GCN Circular 21200

P. Schady (MPE) reports:

We observed the field of GRB 170604A (Swift trigger 755867; Page et al.,
GCN #21191) 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 ESO La Silla
Observatory (Chile).

Observations started at 06:15 UT on 05-06-2017, 11.1 hours the GRB
trigger. They were performed at an average seeing of 1.3" and at an
average airmass
of 2.

We detect a point source within the 1.4" enhanced Swift/XRT error circle
(Page et al., GCN 21196
Not found
 
 
), and consistent with the afterglow position
reported by Izzo et al. (GCN #21192).

Based on a total exposure of 4.6 min in g'r'i'z' and 4 min in JHK, we
estimate preliminary magnitudes (all in AB) of

g' = 20.75 +/- 0.08
r' = 20.46 +/- 0.05
i' = 20.35 +/- 0.06
z' = 20.21 +/- 0.08
J = 19.6 +/- 0.1
H = 19.6 +/- 0.3
K > 19.6

Given magnitudes are calibrated against PanSTARRS and 2MASS field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.03 mag in the direction of the
burst (Schlafly & Finkbeiner 2011).

We acknowledge the excellent support provided by Simona Ciceri in
obtaining these data.