TITLE: GCN CIRCULAR NUMBER: 6492 SUBJECT: GRB 070610: OPTIMA-Burst high-time-resolution optical observations DATE: 07/06/11 00:22:55 GMT FROM: Alexander Stefanescu at MPE A. Stefanescu (1), A. Slowikowska (2)(3), G. Kanbach (1), S. Duscha (1), F. Schrey (1), H. Steinle (1), Z. Ioannou (4) of the OPTIMA-Burst Team report the following: (1)=MPE, Garching, (2)=FORTH, Heraklion (3)=NCAC, Torun (4)=Univ. of Crete "OPTIMA-Burst at the 1.3m Skinakas Observatory, of the University of Crete, Greece slewed to the Swift BAT-errorcircle of GRB 070610 (GCN Circ 6489, C. Pagani et al.) and started observations at 20:53:23UT (57s after the burst, 28s after the notice). We detected a faint transient in the BAT errorcircle. The transient was marginally detected as 19+-0.4 mag in our first 10s white-light image, and was not detected in subsequent CCD frames. The position of the transient is inside the XRT error-circle (GCN 6490, C. Pagani et al.). At 20:57:19 (293s after the burst) observations with the fibre-fed, single-photon-counting high-time-resolution main instrument of OPTIMA-Burst started. A quick-look analysis of this data binned to 1s resolution indicates that the source is barely detectable (below the 2 sigma level) until 21:00:22 (476s after the burst). At that time the source showed a flaring activity of about 7.5 sigma above the background level. The activity lasted about 80s and consisted of three to four overlapping spikes and a slow decay. Further analysis is in progress." This message may be cited.