TITLE: GCN CIRCULAR NUMBER: 18921 SUBJECT: GRB 160121A: GROND Afterglow observations DATE: 16/01/22 08:36:42 GMT FROM: Corentin Delvaux at MPE C. Delvaux (MPE Garching), S. Schmidl (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 160121A (Swift trigger 671231; Hagen et al., GCN #18912) 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 04:09 UT on 22-01-2016, 14.3hrs after the GRB trigger. They were performed at an average seeing of 0.9" and at an average airmass of 1.0. We clearly detect the afterglow at the position reported by Oates et al. (GCN#18917) and Mazaeva et al. (GCN#18920). Based on the first 72 min of total exposures in g'r'i'z' and 60 min in JHK, at a midtime of 04:49 UT, we estimate preliminary magnitudes and upper limits (all in AB system) of g' = 22.6 +/- 0.1 mag r' = 21.2 +/- 0.1 mag, i' = 21.1 +/- 0.1 mag, z' = 21.0 +/- 0.1 mag, J = 19.6 +/- 0.1 mag, H = 19.3 +/- 0.1 mag, K > 19.1 mag. The given magnitudes and limits are derived based on calibrating the images against GROND zeropoints and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.53 in the direction of the burst (Schlafly and Finkbeiner 2011).