TITLE: GCN CIRCULAR NUMBER: 42076 SUBJECT: GRB 251002A: VLT/X-shooter spectroscopic redshift z = 2.178 DATE: 25/10/03 06:54:12 GMT FROM: Andrea Saccardi at CEA/Irfu andrea.saccardi@cea.fr
A. Saccardi (CEA/Irfu), N. Habeeb (U. Leicester), A. de Ugarte Postigo (LAM), B. Schneider (LAM), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), S. D. Vergani (LUX-Paris Obs.), B. C. Rayson (Leicester), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:
We observed the optical counterpart (Palmerio et al., GCN 40061; Turpin et al., GCN 40062; Jelinek et al., GCN 40063; Juliá-Maroto et al. GCN 42064; Moskvitin et al. GCN 42065; Perez-Garcia et al. GCN 42066) of the SVOM/ECLAIRs GRB 251002A (Saccardi et al., GCN 42060) using the ESO/VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
From the 3x30 s acquisition image at 2025-10-03 04:55:20 UT (8.67 hr post-trigger), the optical afterglow is well detected with a magnitude of r = 20.74 +/- 0.12 (AB) calibrated against PanSTARRS nearby stars and not corrected for Galactic extinction.
Our spectra cover the wavelength range 3000-21000 AA and consist of 4 exposures of 600 s each. Observations started on 2025 October 3 at 05:04:11 UT (8.82 hr after the burst).
In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range. From the detection of multiple absorption features, including Lya, S II, Si II, Si II*, O I, O I*, C II, C II*, Si IV, C IV, Fe II, Al II, Zn II, Cr II, Ni II*, Fe II*, Mg II, and Mg I we infer a redshift of z = 2.178.
We acknowledge the prompt response and excellent support of the ESO observing staff in Paranal, in particular Aaron Labdon, Jonathan Smoker and Rodrigo Palominos. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
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