TITLE: GCN CIRCULAR NUMBER: 19710 SUBJECT: GRB 160623A: afterglow spectroscopy by GTC and independent redshift determination DATE: 16/07/14 18:44:42 GMT FROM: Alberto Castro-Tirado at Inst.de Astro. de Andalucia A. J. Castro-Tirado (IAA-CSIC), A. F. Valeev (SAO-RAS), S. Jeong (SKKU), R. Sánchez-Ramírez (IAA-CSIC), V. V. Sokolov (SAO-RAS), P. Ferrero (IAA-CSIC), J. C. Tello (IAA-CSIC), Y. Hu (IAA-CSIC), B.-B. Zhang (IAA-CSIC), A. Pozanenko (IKI-RAS), S. R. Oates (U. of Warwick), S. Geier (GTC) and G. Lombardi (GTC), on behalf of a larger collaboration, report: Following the detection of GRB 160623A by Fermi (Vianello et al. GCNC 19553) and Konus-Wind (Frederiks et al. GCNC 19554), we observed the optical afterglow of GRB 160623A (Pozanenko et al. GCNC 19561) with the 10.4m Gran Telescopio Canarias (GTC), gathering spectra at different epochs: June 25 (1.9 days post-burst) and July 3/4, with both the R1000B and R2500I grisms covering the range 3800-10000 A. At the position of the afterglow, the reddest spectrum (2 x 1200s with R2500I) showed H-alpha and [SII] in emission, from which we determine a redshift z = 0.367, confirming the value proposed by Malesani et al. (GCNC 19708). The bluest range spectrum (1200s) also revealed a marginal detection of H-beta (taking into account the high foreground Galactic extinction in the line of sight). The faint continuum on the first epoch spectrum extended down to 3800 A with no absorption lines being present. Therefore we propose this is the redshift of the GRB 160623A host galaxy. We acknowledge excellent support from the GTC staff.