GCN Circular 8540

S. Loew, T. Kruehler and J. Greiner (all MPE Garching) report on behalf of 
the GROND team: 


We observed the field of GRB 081121 (Swift trigger #335105; Oates et al., 
GCN #8537) 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 00:30 UT on November 22nd, 2008, 3.9 hr after the 
burst at a high airmass of 2.35, and are continuing. 


We detect the optical afterglow reported by Yuan et al. (GCN #8536) and 
Oates & Schady (GCN #8538) clearly in all filters. 


At a midtime of 3.9454 hours after the burst, we estimate the following 
preliminary magnitudes (all in AB system) in stacked images of 4 x 35s 
integration in g'r'i'z' and 24 x 10s integration in JHK: 


g' = 19.1
r' = 18.8
i' = 18.6
z' = 18.5
J = 18.4
H = 18.2
K = 18.1 


with typical errors of 0.2 mag, which are mostly due to a preliminary 
calibration using GROND zeropoints in g'r'i'z' and 2MASS field stars in JHK.