TITLE:   GCN CIRCULAR
NUMBER:  20457
SUBJECT: GRB 170113A: GROND afterglow observations
DATE:    17/01/14 08:00:28 GMT
FROM:    Thomas Kruehler at MPE Garching  <kruehler@mpe.mpg.de>

T. Kruehler (MPE Garching) reports:

I observed the field of GRB 170113A (Swift trigger 732526; Evans
et al. GCN #20446) 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 01:34 UT on 2017-01-14, 15.5 hr after the GRB
trigger. Based on combined images with 22 min of total exposure time
in g'r'i'z' and 18 min in JHK at a mid-time of 01:50 UT on 2017-01-14,
I derive the following preliminary magnitudes and upper limits
(all in the AB system) for the optical/NIR afterglow (Evans et al.,
GCN 20446, Guidorzi et al., GCN 20448, Siegel et al., GCN 20455):

g' = 22.3 +- 0.1 mag
r' = 22.0 +- 0.1 mag
i' = 21.8 +- 0.1 mag
z' = 21.6 +- 0.1 mag
J = 20.9 +- 0.2 mag
H = 20.8 +- 0.3 mag
K > 19.0 mag

Given magnitudes are calibrated against GROND zeropoints and 2MASS
field stars and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of
E_(B-V)=0.099 mag in the direction of the burst (Schlafly &
Finkbeiner 2011).

I acknowledge excellent help in obtaining these data from the
supporting astronomer on La Silla, Regis Lachaume.