A REVERSE-SHOCK MODEL FOR THE EARLY AFTERGLOW OF GRB 050525A

The prompt localization of GRB 050525A by Swift allowed rapid follow-up of the afterglow. The observations revealed that the optical afterglow had a major rebrightening starting at ~0.01 days and ending at ~0.03 days, which was followed by an initial power-law decay. Here we show that this early emission feature can be interpreted as reverse-shock emission superposed by forward-shock emission in an interstellar medium environment. By fitting the observed data, we further constrain some parameters of the standard fireball-shock model: the initial Lorentz factor of the ejecta γ0 > 120, the magnetic energy fraction B > 4 × 10-6, and the medium density n < 2 cm-3. These limits are consistent with those from other very early optical afterglows observed so far. In principle, a wind environment for GRB 050525A is disfavored.

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