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We show that the jet structure of gamma-ray bursts (GRBs) can be investigatedwith the tail emission of the prompt GRB. The tail emission which we consideris identified as a steep-decay component of the early X-ray afterglow observedby the X-ray Telescope onboard Swift. Using a Monte Carlo method, we derive,for the first time, the distribution of the decay index of the GRB tailemission for various jet models. The new definitions of the zero of time andthe time interval of a fitting region are proposed. These definitions forfitting the light curve lead us an unique definition of the decay index, whichis useful to investigate the structure of the GRB jet. We find that if the GRBjet has a core-envelope structure, the predicted distribution of the decayindex of the tail has a wide scatter and has multiple peaks, which cannot beseen for the case of the uniform and the Gaussian jet. Therefore, the decayindex distribution tells us the information on the jet structure. Especially,if we observe events whose decay index is less than about 2, both the uniformand the Gaussian jet models will be disfavored according to our simulationstudy.

Probing the Structure of Gamma-Ray Burst Jets with Steep Decay Phase of their Early X-ray Afterglows