A Comprehensive Analysis of<i>Swift</i>XRT Data. II. Diverse Physical Origins of the Shallow Decay Segment

Abstract

We analyze the properties of the shallow decay segment with a sample of 53 long Swift GRBs. We show that the distributions of its characteristics are log-normal or normal, and its isotropic X-ray energy (E_{iso,X}) is linearly correlated with the prompt gamma-ray energy, but with a steeper photon spectrum except for some X-ray flashes. No significant spectral evolution is observed from this phase to the follow-up phase, and the follow-up phase is usually consistent with the external shock models, implying that this segment likely due to a refreshed external shock. Within this scenario, the data are generally consistent with a roughly constant injection luminosity up to the end of this phase $t_b$. A positive correlation between E_{iso, X} and t_b also favors this scenario. Among the 13 bursts that have well-sampled optical light curves, 6 have an optical break around t_b and the breaks are consistent with being achromatic. However, the other 7 bursts either do not show an optical break or have a break at a different epoch than t_b. This raises a concern to the energy injection scenario, suggesting that the optical and X-ray emissions may not be the same component at least for a fraction of bursts. There are 4 significant outliers in the sample, and this shallow decau phase in these bursts is immediately followed by a very steep decay after t_b, which is inconsistent with any external shock model. The optical data of these bursts evolve independently from the X-ray data. These X-ray plateaus likely have an internal origin and demand continuous operation of a long-term GRB central engine. We conclude that the observed shallow decay phase likely has diverse physical origins(Abridged).