GCN Circular 22819

Tassilo Schweyer and Patricia Schady (MPE Garching) report:

We observed the field of GRB 180620B (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 05:06 UT on 2018-06-21, 13.3 hours after the GRB
trigger. They were performed at an average seeing of 1.6" and at an
average airmass of 1.5.

We found a single, bright point source within the 1.4" Swift-XRT enhanced
error circle reported by Beardmore et al. (GCN #22814) at

RA (J2000.0) = 23:50:05.088
Dec (J2000.0) = -57:57:43.88

with an uncertainty of 0.14" in each coordinate.

Based on a total exposure time of around 30mins we estimate the following
AB magnitudes:

g' = 21.3 +/- 0.1 mag
r' = 20.8 +/- 0.1 mag
i' = 20.4 +/- 0.1 mag
z' = 20.3 +/- 0.1 mag
J = 19.7 +/- 0.1 mag
H = 19.6 +/- 0.2 mag
K = 19.1 +/- 0.4 mag

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.01 in the direction
of the burst (Schlafly & Finkbeiner 2011).

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