[vsnet-grb-info 14335] GRB 131227A: KAIT light curve analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15649
SUBJECT: GRB 131227A: KAIT light curve analysis
DATE:    14/01/01 20:55:53 GMT
FROM:    Weikang Zheng at UC Berkeley  <zwk at astro.berkeley.edu>

WeiKang Zheng, Alexei V. Filippenko, Adam Morgan (UC Berkeley), and
S. B. Cenko (NASA/Goddard Space Flight Center) report on behalf of the
KAIT GRB team:

We analyzed the early time KAIT data of GRB 131227A (Oates et al., GCN
15616). The afterglow is easily detected in our clear and I band
images, but not in V band images (single or co-added). After correcting
for the large Galactic extinction (A_V = 2.8 mag, A_I = 1.5 mag;
Schlegel et al., 2011, ApJ, 737, 103), we found the afterglow had a
color of (V - I) > 1.2 mag at early times (~200 s - 900 s). This is
consistent with the spectral break around 7600 A reported by Cucchiara &
Cenko (GCN 15624), but still not so constraining to say the break is
caused by Lyman-alpha.

Due to lack of emission below 7600 A (either because of extinction
or Lyman-alpha break), we calibrated the KAIT clear image to the i'
band. Our preliminary result shows the afterglow decays as a power law
with alpha = -0.99. The late time upper limit reported by Kann et al.
(GCN 15632) is below the extrapolation of the line. 

The preliminary light curve can be found at the following link:
http://128.32.15.133/kait/grb/131227A/GRB131227A.png