TITLE:   GCN CIRCULAR
NUMBER:  22272
SUBJECT: GRB 171222A: Redshift from 10.4m GTC/OSIRIS
DATE:    17/12/23 07:06:51 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), L. Izzo (HETH/IAA-CSIC), 
D.A. Kann (HETH/IAA-CSIC), C.C. Thoene (HETH/IAA-CSIC), D. A. Perley 
(LJMU), D. Malesani (DARK/NBI), K.E. Heintz (Univ. Iceland, DARK/NBI), 
D. Garcia (GTC) report on behalf of a larger collaboration,

We observed the afterglow of GRB 171222A (Racusin et al. GCN 22265; 
Osborne et al. GCN 22266; Perley et al. GCN 22269) using the 10.4m GTC 
telescope equipped with OSIRIS. Observations consisted of 3x900s exposures 
using grism R1000B, covering the range from 3700 to 7880 AA. The mean time 
of the combined spectrum is 3:11 UT (10.77 hr after the burst).

The combined spectrum shows a power-law continuum over the complete 
spectral range with several weak absorption features. We identify the strongest 
of them as Ly-alpha at a redshift of 2.409, with marginal detections of SiII, SiIV 
and CIV at the same redshift, which we identify as the redshift of the GRB. 
An intervening system at z=1.678 is also identified through the detection of MgII 
absorption.