GCN Circular 6449

J. Greiner, C. Clemens, T. Kruehler, A. Kuepcue-Yoldas, N. Primak, 
G. Szokoly, A. Yoldas (all MPE Garching), S. Klose, U. Laux (Tautenburg Obs), 
and C.C. Thoene (DARK/NBI Copenhagen) report:

We have observed GRB 070521 (trigger 279935, Guidorzi et al. 2007, 
GCN #6431) with the 7-channel imager GROND, mounted at the 
2.2m Max-Planck Institute telescope at La Silla (ESO/Chile). Observations 
started at 06:59 UT, about 8 min. after the GRB trigger, and continued 
for 25 min until the telescope's elevation limit of 20 deg above horizon 
was reached. The observation consisted of a series of 60-second
exposures each in g', r', i, and z', and 10-second exposures in J, H, 
and Ks, respectively. The GRB field was observed through changing,
thick cloud coverage, therefore, only 50% of the images are useful and 
the flux in the remaining images is reduced.

No source is detected within the Swift XRT error circle (GCN #6431).
We derive the following, preliminary 2 sigma upper limits using
SDSS (Cool et al 2007, GCN #6432) and 2MASS photometry of stars within
the field: 
   g' > 22.0 
   r' > 21.7 
   i  > 20.7 
   z' > 20.3
   J  > 20.0 
   H  > 17.9 
   K  > 17.2

The object see by Subaru (Hattori et al, GCN #6444) is not seen
in any of our stacked images.

GROND is presently being commissioned.