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GCN Circular 28739

GRB 201021C: VLT/X-shooter redshift
2020-10-22T08:45:59Z (4 years ago)
Paolo D'Avanzo at INAF-OAB <>
J.-B. Vielfaure (GEPI - Paris Observatory), D. Xu (NAO/CAS), J. Palmerio 
(GEPI - Paris Observatory), P. D'Avanzo (INAF-OAB), D. B. Malesani (DTU Space), 
J. P. U. Fynbo (DAWN/NBI), V. D'Elia (ASI/SSDC), D. A. Kann (HETH/IAA-CSIC) and 
N. R. Tanvir (Univ. Leicester) report on behalf of the Stargate collaboration:

We observed the optical afterglow of GRB 201021C (Lien et al., GCN 
28730; Lipunov et al., GCN 28733) using the ESO VLT UT3 (Melipal) equipped 
with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, 
and consist of 4 exposures by 600 s each. The observation started at 03:29:00 UT on 
2020 Oct 22 (7.02 hr after the GRB).

In a 30 s image taken with the acquisition camera on Oct 22.14 UT, we detect the optical 
afterglow, for which we measure a magnitude r = 19.97 +/- 0.02 (AB, calibrated against 
nearby stars from the SkyMapper catalog; Wolf et al. 2018, PASA, 35, 010).

We clearly detect continuum over the whole X-shooter spectrum.
From detection of multiple absorption features, which we interpret as 
due to Si II, Al II, Al III, Fe II and Mg II, we infer a redshift z = 1.070.
In particular, weak fine structure lines from Fe II* are visible, which 
confirm the association of the absorption system with the GRB. 
We also detect Mg II and Mg I absorption features from an intervening 
system at z = 0.805. 

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Bin Yang and Fernando Selman.
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