The Earliest Optical Observations of GRB 030329

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Abstract
We present the earliest optical imaging observations of the gamma-ray burst GRB 030329 related to SN 2003dh. The burst was detected by the High Energy Transient Explorer 2 satellite on 2003 March 29, at 11:37:14.67 UT. Our wide-field monitoring started 97 minutes before the trigger, and the burst position was continuously observed. We found no precursor or contemporaneous flare brighter than V = 5.1 (V = 5.5) in the 32 s (64 s) timescale between 10:00 and 13:00 UT. Follow-up time series photometries started at 12:51:39 UT (75 s after position notice through the GRB Coordinates Network) and continued for more than 5 hr. The afterglow was RC = 12.35 ± 0.07 at t = 74 minutes after burst. Its fading between 1.2 and 6.3 hr is well characterized by a single power law of the form f(mJy) = [1.99 ± 0.02 (statistic) ± 0.14 (systematic)](t/1 day)-0.890 ± 0.006 (statistic) ± 0.010 (systematic) in the RC band. No significant flux variation was detected, and upper limits are derived as (Δf/f)rms = 3%-5% in minute-to-hour timescales and (Δf/f)rms = 35%-5% in second-to-minute timescales. Such a featureless light curve is explained by the smooth distribution of the circumburst medium. Another explanation is that the optical band was above the synchrotron cooling frequency where the emergent flux is insensitive to the ambient density contrasts. Extrapolation of the afterglow light curve to the burst epoch excludes the presence of an additional flare component at t < 10 minutes as seen in GRB 990123 and GRB 021211.
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