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GCN Circular 2254

Subject
GRB 030329: Optical Observations
Date
2003-05-28T18:42:55Z (21 years ago)
From
Haw Cheng at UNC <hawcheng@physics.unc.edu>
H. Cheng, M. Bayliss, D. Reichart, J. Moran, M. Nysewander, M. Schwartz,
and P. Holvorcem report on behalf of the UNC GRB team of the FUN GRB
collaboration:

We observed the afterglow (Peterson & Price, GCN 1985) of GRB 030329
(Vanderspek et al., GCN 1997) with the 32-inch Tenagra II telescope in
IcRcVIc on April 2nd and 3rd, in IcIc on April 4th and 5th, and in Ic on
April 6th, 7th, 8th, 9th, and 10th.  Between April 2nd and 10th, we find
that the afterglow faded from Ic = 16.9 mag to Ic = 18.9 mag.  Our 1-sigma
error bars are typically less than 0.02 mag.

Fitting a four-parameter model to the data, where the parameters are the
temporal index alpha, the spectral index beta, a normalization parameter,
and a parameter that measures the level of of systematic (i.e.,
non-statistical) fluctuations in the light curve, we find that alpha =
-1.70(+0.13)(-0.14), beta = -1.38(+0.20)(-0.20), and that the fading
afterglow is fluctuating at the 17.5(+3.8)(-3.5)% level at these times.

We note that the measured spectral index is too steep given the measured
temporal index and the post-jet break time relations of Sari, Piran, &
Halpern (1999, ApJ, 519, L17).  Possible explanations include:

1. The light curve had not yet reached its asymptotic temporal index, which
if the afterglow is unextinguished would be alpha = -2.76 +- 0.20 if the
cooling break is redward of the observed bands or alpha = -3.76 +- 0.20 if
the cooling break is blueward of the observed bands; or

2. The light curve is extinguished.  Although this does not appear to be
the case at earlier times, this could be the case at later times if dust
was destroyed within the initial opening angle of the jet, but then the jet
expanded laterally behind undisturbed dust.
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