  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.
