TITLE:   GCN CIRCULAR
NUMBER:  21374
SUBJECT: GRB 170728B: NOT optical afterglow confirmation
DATE:    17/07/29 02:14:13 GMT
FROM:    Kasper Elm Heintz at Univ. of Iceland and DARK/NBI  <heintz@dark-cosmology.dk>

K. E. Heintz (Univ. Iceland and DARK/NBI), N. R. Tanvir (Univ. Leicester), D. A. Perley (LJMU) and R. T. Rasmussen (NOT) report on behalf of a larger collaboration:

We observed the field of the short GRB 170728B (Cenko et al., GCN 21371) using the ALFOSC instrument equipped at the 2.56-m Nordic Optical Telescope (NOT). We obtained a single image with the SDSS r-band consisting of a 60 s exposure at 00:10:53 UT on 29 July, 2017  (i.e., 67.3 minutes after the burst). We confirm the presence of the optical afterglow (P. D'Avanzo et al., GCN 21372/21373) and from the single image we measure a magnitude of r(AB) = 20.9 +/- 0.1 mag calibrated against nearby stars from the Pan-STARRS survey. 

In addition, we obtained spectroscopy of the afterglow consisting of 3x1200 s exposures using grism no. 4, covering a spectral wavelength range of 3200 — 9600 Å, and a slit width of 1.3 arcsec. The trace is faint and at this point we can not robustly determine a redshift. A separate GCN will be submitted later based on the reduced spectroscopic data.

The observations were carried out under good weather conditions at an airmass of ~ 1.6 and a measured seeing of 0.74 arcsec.