TITLE:   GCN CIRCULAR
NUMBER:  18967
SUBJECT: GRB 160131A: GROND Detection of the Optical/NIR Afterglow
DATE:    16/02/01 01:20:14 GMT
FROM:    Fabian Knust at MPE/GROND  <fknust@mpe.mpg.de>

F. Knust, J. Bolmer, J. Greiner (all MPE Garching), and D. A. Kann,  (TLS
Tautenburg) report on behalf of the GROND team:

We observed the field of GRB 160131A (Swift trigger 672236; Page et al.,
GCN #18951) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2m MPG telescope at ESO La Silla
Observatory (Chile).

Observations started at  0:44 UT on 2016-02-01, 16 hrs after the GRB
trigger, and are continuing. They were performed at an average seeing of
1.3" and at an average airmass of 1.1.

We found a single point source within the 2.2" Swift-XRT error circle
reported by Goad et al. (GCN #18958), in agreement with the UVOT afterglow
 (Page et al., GCN #18951) at

RA (J2000.0) = 05 h 12 m 40.32 s
Dec. (J2000.0) = -07d 02' 58.92

with an uncertainty of 0.5" in each coordinate.

Based on the first 4.40 min of total exposures in g'r'i'z' and 4.0 min in
JHK, we estimate preliminary magnitudes (all in AB system) of

g' = 18.7 +/- 0.05 mag,
r' = 18.4 +/- 0.05 mag,
i' = 18.3 +/- 0.05 mag,
z' = 18.2 +/- 0.05 mag,
J  = 17.7 +/- 0.1 mag,
H  = 17.6 +/- 0.1 mag, and
K  = 17.1 +/- 0.1 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)=0.09 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).