A Comprehensive Analysis of Swift XRT Data. I. Apparent Spectral Evolution of Gamma-Ray Burst X-Ray Tails
Astrophysical Journal, 666, pp 1002-1011, 2007
An early steep decay component following the prompt GRBs is commonlyobserved in Swift XRT light curves, which is regarded as the tailemission of the prompt gamma rays. Prompted by the observed strongspectral evolution in the tails of GRBs 060218 and 060614, we present asystematic time-resolved spectral analysis for the Swift GRB tailsdetected between 2005 February and 2007 January. We select a sample of44 tails that are bright enough to perform time-resolved spectralanalyses. Among them 11 tails are smooth and without superimposingsignificant flares, and their spectra have no significant temporalevolution. We suggest that these tails are dominated by the curvatureeffect of the prompt gamma rays due to delay of propagation of photonsfrom large angles with respect to the line of sight. More interestingly,33 tails show clear hard-to-soft spectral evolution, with 16 of thembeing smooth tails directly following the prompt GRBs, and the othersbeing superimposed with large flares. We focus on the 16 clean, smoothtails and consider three toy models to interpret the spectral evolution.The curvature effect of a structured jet and a model invokingsuperposition of the curvature effect tail and a putative underlyingsoft emission component cannot explain all the data. The third model,which invokes an evolving exponential spectrum, seems to reproduce boththe light curve and the spectral evolution of all the bursts, includingGRBs 060218 and 060614. More detailed physical models are called for tounderstand the apparent evolution effect.
Gamma Rays; Bursts