TITLE:   GCN CIRCULAR
NUMBER:  39071
SUBJECT: GRB 250129A: VLT/X-shooter redshift of z = 2.151
DATE:    25/01/29 07:24:10 GMT
FROM:    Benjamin Schneider at MIT <bschn@mit.edu>

B. Schneider (LAM), A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud Univ.), S. D. Vergani (CNRS, Obs. Paris/LUX), N. R. Tanvir (U. Leicester), V. D’Elia (ASI-SSDC and INAF-OAR), A. de Ugarte Postigo (CNRS, OCA and LAM), L. Izzo (INAF-OACn and DARK/NBI) report on behalf of the Stargate collaboration:

We observed the optical afterglow (Francile et al., GCN 39065) of GRB 250129A (Beardmore et al., GCN 39066) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Observations were performed via the rapid-response mode (RRM) system. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4x600 s. The observation mid time was 2025 Jan 29.2528 UT (1.315 hr after the GRB).

In a 5 s image taken with the acquisition camera on Jan 29.237 UT, we detect the optical afterglow, for which we measure a magnitude r =16.72 +- 0.02 AB (calibrated against nearby stars from the Pan-STARRS catalogue).

In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range. From the detection of a broad Lya absorption at ~3830 AA and multiple absorption features, which we interpret as due to N V, Si II, Si II*, O I, O I*, C II, C II*, Al II, Si IV, C IV, Fe II, Fe II*, Mg II, we infer a common redshift of z = 2.151.

We notice the presence of at least one intervening system at z = 1.89, as identified by an additional Lya absorption and multiple absorption lines (C II, Si II, Fe II, Al II, and Mg II).

We acknowledge expert support from the ESO staff in Paranal, in particular Abel de Burgos, Celia Desgrange, Luca Sbordone, Robert Klement, Rodrigo Palomino, and Sam Kim.