GCN Circular 29853
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.