[vsnet-grb-info 28254] GRB 210702A: VLT/X-shooter redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30357
SUBJECT: GRB 210702A: VLT/X-shooter redshift
DATE:    21/07/03 03:58:49 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu at nao.cas.cn>

D. Xu (NAOC), L. Izzo (DARK/NBI), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), D. A. Kann (HETH/IAA-CSIC), S. Y. Fu (NAOC), S. Campana 
(INAF-OAB), V. D'Elia (ASI/SSDC, INAF/OAR), A. J. Levan (Radboud U. 
Nijmegen), B. Milvang-Jensen (DAWN/NBI), D. A. Perley (LJMU), G. 
Pugliese (API, Univ. Amsterdam), R. L. C. Starling (Univ. Leicester), N. 
R. Tanvir (Univ. Leicester), S. Vergani (CNRS - Observatoire de 
Paris/GEPI), and K. Wiersema (Univ. Warwick) report on behalf of the 
Stargate Consortium:

We observed the bright afterglow (Lien et al., GCN #30351; Lipunov et 
al., GCN #30352; Groot et al., GCN #30354; Kuin et al., GCN #30356) of 
GRB 210702A (Lien et al., GCN #30351) using the ESO VLT UT3 (Melipal) 
equipped with the X-shooter spectrograph. Our spectra cover the 
wavelength range 3000-25000 AA, and consist of 4 exposures of 600 s 
each. The observation mid-time was 2021 July 2.997 UT (i.e., 4.80 hr 
after the GRB).

In a 20 s image taken with the acquisition camera on July 02.975 UT, we 
detect the optical afterglow at r' = 16.91 +/- 0.01 mag (AB system), 
measured against three SkyMapper field stars.

In our spectrum, strong continuum is detected over the whole observed 
range. We detect a plethora of metallic absorption features, among them 
Si II, Si II*, C IV, Fe II, Fe II*, Al II, Ni II, Ni II*, Al III, Zn II, 
Mg I, Cr II, Mn II, Mg II, Mg I, and Ca II, all at a common redshift of 
z = 1.160. This is about 1.5 sigma lower than, but is broadly consistent 
with the value reported by Swift/UVOT (Kuin et al., GCN #30355). No 
emission feature is present in the spectrum. The detection of excited 
transitions establishes this as the redshift of the GRB.

There is also an intervening system detected at z = 0.538 showing Mg II 
and Mg I absorption features.

We acknowledge excellent support from the observing staff at Paranal, in 
particular Alain Smette and Bin Yang.