Are X-ray flashes a peculiar class of (soft) gamma-ray bursts ?

Heise et al. (2001) have reported the identification of a new class of sources by Beppo-SAX, the so-called X-ray flashes (XRFs), which have many common properties with gamma-ray bursts (GRBs) but are not detected in the gamma-ray range. In the framework of the internal shock model, we investigate the possibility that XRFs have the same origin than GRBs but are intrinsically softer due to different shock parameters.