X-ray Flashes from Off-axis Gamma-Ray Bursts

Ioka and Nakamura (2001) proposed a simple jet model that is compatible with the peak luminosity-spectral lag relation, the peak luminosity-variability relation and various other relations in the Gamma-Ray Bursts. If the viewing angle is much larger than the collimation angle of the jet in the model by Ioka and Nakamura, for appropriate model parameters we obtain the observational characteristics of the X-ray flashes, such as the peak flux ratio and the fluence ratio between the $\gamma$-ray (50-300 keV) and the X-ray band (2-10 keV), the X-ray photon index, the typical duration and the event rate $\sim 100 {\rm yr}^{-1}$. In our model, if the distance to the X-ray flashes is much larger than $\sim 1$ Gpc (or $z \gtrsim 0.2$) they are too dim to be observed, so the spatial distribution of the X-ray flashes should be homogeneous and isotropic.