TITLE:   GCN CIRCULAR
NUMBER:  29853
SUBJECT: GRB 210420B: Redshift from GTC
DATE:    21/04/21 01:45:21 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

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

We observed the afterglow of GRB 210420B (Moss et al. GCN 
29844; Lipunov et al. GCN 29845; Oksanen GCN 29846; Hu et al. 
GCN 29847; Sun et al. GCN 29848; Leonini et al. GCN 29851; 
de Ugarte Postigo GCN 29852) with OSIRIS at the 10.4m GTC 
telescope, at the Roque de los Muchachos Observatory (La Palma, 
Spain). Observations consisted of 2x600 s spectroscopy with grism 
R1000B, covering the spectral range between 3700 and 7800 AA.

The spectrum shows a strong continuum with weak absorption 
features that we identify as due to FeII, MgII and MgI at a common 
redshift of z = 1.400, which we interpret as the redshift of the GRB.