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GCN Circular 29685

Subject
GRB 210321A: Redshift from OSIRIS/GTC
Date
2021-03-21T22:57:51Z (3 years ago)
From
Antonio de Ugarte Postigo at IAA-CSIC <deugarte@iaa.es>
A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D.A. Kann, 
C. Thoene, M. Blazek, J.F. Agui Fernandez (all HETH/IAA-CSIC), 
S. Geier (GTC, IAC), A. Tejero (GTC) report:

We observed the afterglow of GRB 210321A (Dichiara et al. GCN 
29677; Perri et al. GCN 29678; Siegel et al. GCN 29679; Butler et 
al. GCN 29680; Heintz et al. GCN 29684) with OSIRIS at the 
10.4m GTC telescope at Roque de los Muchachos Observatory, 
in La Palma (Spain). The observation started at 20:28 UT (0.5517 
days after the burst), and consisted in 3x900s with grism R1000B, 
covering the spectral range from 3700 to 7800 AA.

The 30s acquisition image, with a seeing of 0.75���, shows the 
afterglow at a magnitude of r��� = 21.39+/-0.03, using a nearby 
PanSTARRS field star as photometric reference.

The spectral continuum is well detected above 3900 AA, and 
shows very weak lines consistent with several features of FeII, MgII 
and MgI at a common redshift of z = 1.487, which we assume to be 
the likely redshift of the GRB. However, we note that strictly 
speaking this would be a lower limit, with the upper limit being z ~< 2, 
as already noted by the NOT spectroscopy (Heintz et al. GCN 29684).
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