2005ApJ...633.1027S

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 ε_B>4x10^–6, and the medium density n<2/cm3. 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.