[vsnet-grb-info 7976] GRB 090618: OA behavior from KAIT

[GCN Circulars]

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  <weidong at astron.berkeley.edu>

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