The Soft X-Ray Afterglow of Gamma-Ray Bursts: A Stringent Test for the Fireball Model

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Abstract
I consider the recent discovery of a soft X-ray source inside the error box of the gamma-ray burst GB 960720 by the SAX, ASCA, and ROSAT satellites, in terms of the fireball model. I show that the ejecta shell, which after causing the burst is cold and dense but still relativistic, keeps plowing through the interstellar medium, heating up the just-shocked matter, which then emits X-rays. I compute the radiation emitted by this matter. I show that, up to about 2 months after the burst, in the cosmological scenario a soft X-ray (0.1-10 keV) flux of at least ≈ 10-13 erg s-1 cm-2, well within current observational capabilities, is generated, explaining the observations of the three satellites. Instead, in the Galactic Halo scenario a flux 3 orders of magnitude lower is expected. Detection of this nonthermal, declining flux in a statistically significant number of objects would simultaneously establish the fireball model and the cosmological nature of gamma-ray bursts.

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