TITLE:   GCN CIRCULAR
NUMBER:  38820
SUBJECT: GRB 250103B: VLT/X-shooter spectroscopic redshift z = 1.416
DATE:    25/01/04 10:20:17 GMT
FROM:    Andrea Saccardi at CEA/Irfu <andrea.saccardi@cea.fr>

N. Habeeb (Leicester), A. Saccardi (CEA/Irfu), L. Izzo (INAF/OACn and DARK/NBI), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), G. Pugliese (UvA), A. L. Thakur (INAF-IAPS), A. de Ugarte Postigo (CNRS, OCA and LAM), Z.P. Zhu, J. An, D. Xu (NAOC) report on behalf of the Stargate collaboration:

We observed the optical afterglow (Qiu et al., GCN 38813) of GRB 250103B (Evans et al., GCN 38796; Bouchet et al., GCN 38797) 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 1200 s each. The observation mid time was 2025 Jan 4.1283 UT (11.341 hr after the GRB).

In a 60-s acquisition image taken with the z filter (mid time Jan 4.104 UT), we clearly detect the afterglow (Qiu et al., GCN 38813). We measure the following coordinates (J2000):

RA = 03:38:39.335
Dec = -33:44:56.80

with an uncertainty of ~0.4". We measure a magnitude z = 22.5 +- 0.2 AB, calibrated against a single star in common with the Legacy Survey.

In a preliminary reduction of the spectra, we detect a continuum over the entire wavelength range. From the detection of multiple absorption features, which we interpret as due to Mg II and Fe II, we infer a common redshift of z = 1.416. At a consistent redshift, we also detect emission lines due to Halpha, [O III] 5007 AA and the [O II] 3726, 3729 AA doublet. We conclude that this is the redshift of GRB 250103B.

We note that, at the afterglow position, a faint object is seen in the Legacy survey, only confidently detected in the g band, which is likely the GRB host galaxy.

We acknowledge the expert support from the ESO observing staff at Paranal.