  It has now become clear that the radio jet in the giant elliptical galaxy M87
must turn on very close to the black hole. This implies the efficient
acceleration of leptons within the jet at scales much smaller than feasible by
the typical dissipative events usually invoked to explain jet synchrotron
emission. Here we show that the stagnation surface, the separatrix between
material that falls back into the black hole and material that is accelerated
outward forming the jet, is a natural site of pair formation and particle
acceleration. This occurs via an inverse-Compton pair catastrophe driven by
unscreened electric fields within the charge-starved region about the
stagnation surface and substantially amplified by a post-gap cascade. For
typical estimates of the jet properties in M87, we find excellent quantitive
agreement between the predicted relativistic lepton densities and those
required by recent high-frequency radio observations of M87. This mechanism
fails to adequately fill a putative jet from Sagittarius A* with relativistic
leptons, which may explain the lack of an obvious radio jet in the Galactic
center. Finally, this process implies a relationship between the kinetic jet
power and the gamma-ray luminosity of blazars, produced during the post-gap
cascade.
