[vsnet-grb-info 5786] GRB 080319B: Change in power law decay index

[GCN Circulars]

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  <weidong at astron.berkeley.edu>

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