TITLE: GCN CIRCULAR NUMBER: 42939 SUBJECT: EP251202a / GRB 251202A: NOT spectroscopic redshift z = 2.785 DATE: 25/12/02 06:18:17 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> Z.P. Zhu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OACN and DARK/NBI), A. de Ugarte Postigo (LAM), D. Xu, X. Liu, S.Q. Jiang, J. An, L.B. He (NAOC), A. Saccardi (CEA/Irfu), R. H. Rasmussen (NOT and Aarhus) report on behalf of a large collaboration: We observed the optical counterpart (Li et al., GCN 42934) of EP251202a (Zhang et al., GCN 42937), which is likely the same event of Fermi GRB 251202A (Fermi GBM team, GCN 42933), using the Nordic Optical Telescope (NOT), equipped with the ALFOSC camera and spectrograph. We obtained 10x120 s and 9x120 s exposures in the r and z bands, respectively, starting on 2025 Dec 2 at 02:51:32 UT. The afterglow is clearly detected in single images, with an AB magnitude of r = 17.27 +/- 0.01 at a mid time of 1.05 hr after the trigger. A sequence of 3 spectra by 1200 s each was then acquired using grism #4, covering the wavelength range 3500-9500 AA. Our first spectrum started on 2025 Dec 2.156 UT (1.93 hr after the EP/WXT trigger). Continuum is detected across the whole wavelength range. A spectral break is detected around 4610 AA, and the onset of the forest is seen blueward of this wavelength. We match existing features to a low-column density Lyman alpha and C IV (unresolved) at z = 2.785, which we believe to be the redshift of the burst. Intervening systems are also seen in Lyman alpha and C IV at z = 2.450 and 2.356. We acknowledge the use of the grbspec.eu tool to analyse this spectrum. View this GCN Circular online at https://gcn.nasa.gov/circulars/42939. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...