Cosmological X-ray Flashes in the Off-Axis Jet Model
  The <V/Vmax> of the cosmological X-ray flashes detected by Wide Field Cameras on BeppoSAX is calculted 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, the theoretical <V/Vmax> is less than ~0.4, which is consistent with the recently reported observational value 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. 
Ryo Yamazaki (1), Kunihito Ioka (2), Takashi Nakamura (1) ((1)Kyoto   University, (2)Osaka University)