TITLE:   GCN CIRCULAR
NUMBER:  35599
SUBJECT: GRB 240122A: VLT/X-shooter redshift confirmation
DATE:    24/01/23 09:17:34 GMT
FROM:    Andrea Saccardi at Observatoire de Paris <andrea.saccardi@obspm.fr>

A. Saccardi (GEPI, Observatoire de Paris), B. Sbarufatti (INAF-OAB), D. B. Malesani (DAWN/NBI & Radboud Univ.), L. Izzo (INAF-OACn & DARK/NBI), A. de Ugarte Postigo (CNRS/OCA & LAM), S. Campana (INAF-OAB), A. J. Levan (Radboud Univ. & Warwick Univ.), D. Hartmann (Clemson University) report on behalf of the Stargate collaboration:

Following the discovery of GOTO24eu/AT2024apy (Kumar et al. 2024, GCN 35596) within the error box of MAXI/GSC GRB 240122A (Negoro et al. 2024, GCN 35593), we acquired a spectrum of the optical counterpart candidate using the ESO/VLT/UT3 (Melipal) equipped with the X-shooter spectrograph. It covers the wavelength range 3000-21000 AA, and consists of 4 exposures of 1200 s each. Observations started at 02:25:37 UT on Jan 23 2024 (~16 hr after the trigger).

We detect a continuum over the entire wavelength range. From the detection of a rather weak Lya absorption at ~5060 AA and multiple absorption features, which we interpret as being due to SiII, CII, MgII, SiIV and CIV, we infer a common redshift of z = 3.162 (consistent with Thoene et al. 2024, GCN 35598). We conclude this is the redshift of the burst. We also note the presence of additional absorption features likely due to multiple intervening systems, one of them very strong at z = 2.757 with a broad Lya at ~4570 AA. 

We acknowledge the excellent support from the ESO observing staff in Paranal, in particular Felipe Gaete and Jonathan Smoker.