GCN Circular 6564

A. Stefanescu (1), Z. Ioannou (2)(4), G. Kanbach (1), S. Duscha (1), F. 
Schrey (1), A. Slowikowska (2)(3), H. Steinle (1)
((1)=MPE Garching, (2)=FORTH, Heraklion (3)=NCAC, Torun, (4)=Univ. of 
Crete) of the OPTIMA-Burst Team report the following:

OPTIMA-Burst at the 1.3m Skinakas Observatory, of the University of
Crete, Greece observed the Swift XRT error circle of GRB 070621 (GCN
Circ 6560, B. Sbarufatti et al.). Observations started at 23:54:52 UT 
(2233s after the trigger) at an airmass of 2.5.

We see no source at the XRT error circle, neither in single exposures, 
nor in stacked exposures. Data of the exposures are:

t-t_0 (mid exp)  t_exp     R-limit
2263s            60s       20.0         (first exposure)
3271s            1680s     20.9         (stack of all avail. exposures)