The first hours of the optical afterglow from the cosmic gamma-ray burst 030329

Abstract

We present the first results of the observations of the extremely bright optical afterglow of gamma-ray burst (GRB) 030329 with the 1.5m Russian-Turkish telescope RTT150 (TÜBITAK National Observatory, Bakyrlytepe, Turkey). RTT150 was one of the first 1.5m-class telescopes pointed to the afterglow. Observations were started approximately 6 hours after the burst. During the first 5 hours of our observations the afterglow 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 − t 0 ≈ 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 corresponds 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 collimated ultrarelativistic jet. The afterglow flux distribution in radio, optical and x-rays is consistent with synchrotron spectrum. We continue our observations of this unique object with RTT150.

The Detailed Optical Light Curve of GRB 030329

“…We note, however, that our data and those of Uemura et al are in good agreement throughout the overlapping segments. The I-band light curve of Burenin et al (2003), covering the time span between 0.2 and 0.6 days, does not overlap with our I-band data. The zero point for this segment was set by coarsely extrapolating later I-band segments and should therefore be regarded with caution.…”

“…Garnavich, Stanek, & Berlind 2003a;Burenin et al 2003;Price et al 2003b). The change in the powerlaw decline index from $1 to $2, seen both in early optical data and in sparse X-ray observations reported by Tiengo et al (2003), combined with the achromatic nature of the break (Burenin et al 2003), seemed to support an interpretation of this steepening as a ''jet break,'' the manifestation of a conical geometry in the relativistic emitting material (e.g., Rhoads 1997).…”

“…We augmented our data set with external photometry from Burenin et al (2003), Uemura et al (2003), and Bloom et al (2004), kindly provided in digital form by these authors, as well as with the data published by Matheson et al (2003). Each of the external data sources was cross-calibrated with our set by using the procedure described above for the Mount Laguna Observatory data.…”

“…Early optical observations, obtained with small telescopes shortly after the burst, were reported by Uemura et al (2003), , Smith et al (2003), Sato et al (2003), and Urata et al (2004). Burenin et al (2003), Bloom et al (2004), and Matheson et al (2003) presented the results of multiband follow-up campaigns in the optical and near-IR. Greiner et al (2003b) presented optical polarization monitoring of GRB 030329, detecting significant variability.…”

The Polarization Evolution of the Optical Afterglow of GRB 030329

“…During each of the power law decay phases the polarization is of order few percent, different from phase to phase, and variable within the phase, but not in tandem with the "bumps and wiggles" in the light curve. We observe a decreasing polarization degree shortly after the light curve break at 0.4 days (as determined from optical [21,23] and X-ray data [24,25]). Rapid variations of polarization occur 1.5 days after the burst, and could be related to the end of the transition period towards a new power law phase starting at 1.7 days.…”