[vsnet-grb-info 19773] GRB 170903A: Host galaxy redshift from OSIRIS/GTC

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21799
SUBJECT: GRB 170903A: Host galaxy redshift from OSIRIS/GTC 
DATE:    17/09/03 23:16:16 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte at iaa.es>

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), L. Izzo (HETH/IAA-CSIC)
C.C. Thoene (HETH/IAA-CSIC), D.A. Kann (HETH/IAA-CSIC), 
K.E. Heintz (DARK/NBI), N. Castro-Rodriguez (GRANTECAN, IAC, ULL), 
A. Marante (GRANTECAN) report on behalf of a larger collaboration:

We have observed the field of GRB 170903A (Markwardt et al., GCN 21793) 
with OSIRIS at the 10.4m GTC telescope. Observations started at 20:49 UT 
(8.0 hrs after the burst). Observations consisted of an acquisition image in 
i-band and 3x900s spectra with the R1000B grism, which covers the range 
between 3700 and 7880 AA.

The acquisition image shows a single source within the refined XRT error box
(Goad et al., GCN 21797), with an i-band magnitude (AB) of 23.17 +/- 0.15. 
The object appears unresolved at a seeing of 0.6”. Spectroscopy of it shows a 
weak trace covering the complete range above 4000 AA and a single, very 
prominent emission feature. Due to the strength of this feature and the 
non-detection of any other features in the spectral range, we identify it as the 
unresolved [OII] 3727/3729 doublet at a redshift of 0.886, which we propose 
as the redshift of the host galaxy and hence of the GRB.