A COMPREHENSIVE STUDY OF GAMMA-RAY BURST OPTICAL EMISSION. II. AFTERGLOW ONSET AND LATE RE-BRIGHTENING COMPONENTS

Abstract

We continue our systematic statistical study of various components in\ngamma-ray burst (GRB) optical lightcurves. We decompose the early onset bump\nand the late re-brightening bump with empirical fits and analyze their\nstatistical properties. Among the 146 GRBs that have well-sampled optical\nlightcurves, the onset and re-brightening bumps are observed in 38 and 26 GRBs,\nrespectively. It is found that the typical rising and decaying slopes for both\nthe onset and re-brightening bumps are ~1.5 and -1.15, respectively. No early\nonset bumps in the X-ray band are detected to be associated with the optical\nonset bumps, while an X-ray re-brightening bump is detected for half of the\nre-brightening optical bumps. The peak luminosity is anti-correlated with the\npeak time, L_p\\propto t_{p}^{-1.81+/-0.32} and L_p\\propto t_{p}^{-0.83+/-0.17}\nfor the onset and re-brightening bumps, respectively. Both L_p and the\nisotropic energy release of the onset bumps are correlated with E_{\\gamma,\niso}, whereas no similar correlation is found for the re-brightening bumps.\nTaking the onset bumps as probes for the properties of the fireballs and their\nambient medium, we find that the typical power-law index of the relativistic\nelectrons is 2.5 and the medium density profile behaves as n\\propto r^{-1}\nwithin the framework of the synchrotron external shock models. With the medium\ndensity profile obtained from our analysis, we also confirm the correlation\nbetween initial Lorentz factor (\\Gamma_0) and E_{\\gamma, iso} in our previous\nwork. The jet component that produces the re-brightening bump seems to be\non-axis and independent of the prompt emission jet component. Its typical\nkinetic energy budget would be about one order of magnitude larger than the\nprompt emission component, but with a lower \\Gamma_0, typically several tens.\n