En-Wei Liang, Liang Li, He Gao, Bing Zhang, Yun-Feng Liang, Xue-Feng Wu, Shuang-Xi Yi, Zi-Gao Dai, Qing-Wen Tang, Jie-Min Chen, L. Hou-Jun, Jin Zhang, Rui-Jing Lu, L. V. Lian-Zhong, Jian-Yan Wei
We continue our systematic statistical study of various components in gamma-ray burst (GRB) optical lightcurves. We decompose the early onset bump and the late re-brightening bump with empirical fits and analyze their statistical properties. Among the 146 GRBs that have well-sampled optical lightcurves, the onset and re-brightening bumps are observed in 38 and 27 GRBs, respectively. It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are $\sim 1.5$ and in ~1.15, respectively. No early onset bump in the X-ray band is detected to be associated with the optical onset bumps, while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps. The peak luminosity is anti-correlated with the peak time, L_p \propto t_{p}^{-1.86+/- 0.29} for the onset bumps and L_p\propto t_{p}^{-1.05+/- 0.16} for the re-brightening bumps. Both $L_p$ and the isotropic energy release of the onset bumps are correlated with E_{gamma, iso}, whereas no similar correlation is found for the re-brightening bumps. These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs. Taking the onset bumps as probes for the properties of the fireballs and their ambient medium, we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaviors as n \propto r^{-1} within the framework of the synchrotron external shock models. With the medium density profile obtained from our analysis, we also confirm the correlation between initial Lorentz factor (Gamma_0) and E_{iso, gamma} in our previous work. The jet component that produces the re-brightening bump seels to be on-axis and independent of the prompt emission jet component. Its kinetic energy budget would be comparable to the prompt emission component, but with a lower Gamma_0, typically several tens.
View original:
http://arxiv.org/abs/1210.5142
No comments:
Post a Comment