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We analyze the stability of a relativistic double (forward/reverse) shocksystem which forms when the fireball of a Gamma-Ray Burst (GRB) impacts on thesurrounding medium. We find this shock system to be stable to linear globalperturbations for either a uniform or a wind (r^{-2}) density profile of theambient medium. For the wind case, we calculate analytically the frequencies ofthe normal modes which could modulate the early short-term variability of GRBafterglows. We find that perturbations in the double shock system could induceoscillatory fluctuations in the observed flux on short (down to seconds) timescales during the early phase of an afterglow.Comment: ApJ, submitted, 26 pages, 5 figure

Stability of the Forward/Reverse‐Shock System Formed by the Impact of a Relativistic Fireball on an Ambient Medium