GCN Circular 42900
Subject
GRB 251129A / EP251129a: GTC/OSIRIS+ spectroscopic redshift z = 1.460
Event
Date
2025-11-29T20:56:25Z (4 days ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
R. Sanchez-Ramirez, A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, I. Perez-Garcia, E. Fernandez-Garcia and S. Wu (IAA-CSIC), A.M. Garcia Rodriguez (IAC), D. Gonzalez (GTC), C. Hernandez (GTC), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXI) and L. Piro (INAF), on behalf of a larger collaboration, report:
Following the detection of GRB 251129A by SVOM/ECLAIRs (Tan et al., GCN 42877, GCN 42879), Einstein Probe (EP Team et al., GCN 42884), and by additional Swift ToO observations (Evans, GCN 42888), we observed the optical afterglow detected by Angulo et al. (GCN 42880) and further reported (Watson, GCN 42883, Wu et al., GCN 42882, Masi, GCN 42885, Li et al., GCN 42887, Lipunov et al. GCN 42892, Moskvitin et al., GCN 42897 and Guziy et al., GCN 42899), 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 data consisted of 900s exposures using grisms R1000B and R2500I, covering the spectral range between 3,600 and 10,000 A. The observations started on Nov 29, 05:32 UT (i.e. 3.01 h after the burst trigger), and were carried out at high airmass (X~2). From a preliminary reduction, we find a highly absorbed continuum, showing several metal absorption with complex velocity structure. We interpret these lines as coming from CrII, FeII, FeII*, MgII, NiII, SiII, ZnII, all at z=1.460. The detection of iron fine structure lines links this system to the GRB.