GCN Circular 29132

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

We obtained spectroscopy of the optical counterpart (Malesani et al., 
GCN #29117) of the short GRB 201221D (Page et al., GCN #29112) with 
OSIRIS at the 10.4m Gran Telescopio Canarias (Roque de los Muchachos 
Observatory, La Palma, Spain), starting at 1:52 UT (2.757 hr after the 
burst). The observation consisted of 4 x 1200 s exposures with the 
R1000B grism, covering the range between 3700 and 7800 Angstrom.

The spectrum shows a weak continuum over the full spectral range. There 
is a prominent emission feature that we identify as the [OII] 3727/3729 
doublet at a redshift of 1.046. This line happens to lie in the 
atmospheric A band, and therefore must be quite strong to be detectable. 
There are also low significance absorption featuress coincident with 
FeII and MgII at a similar redshift (z = 1.045). We identify this as the 
redshift of GRB 201221D. We note this is a high redshift for a short 
GRB, and it represents one of the few short GRB spectra to show 
afterglow absorption features.