S. Schmidl, A. Rossi (both TLS Tautenburg), and J. Greiner (MPE
Garching) report on behalf of the GROND team:
We observed the field of GRB 130609B (Swift trigger 557828; Krimm et
al., GCN #14841) simultaneously in g'r'i'z'JHK with GROND (Greiner
et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope
at La Silla Observatory (Chile).
Observations started at 09:30 UT on 11th June 2013, 35.9 hours after
the GRB trigger. They were performed at an average seeing of 1.6"
and at an average airmass of 2.2.
We detect a source at the position of the UVOT afterglow (Siegel
et al.; GCN #14857), and based on images taken at a mid-time of
09:45 UT, with total exposures of 25 minutes in g'r'i'z' and 20
minutes in JHK, we estimate preliminary magnitudes (all in AB) of
g' = 22.2 +/- 0.1 mag,
r' = 21.7 +/- 0.1 mag,
i' = 21.1 +/- 0.2 mag,
z' = 20.9 +/- 0.2 mag,
J = 20.6 +/- 0.3 mag,
H = 19.8 +/- 0.4 mag, and
K = 19.4 +/- 0.4 mag.
The source is brighter than what one could expect extrapolating
the early UVOT data (Siegel et al.; GCN #14857) and could be either
a plateau/flare, or due to the contribution of the host galaxy.
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)=0.01 mag
in the direction of the burst (Schlegel et al. 1998).