TITLE:   GCN CIRCULAR
NUMBER:  6437
SUBJECT: GRB 070521: optical limit by  Pi of the Sky
DATE:    07/05/21 09:15:54 GMT
FROM:    Grzegorz Wrochna at Soltan Inst.for Nuclear Studies  <wrochna@fuw.edu.pl>

M.Biskup, M.Cwiok, W.Dominik, G.Kasprowicz, A.Majcher, A.Majczyna,
K.Malek, L.Mankiewicz, M.Molak, K.Nawrocki, L.W.Piotrowski,
M.Sokolowski, J.Uzycki, G.Wrochna, F.Zarnecki
on behalf of "Pi of the Sky" collaboration http://grb.fuw.edu.pl

"Pi of the Sky" apparatus located at Las Campanas Observatory
imaged the region of GRB 070521 from 6:13 UT, i.e. 38 minutes
before the GRB, with 10s exposures (IR-cut filter only).
The most interesting exposure started at 6:51:10.9,
i.e. exactly in coincidence with the GRB trigger.
Variable thin clouds seriously disturbed the observations.
No OT was detected within the XRT error box.

Limits for 10s exposures:
6:50:44-6:50:54   >11.7
6:50:57-6:51:07   >11.9
6:51:10-6:51:20   >12.0 --- GRB trigger
6:51:24-6:51:34   >12.0
6:51:38-6:51:48   >12.2

Limits for 20 coadded images
6:42:51-6:47:17   >12.2
6:47:21-6:51:48   >12.6
6:51:52-6:56:18   >12.5
6:56:22-7:00:53   >12.7