FIRST HOURS OF THE GRB 030329 OPTICAL AFTERGLOW

We present the first results of the observations of the extrem ely bright optical afterglow of gamma-ray burst (GRB) 030329 with the 1.5m Russian-Turkish telescope RTT150 (T ¨ UBITAK National Observatory, Bakyrlytepe, Turkey). RTT150 was one of the first 1.5m-class telescopes pointed to the aftergl ow. Observations were started approximately 6 hours after the burst. During the first 5 hours of our observations the afterg low faded exactly as a power law with index −1.19 ± 0.01 in each of the BVRI Bessel filters. After that, in all BVRI filters simultaneously we observe a steepening of the power law light curve. The power law decay index smoothly approaches the value ≈ −1.9, observed by other observatories later. This power law break occurs at t − t0 ≈ 0.57 days and lasts for ≈ ±0.1 days. We observe no variability above the gradual fading with the upper limits 10‐1% on time scales 0.1‐1000 s. Spectral flux distribution in four BVRI filters corr esponds to the power law spectrum with spectral index α = 0.66 ± 0.01. The change of the power law decay index in the end of our observations can be interpreted as a signature of coll imated ultrarelativistic jet. The afterglow flux distribut ion in radio, optical and x-rays is consistent with synchrotron sp ectrum. We continue our observations of this unique object with RTT150.