TITLE: GCN CIRCULAR NUMBER: 34789 SUBJECT: GRB 230827B: 10.4m GTC spectroscopic redshift DATE: 23/10/02 20:22:32 GMT FROM: Youdong HU at IAA-CSIC <huyoudong072@hotmail.com> R. Sanchez-Ramirez, A. J. Castro-Tirado, Y.-D. Hu, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu, E. Fernandez-Garcia, and I. Perez-Garcia (IAA-CSIC), S. Geier (GTC, IAC), S. B. Pandey (ARIES) and B.-B. Zhang (NJU) on behalf of a larger collaboration, report: Following the detection of AT 2023qxj by ZTF (Li et al. GCNC 34574), GIT (Swain et al. GCN 34576), and AKO (Odeh et al. GCN 34579) as the likely afterglow of Fermi GRB 230827B (Hamburg et al. GCN 34584), also detected in gamma-rays by VLUSAT-2 (Dafcikova et al. GCNC 34585), CALET (Tamura et al. GCNC 34593) and Globug/ISS (Kerr et al. GCNC 34604), we triggered the 10.4m Gran Telescopio de Canarias (GTC) equipped with Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) in La Palma (Spain), starting on Aug 29, 1 UT (~43 h after the Fermi trigger). Spectroscopy was obtained with both the R1000B (3x1200s) and R1000R (1x900s) grisms, covering the 363-1000 nm spectral range. A red continuum is noticeable, and the GTC spectrum clearly shows the Mg II doublet and Mg I absorption lines, implying a redshift of z=0.887, and thus confirming AT 2023qxj as the optical afterglow of GRB 230827B. We thank the staff at GTC for their excellent support. View this GCN Circular online at https://gcn.nasa.gov/circulars/34789. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...