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).