GCN Circular 21050

Jan Bolmer (ESO, Chile), Helmut Steinle and Patricia Schady (both MPE Garching) report:

We observed the field of GRB 170428A (trigger=750298, Beardmore et al., GCN #21042)
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at
the 2.2 m MPG telescope at the ESO La Silla Observatory (Chile).

Observations started at 09:57 UT on 2017-04-28, 43 min after the GRB trigger, and they
were performed at an average seeing of 1.2'' and at an average airmass of 2.2. We detect an
uncatalogued source within the XRT error circle reported by Beardmore et al. (GCN #21047) 
at the position of RA, Dec = 330.07823, 26.91584, which is equivalent to:

RA (J2000): 22h 00m 18.78s
Dec (J2000): +26d 54' 57.0''

with an uncertainty of 0.3 arcsec.

Based on combined images with 17 min of total integration time in g'r'i'z' and 19 min in JHK
at a mid-time of 10:15 UT on 2017-04-28, this source has the following preliminary
magnitudes (all in the AB system):

g' = 21.4 +/- 0.1 mag
r' = 21.3 +/- 0.1 mag
i' = 21.1 +/- 0.1 mag
z' = 21.1 +/- 0.1 mag
J = 20.7 +/- 0.2 mag
H = 20.9 +/- 0.4 mag
K > 20.0 mag

The given magnitudes are derived based on calibrating the images against SDSS in the optical
and 2MASS field stars in the NIR bands. They are not corrected for the Galactic foreground
extinction corresponding to a reddening of E_(B-V)= 0.05 in the direction of the burst (Schlafly
& Finkbeiner 2011).

As noted by Xu et al. (GCN #21048), there already exists a single source with a brightness of
r ~ 22.2 mag, presumably the host galaxy, in the SDSS and Pan-STARRS archival data. We
therefore likely observed the NIR/optical afterglow of the short GRB 170428A.