A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg), and J. Greiner (MPE
Garching) report on behalf of the GROND team:
We observed the field of GRB 111228A (Ukwatta et al., GCN 12737)
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 on La Silla.
Observations started as soon the object was visible over La Silla, at
04:46 UT on December 29, about 13 hr after the burst. We detect the
afterglow (Ukwatta et al., GCN 12737; Xin et al., GCN 12738) in all bands.
At a mean time of December 29, 05:35 UT, in an 8min exposure, we measure
the following preliminary magnitudes (AB system):
g' = 20.17 +/- 0.01,
r' = 19.84 +/- 0.01,
i' = 19.67 +/- 0.02,
z' = 19.50 +/- 0.03,
J = 19.29 +/- 0.07,
H = 18.93 +/- 0.08.
The SED is best fit with a spectral slope of 0.90 +/- 0.05, with no
evidence for additional extinction.
Optical data are calibrated against SDSS and NIR data against 2MASS field
stars. Observations are continuing.