| Abstract: | We assume that internal shocks of gamma-ray bursts (GRBs) consist of multiple sub-jets with a collimation half-angle of about several times gamma-1i, where gammai is the Lorentz 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 precursors and/or postcursors as well as an afterglow. |
