TITLE: GCN CIRCULAR NUMBER: 7438 SUBJECT: GRB 080319B: Change in power law decay index DATE: 08/03/19 11:36:37 GMT FROM: Weidong Li at UC Berkeley KAIT/LOSS W. Li, R. Chornock, D. A. Perley, and A. V. Filippenko, University of California at Berkeley, on behalf of the KAIT GRB team: We have analyzed the KAIT data on GRB 080319B (GCN 7472, 7428) as reported in GCN 7430. Preliminary reduction of the unfiltered data, calibrated to SDSS observations of the field, shows a smooth light curve from t = 20 to 120 minutes after the BAT trigger, with an apparent change in power-law decay index at t ~ 35 minutes. The power-law decay index is measured to be -1.89 +/- 0.04 between t = 20.1 to 26.3 minutes after the BAT trigger, and -1.23 +/- 0.02 between t=60.9 to 117.2 minutes after the BAT trigger. The shallower decay is also observed at t ~ 80 minutes as noted in GCN 7435. We note this change in power-law decay index is reminiscent of what has been observed in GRB 990123 (e.g. Akerlof et al. 1999, Nature 398, 400) and GRB 021211 (e.g. Li et al. 2003, ApJ 586, L9), and suggests a transition from reverse shock to forward shock emission. Selected KAIT unfiltered photometry, all unfiltered data calibrated to the R band via the SDSS calibration: t(start; seconds after BAT trigger) exp mag merr 1207 20.0 14.110 0.008 1394 20.0 14.417 0.011 1576 20.0 14.645 0.011 1991 20.0 15.063 0.011 2912 20.0 15.710 0.014 3770 20.0 16.053 0.015 4380 20.0 16.247 0.014 5005 20.0 16.415 0.019 6820 20.0 16.843 0.017