TITLE:   GCN CIRCULAR
NUMBER:  21240
SUBJECT: GRB 170607A: Redshift from GTC/OSIRIS
DATE:    17/06/10 07:33:46 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC), 
C.C. Thoene (IAA-CSIC), Z. Cano (IAA-CSIC), D. A. Kann (IAA-CSIC), 
R. Scarpa (GTC), and V. Herrera (GTC) report on behalf of a larger 
collaboration:

We observed the afterglow of GRB 170607A (D’Ai et al. GCN 21214, 
Heintz et al. GCN 21217, Lipunov et al. GCN 21219, Volnova et al. 
GCN 21237) using OSIRIS at the 10.4m GTC telescope. Observation 
consisted of 3x900s using grism R1000R, which covers the range 
between 5100 and 10100 Å, at a mean epoch of 10.1955 June 2017 
UT (2.2222 days after the GRB).

The acquisition image still shows a bright afterglow at r(AB) = 20.70 
+/- 0.05 mag, as compared to SDSS field stars. In spite of the strong 
contribution from the afterglow, no clear absorption features can be 
detected in this spectral range. However, there are strong emission 
lines due to [OII], [OIII], and H-beta at a common redshift of 0.557, 
which we identify as the redshift of the GRB.