TITLE:   GCN CIRCULAR
NUMBER:  29944
SUBJECT: GRB 210504A: VLT X-shooter redshift
DATE:    21/05/05 07:36:20 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. Xu (NAOC), P. Schady (Univ. Bath), K. E. Heintz (Univ. Iceland and 
DAWN/NBI), D. B. Malesani (DTU Space), B. Sbarufatti (PSU), G. Pugliese 
(API, Univ. Amsterdam), D. A. Perley (LJMU), V. D'Elia (ASI/SSDC, 
INAF/OAR), D. A. Kann (HETH/IAA-CSIC), and J. P. U. Fynbo (DAWN/NBI/DTU) 
report on behalf of the Stargate consortium:

We observed the optical afterglow (Breeveld & Beardmore, GCN 29933; 
Heintz et al., GCN 29937) of GRB 210504A (Beardmore et al., GCN 29929; 
Lien et al., GCN 29330) 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 of 1200 s each (8 x 600 s in 
the NIR). The observation mid-time was 2021 May 05.06 UT (11.6 hr after 
the GRB).

In a 60 s image taken with the acquisition camera on May 05.02 UT, we 
detect the optical afterglow, for which we measure an AB magnitude r' = 
21.11 +- 0.04 mag (calibrated against nearby stars from the Pan-STARRS 
catalog).

We clearly detect continuum over the wavelength range of the entire 
spectrum. A trough is visible around 3740 AA, which we identify as due 
to H I. From the detection of several absorption features, which we 
interpret as due to Si II, C II, C IV, Fe II, Al II, Mg II, among 
others, as well as the Lyman forest, we infer a redshift z = 2.077.

We acknowledge excellent support from the observing staff at Paranal, in 
particular Steffen Mieske, Ditte Slumstrup and Diego Parraguez.