TITLE: GCN CIRCULAR NUMBER: 21298 SUBJECT: GRB 170705A: Redshift from 10.4m GTC/OSIRIS DATE: 17/07/05 23:48:25 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D.A. Kann (IAA-CSIC), L. Izzo (IAA-CSIC), C.C. Thoene (IAA-CSIC) report on behalf of the HETH group: We observed the afterglow of GRB 170705A (Starling et al. GCN 21289, Lipunov et al. GCN 21291, Butler et al. GCN 21293, Emery et al. GCN 21294) with OSIRIS at the 10.4m GTC telescope. The observation consisted of 3x900s using grism R1000B, covering the range between 3700 and 7880 AA at a mean epoch of July 5.9350 2017 UT (19.68 hr after the burst). The acquisition shows the afterglow at a magnitude of r(AB) = 20.20 +/- 0.10 mag, as compared to SDSS field stars. The spectrum shows continuum across all the spectral range, although the signal to noise ratio of the bluest part is low because of the bright sky due of the nearby almost full Moon. We detect weak spectral absorption features due to CIV, AlII, AlIII, FeII and FeII* at a common redshift of 2.010, which we identify as the redshift of the GRB.