TITLE: GCN CIRCULAR NUMBER: 9517 SUBJECT: GRB 090618: OA behavior from KAIT DATE: 09/06/18 11:21:26 GMT FROM: Weidong Li at UC Berkeley KAIT/LOSS W. Li, D. A. Perley, and A. V. Filippenko, University of California at Berkeley, on behalf of the KAIT GRB team: We have analyzed the KAIT observations on GRB 090618 (GCN 9512) as reported in GCN 9514. Our first 2s unfiltered observation started at 08:29:44 UT, 75 s after the BAT trigger. The OA as reported in GCN 9512, 9513, 9514, and 9515 is clearly detected. Subsequent observations show that the OA declined and reached a bottom at t=92 s, then brightened and reached a peak at t=120 s. After the peak, the OA declined as a smooth power-law, with an obvous change of decay index at t ~ 600 s (from alpha = -1.08 +/- 0.05 to -0.70 +/- 0.02). Selected photometry, all unfiltered calibrated to the R band via USNO B1: t_mid(seconds) exp(s) mag mag_err --------------------------------------- 76 2.0 13.61 0.03 92 2.0 13.99 0.04 117 2.0 13.50 0.03 237 20.0 14.02 0.02 436 20.0 14.73 0.02 729 20.0 15.26 0.02 2601 20.0 16.22 0.02 7329 120.0 17.02 0.03