Broad-band (2-400 keV) Spectra of Gamma-Ray Bursts and X-Ray Flashes based on HETE-2 Observations

One of the primary goals of HETE-2 is the spectroscopy of the prompt emission of gamma-ray bursts (GRBs) in a wide energy range. A large overlapping energy range of WXM and FREGATE ensures reliable spectral measurements. Thanks to the enhanced sensitivity in the X-ray range, HETE-2 has detected large numbers of X-ray rich events in addition to the classical GRBs. We have examined the spectral properties of these cosmological bursts localized with HETE-2. They have much variety in their properties, with the spectral peak energy ranging from a few keV to ∼400 keV, and the gamma-ray (30-400 keV) fluence from ∼ 10 -8 to ∼ 10-4 erg cm-2. According to the ratio of fluence in the X-ray range (2-30 keV) to that in the gamma-ray range (30-400 keV), we have classified the localized cosmological bursts into three classes, hard GRBs, X-ray rich GRBs and X-ray flashes. When the nature of the X-ray rich GRBs and X-ray flashes are studied systematically, we find that they have many properties in common with the classical GRBs. In fact, there is no clear separation in properties of these three classes, suggesting that they are a single phenomenon. The relation between the spectral peak energy and the isotropic-equivalent radiated energy (both in the source frame) has been confirmed for the HETE-localized GRBs and extended for XRFs