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 similar to 0.01 days and ending at similar to 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 gamma(0) > 120, the magnetic energy fraction epsilon(B) > 4 x 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.