GCN Circular 28717

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), M. Blazek, J. F. Agui Fernandez, C. C. Thoene (all 
HETH/IAA-CSIC), G. Lombardi (GRANTECAN, IAC) and A. Tejero (GRANTECAN) 
report:

We observed the afterglow (Ambrosi et al., GCN#28696; Adachi et al., 
GCN#28697; Fu et al., GCN#28701; Belkin et al., GCN#28703; Pankov et 
al., GCN#28708; Kumar et al., GCN#28711; Paek et al., GCN#28713) of GRB 
201020A (Ambrosi et al., GCN#28696; Sakamoto et al., GCN#28705; Bissaldi 
et al., GCN#28706) with the 10.4m GTC telescope, at Roque de los 
Muchachos Observatory (La Palma, Spain) equipped with OSIRIS. The 
observation started on 20 October 2020 at 20:08 UT (14.34 hrs after the 
GRB onset) and consisted of 4 x 900s with the R1000B grism, covering the 
wavelength range between 3700 and 7800 AA. Observations were taken under 
inclement conditions, with bad seeing and thick clouds passing through.

The spectrum shows a clear continuum, consistent with the Lyman forest, 
Ly-alpha, SiII, OI, CII, SiIV, CIV, FeII, FeII*, and AlII, all at a 
common redshift of z = 2.903, which we identify as the redshift of the 
GRB.