1203.2332 (Liang Li et al.)

A Comprehensive Study of Gamma-Ray Burst Optical Emission: I. Flares and Early Shallow Decay Component    [PDF]

Liang Li, En-Wei Liang, Qing-Wen Tang, Jie-Min Chen, Shao-Qiang Xi, Hou-Jun LV, Bing Zhang, Jin Zhang, Shuang-Xi Yi, Rui-Jing Lu, Lian-Zhong LV, Jian-Yan Wei

Well-sampled optical lightcurves of 146 GRBs are complied from the literature. Fitting the lightcurves with the superposition of multiple broken power law functions, we identify eight possible emission components that may have distinct physical origins. We summarize the results in a "synthetic" optical lightcurve. In this paper we focus on a statistical analysis of optical flares and an early optical shallow-decay component, both are likely related to a long-term central engine activity. Twenty-four optical flares are obtained from 19 GRBs. The isotropic flare peak luminosity is correlated with that of gamma-rays. The flares peak at from tens of seconds to several days post the GRB trigger. Later flares tend to be wider and dimmer. The fraction of GRBs with detected optical flares is much smaller than that of X-ray flares. Associated X-ray flares are observed for 4 optical flares, and the optical flares usually lag behind the corresponding X-ray flares. An optical shallow decay segment is observed in 39 GRBs. Their break times range from tens of seconds to several days post the GRB trigger. The break luminosity is anti-correlated, similar to that derived from X-ray flares. The detection fraction of the optical shallow decay component is comparable to that in the X-ray band. 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 the gamma-rays, the optical flares are also related to the erratic behavior of the central engine. The shallow decay component, on the other hand, is likely due to energy injection into the blastwave, possibly related to a long-lasting spinning-down central engine or piling up of flare materials onto the blastwave. Mixing of different emission components may be the reason of the diverse chromatic afterglow behaviors as observed for Swift GRBs.

View original: http://arxiv.org/abs/1203.2332