TITLE: GCN CIRCULAR NUMBER: 21059 SUBJECT: GRB 170428A: GTC spectroscopic redshift of candidate host galaxy DATE: 17/05/02 17:43:47 GMT FROM: Zach Cano at U of Iceland ​​L. Izzo, Z. Cano (IAA-CSIC), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D. A. Kann, C. Thoene (IAA-CSIC), and S. Geier (IAC, GRANTECAN) report on behalf of a larger collaboration: We observed the field of short-duration GRB 170428A (Beardmore et al., GCN Circ. 21042) with OSIRIS mounted on the 10.4-m GTC telescope on La Palma (Spain) starting at 04:31:49 UT on 01 May 2017, at a mean time of +2.82 days from the GRB detection. The observations consisted of 4 x 900 s spectra using grism R1000R, which covers the wavelength range between 5100 and 10000 AA. In our r-band acquisition image, (30 second exposure time), we see the host candidate of Xu et al. (GCN Circ. 21048), which is also slightly visible in the r-band Pan-STARRS image. The slit was centred on the host galaxy candidate, and in the faint trace we detect emission lines corresponding to Halpha, [OIII] 5007, Hbeta, and [OII] 3726,3729, all at a common redshift of z=0.454. The angular separation between the candidate host and the GRB position determed by GROND (Bolmer et al., GCN Circ. 21050) is 1.2'' +- 0.3'', which at this redshift implies an offset of 6.9 kpc +- 1.7 kpc. GTC finder chart of the AG position and candidate host: http://www.iaa.es/~deugarte/GRBs/170428A/GTC.jpeg