TITLE:   GCN CIRCULAR
NUMBER:  29806
SUBJECT: GRB 210411C: VLT/X-shooter redshift
DATE:    21/04/12 10:46:55 GMT
FROM:    Daniele B Malesani at DTU Space  <malesani@space.dtu.dk>

A. de Ugarte Postigo (HETH/IAA-CSIC and DARK/NBI), B. Sbarufatti (PSU), 
D. Xu (NAO/CAS), J.-B. Vielfaure (APC, Paris University), D. B. Malesani 
(DTU Space), D. A. Kann (HETH/IAA-CSIC) V. D'Elia (ASI/SSDC, INAF/OAR), 
P. J. Pessi (FCAG/ESO), report on behalf of the Stargate consortium:

We observed the optical afterglow of GRB 210411C (Sbarrato et al., GCN 
29794) 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 1200 s each (8x600 s in the NIR). The 
observation mid time was 2021 Apr 12.33 UT (16.9 hr after the GRB).

In a 60 s image taken with the acquisition camera on Apr 12.294 UT, we 
detect the optical afterglow, for which we measure an AB magnitude r = 
21.69 +- 0.04 (calibrated against nearby stars from the SkyMapper 
catalog; Wolf et al. 2018, PASA, 35, 010; 
https:doi.org/10.4225/41/593620ad5b574).

We clearly detect continuum over the wavelength range 3520 - 17500 AA. A 
trough is visible around 4650 AA, which we identify as due to H I. From 
the detection of several narrow absorption features, which we interpret 
as due to, among others, Si II, Si II*, O I, C II, Si IV, C IV, Fe II, 
Al II, we infer a redshift z = 2.826. We also identify the FeII* 2396 
fine-structure transition, though at low S/N.

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

[GCN OPS NOTE(12apr2021):  Per author's request, GRB in the SUBJECT line
was changed from 210411A" to the correct name "210411C".]