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Did Swift measure GRB prompt emission radii?

Maxim Lyutikov
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Abstract

The Swift X-Ray Telescope often observes a rapidly decaying X-ray emission stretching to as long as $ t \sim 10^3$ seconds after a conventional prompt phase. This component is most likely due to a prompt emission viewed at large observer angles $θ> 1/Γ$, where $θ\sim 0.1$ is a typical viewing angle of the jet and$Γ\geq 100$ is the Lorentz factor of the flow during the prompt phase. This can be used to estimate the prompt emission radii, $r_{em} \geq 2 t c/θ^2 \sim 6 \times 10^{15}$ cm. These radii are much larger than is assumed within a 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. due to 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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Maxim Lyutikov. 2006-01-24. Did Swift measure GRB prompt emission radii?. https://doi.org/10.1111/j.1745-3933.2006.00161.x

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