GCN Circular 18921

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).