TITLE:   GCN CIRCULAR
NUMBER:  22818
SUBJECT: GROND observations of GRB 180620A
DATE:    18/06/21 11:16:12 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady@mpe.mpg.de>

Tassilo Schweyer and Patricia Schady (MPE Garching) report:

We observed the field of GRB 180620A (Swift trigger 843211; Evans et al.,
GCN #22807) 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 04:35 UT on 2018-06-21, 20 hours after the GRB
trigger. They were performed at an average seeing of 1.4" and at an
average airmass of 1.7.

We detect a faint point source consistent with the position of the
reported X-ray and optical afterglow (Elenin et al., GCN #22800; Osborne
et al., GCN #22802; Breeveld et al., GCN #22805). Based on a total
exposures time of around 30mins we estimate the following AB magnitudes:

g' = 23.9 +/- 0.1 mag
r' = 23.4 +/- 0.1 mag
i' = 23.1 +/- 0.1 mag
z' > 23.2 mag
J > 21.2 mag
H > 20.5 mag
K > 19.3 mag

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)=0.11 in the direction of the burst
(Schlafly & Finkbeiner 2011).

We thank Markus Rabus for the excellent support from La Silla.