GCN Circular 42086
Subject
GRB 251003A: GTC/OSIRIS+ spectroscopic redshift z = 4.412
Event
Date
2025-10-03T10:34:36Z (5 days ago)
From
Ruben Sanchez-Ramirez at IAA-CSIC <ruben@iaa.es>
Via
Web form
R. Sanchez-Ramirez, A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), Y.-D. Hu (GXI), R. Scarpa (GTC, IAC), D. Gonzalez (GTC), A. Cabrera-Lavers (GTC, IAC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), J. Becerra-Gonzalez (IAC), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report:
Following the detection of GRB 251003A by Fermi (Fermi Team, GCN 42069) and Swift (Beardmore et al. GCN 42070, Evans et al. GCN 42071), we observed the optical afterglow (Aceituno et al., GCN 42072, Moreno Mendez et al., GCN 42073, Strausbaugh et al., GCN 42074) 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 consisted of 2x900s exposures using grism R1000B, and 1x900s using R2500I, covering the spectral range between 3,600 and 10,000 A. The observations started on Oct 3, 03:12 UT (i.e. 1.22 h after the burst trigger).
From a preliminary reduction, we find a strong DLA at ~6500A with its corresponding Lyman break at ~5000A. Several metal absorption lines are detected as well, which we interpret as coming from SiII, SiII*, CII, SiIV, CIV, FeII, FeII*, NiII, AlII, AlIII, all at z=4.412. The detection of fine structure lines links this system to the GRB.