TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5623 SUBJECT: GRB 060926: OPTIMA-Burst -- further observations DATE: 06/09/27 00:40:02 GMT FROM: Alexander Stefanescu at MPE A. Stefanescu, F. Schrey, S. Duscha, G. Kanbach, M. Muehlegger, N. Primak, H. Steinle (MPE Garching) of the OPTIMA-Burst Team report the following: We have observed the OT of GRB 060926 under unfavorable weather conditions until it became unobservable at ~18:40 UT. To make comparision easier with GCN 5618 (U. Kiziloglu et al.), we have callibrated our observations against the R2-magnitudes of 4 nearby USNO-B1 stars (1030-0324059, 1030-0324042, 1030-0324039, 1030-0324046). We estimate the magnitude of the OT as follows: t-t_BAT exp-time [min] (mid-exp) [s] R_mag ----------------------------------------------- 35.4 600 20.3 +-0.5 59.7 600 20.6 +-0.5 72.7 600 20.7 +-0.7 90.7 2x 1200 21.0 +-0.7 101.2 1200 21.1 +-0.7 We fitted these values with a powerlaw with a decay index of 0.73 +-0.1. This is significantly shallower than the powerlaw between the MASTER observations (GCN 5619, V. Lipunov et al.) and ours, with a decay index of 0.90 +-0.06 - allthough this could be due to incompatibilities in zero-point. We thank J. Papamastorakis of the University of Crete and his team for the possibility to use the 1.3m Telescope at Skinakas Observatory for a prolonged period of time.