TITLE:   GCN CIRCULAR
NUMBER:  41955
SUBJECT: GRB 250920C: VLT redshift confirmation z = 1.399
DATE:    25/09/23 16:00:11 GMT
FROM:    Valentina Abril-Melgarejo at Obs. Paris/LUX <valentina.abril-melgarejo@obspm.fr>

L. Izzo (INAF/OACn and DARK/NBI), D. Xu (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), V. D’Elia (ASI/SSDC), report on behalf of the Stargate collaboration:

We observed the optical afterglow (Wortley et al., GCN 41907) of GRB 250920C (Gupta et al., GCN 41904), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. The mid time of our observation was Jan 21.209 UT (10.41 hr after the GRB trigger), and consisted of 4 exposures of 19 min each.

We extracted a 1D spectrum using a circular aperture of 0.4 arcsec radius, centered on the afterglow emission. In the reduced spectrum, which covers the wavelength range 4750 - 9330 AA, we detect clear continuum and a number of absorption features, which we identify, among others, as Mg II, Mg I, Fe II, Fe II*, Ni II*, Cr II, Zn II, Mn II, all at a common redshift z = 1.399. The detection of Fe II fine-structure lines cements the association of the GRB with this absorption system. Furthermore, an emission line consistent with [O II] at the same redshift is also detected, likely from the underlying host galaxy (Wortley et al., GCN 41907).

Our result is thus fully consistent, and strengthens, the previous reports by GTC (Sanchez-Ramirez et al., GCN 41928) and SALT (De Wet et al.. GCN 41952).

We acknowledge expert support from the observing staff in Paranal, in particular Boris Haeussler, Enrico Congiu, Ana Jimenez Gallardo, and Marcelo Lopez.