General properties of X-Ray Riches and X-Ray Flashes in comparison with Gamma-Ray Bursts

The X-Ray Flashes (XRFs) and X-Ray Riches (XRRs) are two subclasses of Gamma-Ray Bursts (GRBs), which have respectively no detection in the gamma-ray energy and very faint gamma to X-ray fluence. To investigate their nature we compiled a sample of 54 events observed by BeppoSAX and HETE-2, available in literature and from the web. To classify XRRs/XRFs for those two experiments, we adopted the same spectral hardness ratio. We studied their prompt emission in the X and. range and their spectral parameters and compared them with those of GRBs. We find XRRs/XRFs are characterized by a significantly smaller value of E-peak while the spectral slopes alpha and beta are quite similar. We analysed also the optical and X-ray afterglow fluxes and their ratio and compared them with that obtained for GRBs. We find that the distribution of X-ray flux of XRR/XRF afterglow is consistent with that of GRBs, which is incompatible with the off-axis model. For example, in the inhomogeneous jet model it implies that the observer angle for an XRR/XRF is at most 2 degrees. It is also not explained by the high redshift scenario.