TITLE:   GCN CIRCULAR
NUMBER:  29432
SUBJECT: GRB 210204A: redshift revision of ZTF21aagwbjr/AT2021buv (correction to GCN 29411)
DATE:    21/02/08 18:20:17 GMT
FROM:    Daniele B Malesani at DTU Space  <malesani@space.dtu.dk>

D. Xu (NAOC), 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), J.-B. Vielfaure (APC), report on 
behalf of the Stargate consortium:

Off-line reanalysis of our data of GRB 210204A (ZTF21aagwbjr/AT2021buv; 
Kool et al., GCN 29405) made us realise that we had used an incorrect 
data set for our previous GCN circular (Izzo et al., GCN 29411). We 
report here the correct analysis and results, and urge users to 
disregard GCN 29411. We regret for any confusion created by this mishap.

Our data of GRB 210204A were acquired with the X-shooter spectrograph 
installed on the ESO VLT UT3 (Melipal) telescope. Four spectra by 600 s 
each were secured, starting on 2021 Feb 6.08 UT (1.79 days after the 
trigger) and covering the wavelength range 3000-21000 AA.

Continuum is clearly detected across the entire trace. A number of 
absorption features are detected; we identify, among others, Al II, Zn 
II, Fe II, Mg II, Mg I, Ca II H and K at a common redshift z = 0.876. In 
particular, fine-structure lines from Fe II* are also visible, which 
confirms the association of this absorption system with the GRB. 
Emission lines due to [O II], [O III], H beta and H alpha are also 
detected at z = 0.876.

We thus conclude that ZTF21aagwbjr/AT2021buv is at redshift z = 0.876.

Three intervening Mg II absorbers are also detected, at redshift 0.71, 
0.66, and 0.57.

The revision of the redshift value does not affect our previous 
conclusion that ZTF21aagwbjr/AT2021buv is the likely afterglow of GRB 
210204A (Fermi GBM Team, GCN 29390; Li et al., GCN 29392; Hurley et al., 
GCN 29408; Waratkar et al., GCN 29410): at z = 0.876, the transient 
reached an absolute magnitude of ~ -26.7 (Kool et al., GCN 29405), 
within the range of long GRB afterglows. The set of absorption features 
(including those due to fine-structure transitons) is also typical of 
GRB environments.

We recompute the isotropic-equivalent gamma-ray energy using the fluence 
reported by Konus-Wind (Frederiks, GCN 29424) and the correct redshift 
value, which turns to be ~8.7*10^52 erg.

We acknowledge the observing staff at Paranal, in particular Zahed Wahhaj.