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 _{γ, iso}. The relation between the isotropic luminosities of the flares and gamma rays follows L ^F _{R, iso} ∞ L ^{1.11 ± 0.27} _{γ, iso}. Later flares tend to be wider and dimmer, i.e., w ^F ∼ t ^F _p/2 and L ^F _{R, iso} ∞ [t ^F _p/(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 γ-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.