TITLE:   GCN CIRCULAR
NUMBER:  18887
SUBJECT: GRB 160117B: GROND optical/NIR afterglow observations
DATE:    16/01/18 10:54:19 GMT
FROM:    Thomas Kruehler at MPE Garching  <kruehler@mpe.mpg.de>

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

We observed the field of GRB 160117B (Sonbas et al., GCN 18875) 
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 00:54 UT on 2016-01-18, about 11 hours after the 
GRB trigger. The optical afterglow (Sonbas et al., GCN 18875; Yurkov et 
al., GCN 18877; Kruehler et al., GCN 18878, Malesani et al.
GCN 18884, Hentunen et al. GCN 18885, de Ugarte Postigo et al. GCN 
18886) is clearly detected in our images.

Based on 29 min of total exposure in g'r'i'z' and 24 min in JHK at a 
midtime of 01:12 UT on 2016-01-18, we estimate the following preliminary 
magnitudes (all in the AB system):

  g' = 20.8 +- 0.1 mag
  r' = 20.7 +- 0.1 mag
  i' = 20.6 +- 0.1 mag
  z' = 20.4 +- 0.1 mag
  J  = 20.0 +- 0.2 mag
  H  = 19.8 +- 0.2 mag
  K  > 18.9 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)=0.058 mag 
in the direction of the burst (Schlafly & Finkbeiner 2011).