TITLE: GCN CIRCULAR NUMBER: 16968 SUBJECT: GRB 141026A: GTC spectroscopy DATE: 14/10/28 16:59:55 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC A. de Ugarte Postigo (IAA-CISC, DARK/NBI), C.C. Thoene (IAA-CSIC), N.R. Tanvir (U. Leicester), J. Gorosabel (UPV, IAA-CSIC), J. Fynbo (DARK/NBI), G. Lombardi (GTC), D. Reverte-Paya (GTC), and D. Perez (GTC) report: We observed the afterglow of GRB 141026A (Hagen et al. GCN 16950, Perley GCN 16951, Littlejohns et al. GCN 16952, Varela et al. GCN 16953, Gorosabel et al. GCN 16954, Perley et al. GCN 16955, Beardmore et al. GCN 16956, Butler et al. GCN 16962,Moskvitin et al. GCN 16966) with OSIRIS at the 10.4m Gran Telescopio Canarias. Observations consisted of z-band imaging, and spectroscopy using the R1000R grism (covering from 5100 to 10000 A with a resolution of ~1000). The spectroscopic observations were delayed due to bad weather and had a mean epoch of 6:12 UT (3.59 hours after the burst), and consisted of 2x900s exposures. Conditions were not optimal, as the airmass at that time was ~1.8 and most of the observation was performed during twilight. The spectrum shows a low S/N continuum, which is however, visible between 5100 and 9500 A. Some absorption features seem to be present, and in particular we identify a broad feature at 5300 A, with an observed equivalent width of ~30 A. This could correspond to a weak Ly-alpha (log(N_H/cm^-2) ~< 20) absorption at z=3.35. However, no other features corresponding to that same redshift are seen, which is not unexpected for a low density line of sight that would correspond to such a Ly-alpha feature and a low S/N spectrum. Further absorptions could correspond to intervening MgII absorbers. This redshift solution is supported by the photometric drop seen by GROND in the g-band (Varela et al. GCN 16953), which would correspond to a similar photometric redshift.