TITLE:   GCN CIRCULAR
NUMBER:  41928
SUBJECT: GRB 250920C: GTC/OSIRIS+ spectroscopic redshift z = 1.40
DATE:    25/09/21 09:47:37 GMT
FROM:    Ruben Sanchez-Ramirez at IAA-CSIC <ruben@iaa.es>

R. Sanchez-Ramirez, A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), A. M. Garcia-Rodríguez (GTC, IAC), A. Tejero-Caro (GTC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXI), A. Nicuesa Guelbenzu (Tautenburg Obs.), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report:

Following the detection of GRB 250920C by Fermi (Fermi Team, GCN 41903; Mailyan et al. GCN 41907), Swift (Gupta et al. GCN 41904, 41924), we observed the optical afterglow (Wortley et al. GCN 41907, Strausbaugh et al. GCN 41911, Ghosh et al. GCN 41913, Kuin et al. GCN 41920, Mohan et al. GCN 41925) with the 10.4m GTC telescope, at the Spanish Roque de los Muchachos Observatory, on the island of La Palma, equipped with the OSIRIS+ instrument.

The spectroscopic observations were performed at a high airmass and consisted of 3x900s exposures using grism R1000B, with a spectral coverage between 3,600 and 7,700 A and a resolving power of  ~600. The observations started on Sep 21, 04:29 UT (i.e. 13.07 h after the burst trigger).

From a preliminary reduction, we find a low SNR continuum over all the wavelength range but the bluest extreme, most probably due to atmospheric extinction. The extracted spectrum shows several absorption features that we interpret as AlII, AlIII, ZnII, FeII, MgII, and MgI at z=1.40, which we propose as the redshift of GRB 250920C.