TITLE: GCN CIRCULAR NUMBER: 6564 SUBJECT: GRB070621: OPTIMA-Burst optical upper limit DATE: 07/06/22 02:41:36 GMT FROM: Alexander Stefanescu at MPE 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)