GCN Circular 8719

GRB 081221: GROND observations 

P. Afonso, T. Kruehler, J. Greiner, C. Clemens (all MPE Garching) and A.
Updike (Clemson University) report on behalf of the GROND team: 

GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile) imaged the field of GRB 081221
(Swift trigger 337889, Hoversten et al., GCN #8687) on two epochs
simultaneously in g'r'i'z'JHK. This burst was also detected by Konus-Wind
(Golenetskii et al. GCN #8694) and the Fermi GBM (Wilson-Hodge, GCN
#8704). 

The first epoch started on 2008-12-22 at 01:33:08 UT roughly 9 hours after
the trigger and consisted of 16 images of 375s exposure in g'r'i'z' and
480 images of 10s exposure in JHK. An equal amount of images were obtained
during the second epoch in the following night from 01:19:56 UT on, 33
hours after the burst. 

Inside the refined XRT error circle (Stroh & Hoversten, GCN #8701) we
detect in both epochs the object reported by Tanvir et al.(GCN #8698) and
Perley et al. (GCN #8711) in g'r'i', but not in z'JHK. 

No variability between the two epochs is evident, and no other source is
detected inside the XRT error circle. 

Preliminary VEGA magnitudes from the second epoch calibrated against
USNO-B field stars and upper limits are as follows: 

g' = 25.2 +/- 0.3
r' = 24.7 +/- 0.2
i' = 24.4 +/- 0.4
z' > 24.0
J > 21.0
H > 19.8
K > 19.0 

Given the large errors due to image statistics, photometric calibration
and complicated photometry as pointed out by Perley et al. (GCN #8711),
these values are consistent with what is reported in GCNs #8698 and #8711.