Cosmological X-Ray Flashes from Off-Axis Jets

The <V/Vmax> of the cosmological X-ray flashes detected by WFC/BeppoSAX is calculated theoretically in a simple jet model. The total emission energy from the jet is assumed to be constant. We find that if the jet opening half-angle is smaller than 0.03 radian, off-axis emission from sources at z<~4 can be seen. The theoretical <V/Vmax> is less than 0.4, which is consistent with the observational result of 0.27+/-0.16 at the 1-sigma level. This suggests that the off-axis GRB jet with the small opening half-angle at the cosmological distance can be identified as the cosmological X-ray flash.