GCN Circular 29450

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

We observed the afterglow of GRB 210210A (Laha et al., GCN #29444, 
Lipunov et al., GCN #29447) with OSIRIS at the 10.4m Gran Telescopio 
Canarias (Roque de los Muchachos Observatory, La Palma, Canary Islands, 
Spain), starting at 2021-02-10 06:18:28.465 UT (4.300 hours after the 
trigger). The source is clearly detected. We obtained 3 x 900 s 
spectroscopy with the R1000B grism, covering the spectral range from 
3700 to 7800 AA.

 From the 30 s image starting at 2021-02-10 06:14:20.350 (0.1763 days 
after the trigger), we measure r' = 18.84 +/- 0.03 mag (AB) vs. a nearby 
PanSTARRS star. We note this indicates a slow decay compared with the 
initial Swift detection (Laha et al., GCN #29444). Further photometric 
follow-up is encouraged.

The spectrum shows a strong continuum over the full spectral range with 
several absorption lines. We identify these as FeII, MgII, and MgI, at a 
mean redshift of z = 0.715. At this redshift, we also detect OII in 
emission, which allows us to identify this as the redshift of the GRB. 
We also find an intervening system in MgII and MgI at z = 0.481.