Radiation front sweeping the ambient medium of gamma-ray bursts

Gamma-ray bursts (GRBs) are emitted by relativistic ejecta from powerful cosmic explosions. Their light curves suggest that the gamma-ray emission occurs at early stages of the ejecta expansion, well before it decelerates in the ambient medium. If so, the launched c-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 approximate to 10(16)E(54)(1/)2 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 R<R-load decelerates at producing a lot of soft radiation. (2) The light curve of soft emission peaks at t(peak) approximate to 40(1+z) E-54(1/2)(Gamma(ej)/100)(-2) s, where is the Lorentz factor of the ejecta. Given measured redshift z and t(peak), one finds Gamma(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.