GCN Circular 8046
Subject
GRB 080727B: KAIT photometry and an early lightcurve break
Date
2008-07-28T23:49:10Z (17 years ago)
From
Weidong Li at UC Berkeley KAIT/LOSS <weidong@astron.berkeley.edu>
W. Li, R. Chornock, D. A. Perley, J. S. Bloom, and A. V. Filippenko,
University of California at Berkeley, on behalf of the KAIT GRB team;
D. A. Kann, TLS Tautenburg, report:
We have analyzed the KAIT observations of the OA of GRB 080727B
as reported in Li et al. (GCN 8024). The imaging sequence started
with 10 x 2 s unfiltered exposures, with the OA detected on each
frame. A series of 20 s V, I, and unfiltered images were then
obtained, and the OA was detected in the first couple of I and
unfiltered images. Additional photometry (calibrated to USNO B1):
UT start exptime filter mag err
08:14:46 2 s clear 18.98 0.28
08:16:03 20 s clear 19.64 0.07
08:17:35 20 s clear 20.68 0.23
Our GRB pipeline used more than 100 USNO B1 stars in calculating the
photometric zero point. However, we suspect that the USNO B1 calibration
for the GRB field may be significantly off. Based on our experience in
the limiting magnitude of the KAIT 2 s and 20 s unfiltered images, we
estimate that the above photometry and those reported in GCN 8024 should
be made brighter by 1.0 - 2.0 mag. The corrected KAIT photometry, together
with the REM NIR detection (D'Avanzo et al. GCN 8037), suggests an
optical - NIR color of R-H (Vega) = 1.2 to 2.2 mag, which is consistent
with the SED fit from the GROND observations (predicted R-H (AB) = 1.0 mag;
Kruehler et al. GCN 8045).
The KAIT unfiltered light curve of GRB 080727B can be well fit by
a broken power-law with an index of 1.08 +/- 0.07 between t = 49 s
to 169 s, and 1.88 +/- 0.21 between t= 169 s and 524 s. Such an
early break in the light curve is quite extraordinary.
We plan to obtain photometric calibrations of the GRB field in the
upcoming photometric nights at Lick Observatory.