GCN Circular 20312

P. Wiseman, T. Kruehler, and J. Greiner (all MPE Garching) report on
behalf of the GROND team:

We observed the field of GRB 161220A (Swift trigger 727610; D'Elia et al.,
GCN #20302) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla
Observatory (Chile).

Observations started at 00:36 UT on 2016-12-21, 16 hrs after the GRB
trigger. They were performed at an average seeing of 0".9, and at an
average airmass of 1.2.

Similar to Marshall & D'Elia (GCN #20307), we detect no credible
optical/NIR afterglow consistent with the XRT position. Based on combined
images with 14.5 min of total exposures in g'r'i'z'
and 12.0 min in JHK at a mid-time of 00:45 UT on 2016-12-21,
we derive the following upper limits (all in AB system).

g' > 22.7 mag
r' > 22.7 mag
i' > 21.3 mag
z' > 21.8 mag
J > 20.8 mag
H > 20.4 mag
K > 17.2 mag

Given magnitudes are calibrated against USNO as well as 2MASS field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V) = 0.005 mag in the direction of
the burst (Schlafly & Finkbeiner 2011).