  The dynamical properties of an electron--positron--photon plasma created by
the vacuum polarization process occurring around a charged gravitationally
collapsing core of an initially neutral star are examined within the framework
of General Relativity and Quantum Field Theory. The Reissner--Nordstr\"{o}m
geometry is assumed to apply between the collapsing core and the oppositely
charged remnant of the star. The appearance of a separatrix at radius
$\bar{R}$, well outside the asymptotic approach to the horizon, is evidenced.
The neutral electron--positron--photon plasma created at radii $r>\bar{R}$
self-propels outwards to infinity, following the classical PEM--pulse analysis.
The plasma created at $r<\bar{R}$ remains trapped and follows the gravitational
collapse of the core only contributing to the reduction of the electromagnetic
energy of the black hole and to the increase of its irreducible mass. This
phenomenon has consequences for the observational properties of gamma--ray
bursts and is especially relevant for the theoretical prediction of the
temporal and spectral structure of the short bursts.
