GCN Circular 43204
Subject
GRB 251222A / EP251222b: VLT/X-shooter spectroscopic redshift z = 3.171
Event
Date
2025-12-23T05:41:55Z (3 days ago)
From
Andrea Saccardi at CEA/Irfu <andrea.saccardi@cea.fr>
Via
Web form
A. Saccardi (CEA/Irfu), A. L. Thakur (INAF-IAPS), Z.P. Zhu, J. An (NAOC), D. B. Malesani (DAWN/NBI and Radboud), L. Cotter, A. Martin-Carrillo (UCD), report on behalf of the Stargate collaboration:
We observed the optical/NIR counterpart (Palmerio et al., GCN 43189; An et al., GCN 43194; Calapai et al., GCN 43195; Durbak et al., GCN 43197; Li et al., GCN 43203) of the GRB 251222A / EP251222b detected by Fermi/GBM (Fermi GBM team, GCN 43186), SVOM/ECLAIRs (Yang et al., GCN 43188), EP/WXT (Guo et al., GCN 43201) and SVOM/GRM (Wang et al., GCN 43202) using the ESO/VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
Our spectra cover the wavelength range 3000-21000 AA and consist of 2 exposures of 600 s each. Observations started on 2025 December 23 at 03:38:43 UT (10.55 hr after the burst).
In a 15-s acquisition image secured in the r band (10.45 hr after trigger), we measure r = 19.91 +/- 0.04 (AB), calibrated against nearby stars from Pan-STARRS.
In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range and a trough due to Lya absorption visible at ~5070 AA. From the detection of multiple absorption features, including S II, Si II, Si II*, O I, O I*, Ni II, Si IV, C IV, Fe II, Fe II*, Al II, Al III, Cr II, Ni II*, Mg II, and Mg I we infer a redshift of z = 3.171. At the same redshift we also detect absorption features consistent with Ly-beta, Ly-gamma, and the Lyman-limit.
We acknowledge the excellent support of the ESO observing staff in Paranal. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).