Afterglow emission from pair-loaded blast waves in gamma-ray bursts

The MeV radiation front of gamma-ray bursts creates copious e+- pairs as it propagates through an ambient medium. The created pairs enrich the leptonic component of the medium by a large factor at distances R < R_load ~ 10^{16} cm from the burst center. The following blast wave sweeps up the pair-rich medium and then emits the observed afterglow radiation. We find that the afterglow has a "memory" of e+- loading outside R_load. The e+- pairs remain in the swept-up material and slowly cool down by emitting synchrotron radiation. They are likely to dominate the blast-wave emission in IR, optical, and UV bands during the first minutes of the observed afterglow. The expected e+- radiation is described by a simple formula, which is derived analytically and checked by numerical integration of synchrotron emission over the blast material; a suitable Lagrangian formalism is developed for such calculations. The main signature of e+- radiation is its flat ("white") spectrum in a broad range of frequencies from IR to UV and possibly soft X-rays. This radiation can be detected by Swift satellite, which would enable new observational tests for the explosion physics.