GCN Circular 2064

Weidong Li, Ryan Chornock, Saurabh Jha, and Alexei V. Filippenko (UC Berkeley)
report:

We selected from the GCN literature all available R-band measurements of
the GRB 030329 afterglow. We fitted a smoothly broken power-law model
(Beuermann et al. 1999, A&A 352, L26; Stanek et al. 2001, ApJ 563, 592)
to the data within 24 hours of the burst, and obtained the following
parameters: alpha_1 = -0.75 +/- 0.06, alpha_2 = -1.90 +/- 0.05, t_break
= 0.44 +/- 0.04 days, and s = 5 +/- 2. Our fitting parameters are similar
to those reported by Zeh et al. (GCN 2047
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) except for alpha_2 (ours is much 
steeper). A plot showing the fit can be found at

http://astron.berkeley.edu/~bait/grb/grb030329.fit.ps

while a table of GCN R-band photometry of GRB 030329 can be found at

http://astron.berkeley.edu/~bait/grb/gcn030329
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.r.dat

which includes citations for all the data points used in the plot.

The substantial re-brightening reported by Lipkin et al. (GCN 2045
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) and
Burenin et al (GCN 2046
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) is apparent in the plot. Applying the above fitting
parameters to epochs after t = 24 hours, we found that the residual component
brightened steeply starting from t = 22 hours and reached a peak of 16.8 mag
at t = 40 hours, then declined gradually thereafter. Attempts to fit this
component with the R-band light curve of the hypernova SN 1998bw all failed;
thus the re-brightening may not be caused by a SN component.

Analysis of the color of the GRB from KAIT photometry during this phase
suggests the afterglow has a roughly constant color of (B-V) = 0.26 +/-
0.02 mag, (V-R) = 0.36 +/- 0.02 mag, and (V-I) = 0.78 +/- 0.02. (The data
of Fitzgerald & Orosz, GCN 2056
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, suggests a constant (B-V) = 0.39 +/- 0.02
mag; there is an offset from the KAIT B photometry, perhaps due to a color
term). The constant color during the phase of the re-brightening is additional
evidence against a SN component explanation, in which significant color
changes are expected when the SN rises to the maximum.

Density inhomogeneities and extra energy sources are possible explanations 
for the re-brightening. 

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