TITLE:   GCN CIRCULAR
NUMBER:  8510
SUBJECT: GRB 081109: GROND Detection of the Optical/NIR Afterglow Candidate
DATE:    08/11/10 03:34:52 GMT
FROM:    Christian Clemens at MPE  <cclemens@mpe.mpg.de>

C. Clemens, T. Kruehler, J. Greiner (all MPE Garching) and G. Szokoly 
(Eoetvoes Univ., Budapest and MPE) report on behalf of the GROND team:

We observed the field of GRB 081109 (Swift trigger #334112; Immler et al., GCN 
#8500) 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).
 
Observations started at 00:08 UT on November 10th, 2008, 17.1 hr after the GRB 
trigger, and are continuing. They were performed during astronomical twilight 
at an average seeing of 1.0" and at an average airmass of 1.1 as well as 
under bright Moon.
 
Based on the first 7.7 min of total exposures in g'r'i'z' and 8.0 min in JHK, 
we detect the afterglow candidate reported by D'Avanzo et al. (GCN #8501) and 
estimate preliminary magnitudes (all in AB system) of
 
g' > 22.8
r' ~ 22.8 +/- 0.3
i' ~ 21.8 +/- 0.15
z' ~ 21.4 +/- 0.15
J ~ 20.4 +/- 0.10
H ~ 19.7 +/- 0.15
K ~ 18.9 +/- 0.20
 
These magnitudes are calibrated against GROND zeropoints as well as 
2MASS field stars.
 
The spectral energy distribution is very red, and requires an extinction
of A_V > 0.5. With this strong extinction, and our g'-band limit being
compromised by the nearby full Moon, we cannot derive a redshift
at this stage.