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 (E(p)) distribution of the nuF(nu) spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57 bursts observed by High Energy Transient Explorer (HETE-2) French Gamma Telescope and discuss its implications for the jet structure models. Combining the observed E(p) distribution of HETE-2 GRBs/XRFs with that of BATSE GRBs, we find that the observed E(p) distribution of GRBs/XRFs is a bimodal one with peaks of less than or similar to30 keV and similar to160-250 keV. According to the recently discovered equivalent-isotropic energy-E(p) 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 similar to15 and similar to200 keV. We argue that future observations of the peak of similar to15 keV in the E(p) 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.