A COMPREHENSIVE STUDY OF GAMMA-RAY BURST OPTICAL EMISSION. I. FLARES AND EARLY SHALLOW-DECAY COMPONENT

Well-sampled optical light curves of 146 gamma-ray bursts (GRBs) are compiled from the literature. By empirical fitting, we identify eight possible emission components and summarize the results in a synthetic light curve. Both optical flare and early shallow-decay components are likely related to long-term central engine activities. We focus on their statistical properties in this paper. Twenty-four optical flares are obtained from 19 GRBs. The isotropic R-band energy is smaller than 1% of E-gamma,E-iso. The relation between the isotropic luminosities of the flares and gamma rays follows L-R,iso(F) proportional to L-gamma,iso(1.11 +/- 0.27). Later flares tend to be wider and dimmer, i.e., w(F) similar to t(p)(F)/2 and L-R,iso(F) proportional to [t(p)(F)/(1 + z)](-1.15 +/- 0.15). The detection probability of the optical flares is much smaller than that of X-ray flares. An optical shallow-decay segment is observed in 39 GRBs. The relation between the break time and break luminosity is a power law, with an index of -0.78 +/- 0.08, similar to that derived from X-ray flares. The X-ray and optical breaks are usually chromatic, but a tentative correlation is found. We suggest that similar to the prompt optical emission that tracks gamma-rays, the optical flares are also related to the erratic behavior of the central engine. The shallow-decay component is likely related to a long-lasting spinning-down central engine or piling up of flare materials onto the blast wave. Mixing of different emission components may be the reason for the diverse chromatic afterglow behaviors.