TITLE:   GCN CIRCULAR
NUMBER:  37469
SUBJECT: GRB 240912A: Redshift from OSIRIS+/GTC
DATE:    24/09/12 06:32:02 GMT
FROM:    Antonio de Ugarte Postigo at LAM/OCA, CNRS <deugarte@oca.eu>

A. de Ugarte Postigo (LAM/OCA, CNRS), J. F. Agui Fernandez (CAHA), C. C. Thoene (ASU-CAS), N. A. Rakotondrainibe (LAM),  N. R. Tanvir (U. Leicester), L. Izzo (INAF-OACn & DARK/NBI), S. Geier (GTC), G. Lombardi (GTC), J. P. U. Fynbo (DAWN/NBI), A. Martin-Carrillo (UCD),  A. Garcia Rodriguez (GTC), A. Perez Romero (GTC) report,

We observed the afterglow of GRB 240912A (Fermi GBM team GCN 37465; Evans et al. GCN 37466) using OSIRIS+ on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain). The observation consisted in 2 acquisition images in r-band, followed by a single 600 s spectra obtained just before the twilight became too bright to continue with observations. The spectrum was obtained with grism R1000B, covering the spectral range between 3650 and 7800 AA at a resolving power of 600. 

The acquisition image, obtained at 2024-09-12T05:34:23 UT, 3.798 hrs after the burst, shows the afterglow at r(AB) = 19.15 +/- 0.08 mag, as compared to SDSS field stars. The spectrum has a strong continuum throughout the complete range with absorption features due to AlII, AlIII, FeII, MnII, MgII and MgI at a common redshift of 1.234, which we propose as the redshift of GRB 240912A.