GCN Circular 8745

GRB 081228: detection by GROND 

P. Afonso (MPE), S. Klose (Tautenburg), T. Kruehler, (MPE), J. Greiner
(MPE), A. Updike (Clemson University), A. Kupcu-Yoldas(ESO), A. Yoldas 
(MPE), and G. Szokoly (E��tv��s Univ. Budapest) 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 081228
(Swift trigger 338338, Page et al., GCN #8742) simultaneously in
g'r'i'z'JHK. 

Inside the enhanced XRT error circle (position, RA (J2000): 02h 37m 50.77s
Dec (J2000): +30d 51' 09.5"), reported by Goad et alia, GCN #8744, we
detect an object in the g'r'i'z' bands, but not in JHK. 

Observations started at 01h26 UT, 9 min after the burst. 

We used 4 images of 375 s exposure in g'r'i'z' and 120 images of 10s
exposure in JHK. The images midtimes for g'r'i'z' and JHK are respectively
02h23m59s and 02h24m20s UT, or 1h6m19s and 1h6m40s after the burst. 

Observations were done at high air mass and seeing bigger than 1''. 

The object is detected in the g'r'i'z' bands, implying a redshift smaller
than 3.5. The detection is fainter in the g' band. 

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

g' = 23.73 +/- 0.11
r' = 21.98 +/- 0.04
i' = 21.42 +/- 0.06
z' = 21.80 +/- 0.08
J > 19.6
H > 18.4
K > 17.6 

No correction for galactic extinction was done.