VLT/X-shooter redshift of 1.466 for ZTF21aagwbjr/AT2021buv, strengthening its association with GRB 210204A

We observed the fast optical transient ZTF21aagwbjr/AT2021buv (Kool et al., GCN 29405), likely associated with the Fermi, GECAM, and IPN GRB 210204A (The Fermi GBM team, GCN 29390; F. Kunzweiler et al., GCN 29391; Li et al., GCN 29392; Hurley et al., GCN 29408) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Spectroscopic observations started at 01:38 UT on 06 February 2021, i.e. ~1.8 days after the burst detection and they consisted of 4 x 600 s exposures. In the resulting spectrum, we find a series of strong absorption lines that we identify as Mg II, Si IV, C IV, and Ca II at a common redshift of 1.466. At this redshift, we also detect emission lines of [O II], [O III] and H-alpha Balmer line. We therefore conclude that z = 1.466 is the redshift of ZTF21aagwbjr/AT2021buv. The measured redshift implies a high intrinsic brightness (absolute magnitude ~ 28 at an epoch of 45 min after the GRB; Kool et al., GCN 29405), which is typical of long GRB afterglows. This, coupled with the observed absorption lines set, typical of long GRB environments, further strengthens the association between ZTF21aagwbjr/AT2021buv and GRB 210204A. Further analysis is undergoing. We acknowledge support from the ESO observing staff at Paranal, in particular Zahed Wahhaj.