Multiple Sub-Jet Model of Gamma Ray Bursts and Possible Origin of X-ray Precursors and Post-cursors

We assume that internal shocks of Gamma Ray Bursts (GRBs) consist of multiple sub-jets with a collimation half-angle $\sim {\rm several }\times\gamma_i^{-1}$, where $\gamma_i$ is the Lorenz factor of each sub-jet. If by chance a sub-jet is first emitted off-axis from the line of sight, the observed peak energy can be in the X-ray region. Next if by chance a subsequent sub-jet is emitted along the line of sight, then the peak energy will be in the gamma ray region and the gamma ray may arrive after the X-ray precursor from the former sub-jet depending on parameters. This model predicts a new class of GRBs with extremely weak and short gamma ray emission but X-ray precursor and/or post-cursor as well as an afterglow.Comment: 6 pages, Latex, accepted for publication in Astrophysical Journal Letter