Apparent spectral evolution of GRB x-ray tails

Abstract

A strong spectral evolution feature is commonly observed in the early steep decay component of Swift XRT light curves. We present a systematic time-resolved spectral analysis for the GRB X-ray tails observed with Swift/XRT. About one-third of the tails, which are smooth and without superimposing significant flares, have no significant spectral evolution. They can be interpreted by the curvature effect of the emission with a power law spectrum. More interestingly, two thirds of the tails show clear hard-to-soft spectral evolution, with half of them being smoothly superimposed with large flares. A toy model, which invokes an evolving exponential spectrum is proposed. It can reproduce both the light curve and the spectral evolution for those tails that have no significant flares, including GRBs 060218 and 060614. © 2008 American Institute of Physics.