The Peak Energy Distribution of the Νfν Spectra and the Implications for the Jet Structure Models of Gamma-ray Bursts

We study the peak energy (Ep) distribution of the νFν spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57 bursts observed by High Energy Transient Explorer 2 (HETE2) French Gamma Telescope and discuss its implications for the jet structure models. Combining the observed Ep distribution of HETE–2 GRBs/XRFs with that of BATSE GRBs, we find that the observed Ep distribution of GRBs/XRFs is a bimodal one with peaks of ∼< 30 keV and ∼ 160−250 keV. According to the recently-discovered equivalent-isotropic energy-Ep relationship, such a bimodal distribution implies a two-component structure of GRB/XRF jets. A simple simulation analysis shows that this structured jet model does roughly reproduce a bimodal distribution with peaks of ∼ 15 and ∼ 200 keV. We argue that future observations of the peak of ∼ 15 keV in the Ep distribution would be evidence supporting this model. Swift, which covers an energy band of 0.2–150 keV, is expected to provide a key test for our results. Subject headings: gamma rays: bursts—gamma rays: observations—ISM: jets and outflows—methods: statistical