TITLE:   GCN CIRCULAR
NUMBER:  38934
SUBJECT: GRB 250114A: VLT/X-shooter spectroscopic redshift z = 4.732
DATE:    25/01/14 10:32:04 GMT
FROM:    Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>

D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), S. D. Vergani (GEPI/Obs. de Paris), P. D’Avanzo (INAF/OABr), A. de Ugarte Postigo (CNRS, OCA and LAM), K. E. Heintz (DAWN/NBI), L. Izzo (INAF/OACn and DARK/NBI), A. J. Levan (Radboud), B. Milvang-Jensen (DAWN/NBI), B. Schneider (LAM), N. R. Tanvir (Leicester), A. L. Thakur (INAF/IAPS) report on behalf of the Stargate collaboration.

We observed the optical afterglow of GRB 250114A (Klingler et al., GCN 38927; An et al., GCN 38929; Malesani & Izzo, GCN 38930; Francile et al., GCN 38931; Brivio et al., GCN 38933) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4 exposures by 600 s each. The observation mid time was 2025 Jan 14.3549 UT (0.962 hr after the GRB).

In a 30 s image taken with the acquisition camera on Jan 14.339 UT, we detect the optical afterglow, for which we measure a magnitude r = 21.58 +- 0.12 AB (calibrated against two nearby stars from the Pan-STARRS catalog).

Continuum is clearly detected in the red end of the VIS arm, showing a strong break at 7000 AA, which we interpret as the Lyman break and the onset of the Lyman-alpha forest. From the detection of multiple absorption features, which we interpret as due to Si II 1260, Si II* 1264, O I 1302, C IV 1548,1540, Si II* 1264, and Mg II 2796,2803 (among others), we infer a redshift of z = 4.732.

An intervening system at z = 1.642 is also identified via the Mg II 2796,2803 doublet.

We acknowledge expert support from the ESO observing staff at UT3, in particular Rodrigo Palominos and Ana Jimenez.