Curvature Effect of a Non-power-law Spectrum and Spectral Evolution of Grb X-ray Tails

The apparent spectral evolution observed in the steep decay phase of many GRB early afterglows raises a great concern of the high-latitude “curvature effect” interpretation of this phase. However, previous curvature effect models only invoked a simple power law spectrum upon the cessation of the prompt internal emission. We investigate a model that invokes the “curvature effect” of a more general non-power-law spectrum and test this model with the Swift/XRT data of some GRBs. We show that one can reproduce both the observed lightcurve and the apparent spectral evolution of several GRBs using a model invoking a power-law spectrum with an exponential cut off. GRB 050814 is presented as an example. Subject headings: gamma-rays: bursts