The Earliest Optical Observations of Grb 030329

We present the earliest optical imaging observations of GRB 030329 related to SN 2003dh. The burst was detected by the HETE-2 satellite at 2003 March 29, 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 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 GCN) and continued for more than 5 hours. The afterglow was Rc = 12.35 ± 0.07 at t = 74 min after burst. Its fading between 1.2 and 6.3 hours is well characterized by a single power-law of the form f (mJy) = (1.99 ± 0.02(statistic) ± 0.14(systematic)) × (t/1 day) in Rc-band. No significant flux variation was detected and upper limits are derived as (∆f /f) RMS = 3 − 5% in minutes to hours timescales and (∆f /f) RMS = 35 − 5% in seconds to minutes timescales. Such a featureless lightcurve is explained by the smooth distribution of circumburst medium. Another explanation is that the optical band was above the synchrotron cooling frequency where emergent flux is insensitive to the ambient density contrasts. Extrapolation of the afterglow lightcurve to the burst epoch excludes the presence of an additional flare component at t < 10 minutes as seen in GRB 990123 and GRB 021211. 1. introduction A long (≃50 s) duration gamma-ray burst of extreme The burst direction was determined by ground analysis and distributed through GCN (Barthelmy, et al. 1995) at 12:50:24 UT (73 minutes after trigger). Identification of an optical afterglow was reported by Peterson & Price (2003), followed by Torii (2003) and Ue-mura (2003). The redshift of the afterglow was measured as z = 0.1685 from high resolution spectroscopy with the VLT (Greiner, et al. 2003). As the afterglow faded, a supernova component (SN 2003dh) emerged which was clearly observed both photometrically and spectroscopi-cally (Stanek, et al. 2003; Kawabata, et al. 2003; Hjorth, et al. 2003). A dwarf host galaxy with an absolute magnitude similar to the Small Magellanic Cloud was found with the HST observations (Fruchter, et al. 2003). In this Letter, we report the earliest phase behavior of the optical afterglow of GRB 030329. We use a luminosity distance of 800 Mpc and the prompt gamma-ray energy output, …