Are XRFs Really Off‐Jet GRBs?

Gamma‐Ray Bursts (GRBs) are widely thought to originate from collimated jets of material moving at relativistic velocities. Emission from such a jet should be visible even when viewed from outside the angle of collimation. Such off‐jet emission has been proposed as an explanation for X‐Ray Flashes (XRFs). I summarize recent work on the special relativistic transformation of the burst quantities Eiso and Epeak as a function of viewing angle, in the context the top‐hat shaped variable opening‐angle jet model. The resulting formulae serve as input for a Monte Carlo population synthesis method. I find that the XRFs seen by HETE‐2 and BeppoSAX cannot be easily explained as classical GRBs viewed off‐jet. For γ 300 for all bursts or unless there is a strong i...