2002ApJ...565..808B

Gamma-ray bursts (GRBs) are emitted by relativistic ejecta from powerful cosmic explosions. Their light curves suggest that the γ-ray emission occurs at early stages of the ejecta expansion, well before it decelerates in the ambient medium. If so, the launched γ-ray front must overtake the ejecta and sweep the ambient medium outward. As a result, a gap is opened between the ejecta and the medium that surfs the radiation front ahead. Effectively, the ejecta moves in a cavity until it reaches a radius R_gap~10¹⁶E^1/2₅₄ cm, where E is the isotropic energy of the GRB. At R=R_gap the gap is closed and a blast wave forms and collects the medium swept by radiation. Further development of the blast wave is strongly affected by the leading radiation front: the front plays the role of a precursor where the medium is loaded with e^± pairs and preaccelerated. It impacts the emission from the blast at R<R_load=5R_gap (the early afterglow). A spectacular observational effect results: GRB afterglows should start in optical/UV and evolve fast (less than minutes) to a normal X-ray afterglow. The early optical emission observed in GRB 990123 may be explained in this way. The impact of the front is especially strong if the ambient medium is a wind from a massive progenitor of the GRB. In this case three phenomena are predicted. (1) The ejecta decelerates at R<R_load, producing a lot of soft radiation. (2) The light curve of soft emission peaks at t_peak~40(1+z)E^1/2₅₄(Γ_ej/100)^–2 s, where Γ_ej is the Lorentz factor of the ejecta. Given measured redshift z and t_peak, one finds Γ_ej. (3) The GRB acquires a spectral break at 5-50 MeV because harder photons are absorbed by radiation scattered in the wind. A measurement of the break position will determine the wind density.