Did Swift measure gamma-ray burst prompt emission radii?

The Swift X-Ray Telescope often observes a rapidly decaying X-ray emission stretching to as long as t similar to 10(3) s after a conventional prompt phase. This component is most likely due to a prompt emission viewed at large observer angles theta > 1/Gamma, where theta similar to 0.1 is a typical viewing angle of the jet and Gamma >= 100 is the Lorentz factor of the flow during the prompt phase. This can be used to estimate the prompt emission radii, r(em) >= 2tc/theta(2) similar to 6 x 10(15) 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.