were added to the list.]