Radiation Front Sweeping the Ambient Medium of Gamma-ray Bursts

Prompt emission of gamma-ray bursts (GRBs) is likely generated at early stages of the ejecta expansion, possibly by internal shocks. The γ-ray front overtakes the ejecta and sweeps the ambient medium. As a result a gap is opened between the ejecta and the medium that surfs the front ahead. Effectively, the ejecta move in a cavity until they reach a radius Rgap ≈ 3 × 1015E 53 cm where E is the isotropic energy of the GRB. At R = Rgap the gap disappears, the blast wave forms and collects the medium behind the radiation front. At radii Rgap < R < Rload ≈ 1.5× 1016E 53 cm the blast wave propagates in e±-rich and preaccelerated material. (The medium is e±-loaded in the radiation front: here the scattered GRB photons are decollimated and turn into e± via γ − γ reaction.) The e± enrichment and preacceleration of the ambient medium should cause a spectacular observational effect: GRB afterglows should start in optical/UV and evolve fast (< min) to a normal X-ray afterglow. The early soft emission has an unusual light curve which can overlap with the prompt GRB. If the ambient medium is normal ISM then less than 1% of the blast wave energy is dissipated at the early soft stage. If the medium is a wind from a massive progenitor then almost all the energy is dissipated at this stage. In the massive progenitor scenario, we predict two phenomena: (1) The early soft light curve peaks at tpeak ≈ 12E 53 s if the Lorentz factor of the ejecta is Γej = 100 and tpeak scales as Γ −2 ej . The peak position weakly depends on the wind parameters and offers a way to measure Γej. (2) The prompt GRB should have a spectral break at 10− 100 MeV because harder photons are absorbed by radiation scattered in the wind. The measurement of the break position will determine the wind density. Subject headings: Cosmology: miscellaneous — gamma-rays: bursts — radiation mechanisms: nonthermal — scattering — shock waves