2006MNRAS.369L...5L

The Swift X-Ray Telescope often observes a rapidly decaying X-ray emission stretching to as long as t ∼ 10³ s after a conventional prompt phase. This component is most likely due to a prompt emission viewed at large observer angles θ > 1/Γ, where θ ∼ 0.1 is a typical viewing angle of the jet and Γ ≥ 100 is the Lorentz factor of the flow during the prompt phase. This can be used to estimate the prompt emission radii, r_em≥ 2t c/θ² ∼ 6x10¹⁵ cm. These radii are much larger than is assumed within the framework of a fireball model. Such large emission radii can be reconciled with a fast variability, on time-scales as short as milliseconds, if the emission is beamed in the bulk outflow frame, e.g. because of a random relativistic motion of `fundamental emitters'. This may also offer a possible explanation for X-ray flares observed during early afterglows.