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GCN Circular 29411

Subject
VLT/X-shooter redshift of 1.466 for ZTF21aagwbjr/AT2021buv, strengthening its association with GRB 210204A
Date
2021-02-06T11:15:41Z (3 years ago)
From
Maryam Arabsalmani at University of Melbourne <mar.arabsalmani@gmail.com>
L. Izzo (DARK/NBI), J. P. U. Fynbo (DAWN/NBI), D. A. Kann (HETH/IAA-CSIC),
S. D. Vergani (GEPI, Observatoire de Paris), D. B. Malesani (DTU Space), M.
Arabsalmani (ICRAR/UWA), A. Rossi (INAF-OAS), G. Pugliese (API, Univ.
Amsterdam), D. Xu (NAOC), report on behalf of the Stargate consortium:

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.
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