  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.
