GCN Circular 10271

Adria Updike (Clemson University), Andrea Rossi (Tautenburg Obs.) and
Jochen Greiner (MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 091208B (Swift trigger 378559, Pagani et al.,
GCN #10256) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla
Observatory (Chile).

First observations started at 01:07 UT on Dec 09, 15h17m after the GRB 
trigger,
for a total integration time of 7.7 min in g'r'i'z' and 8min in JHK.

For the afterglow reported by Pagani et al. and
de Ugarte Postigo et al. (GCN #10255) we estimate the
following preliminary magnitudes (in the AB system):

g' = 22.64  +- 0.15
r' = 21.88  +- 0.14
i' = 21.65  +- 0.17
z' = 21.35  +- 0.17
J  = 20.61  +- 0.10
H  = 20.5   +- 0.3
K analysis on going

These magnitudes are in agreement with a afterglow at redshift z<3.5.
After correction for the different filter and magnitude system,
a comparison with other observation (Xu et al.,GCN #10269 and Cano et
al., GCN #10262) confirms that the afterglow is decaying with the slope
of 0.6-0.7 reported by Xu et al. .

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars. The errors take in account zero point uncertainties. 
Magnitudes are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.05 mag in the
direction of the burst (Schlegel et al. 1998).