| Abstract
| - Abstract. 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, rem ⩾ 2t c/θ2 ∼ 6 × 1015 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.
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