THE COLLISIONAL ACCELERATION OF RELATIVISTIC ELECTRONS IN THE CENTRAL REGIONS OF ACTIVE GALACTIC NUCLEI

The acceleration of electrons to relativistic energies by collision with relativistic protons is examined in the environment of the compact central regions of active galactic nuclei. Coulomb collisions between the protons and cold electrons form a prominent scattering mechanism: only large-angle scatterings result in relativistic boosts to the electron energy. The cold electrons could be the end product of a pair cascade, such as is found in models of the X-ray and gamma-ray AGN continuum, while the protons are injected into the central regions of AGN by some acceleration process such as diffusive shock acceleration. The resulting electron excitation spectrum is determined, and is found to be generally of a power-law form. Coulomb scattering proves to be an efficient acceleration mechanism, and can provide a significant supply of non-thermal electrons for typical AGN conditions. If proton cooling in the collisions is included, very flat electron spectra can be obtained, otherwise the electron injection has an index of at least two. Bremsstrahlung collisions between the protons and the cold electrons are also investigated and are found to contribute insignificantly to the accelerated electron population unless the proton Lorentz factor exceeds about 10(4). For a power-law distribution of protons, the Coulomb process is more efficient than bremsstrahlung at accelerating electrons to energy-epsilon except when epsilon greater-than-or-similar-to 10(7) m(e)c2. Other modes of proton cooling that are important for producing higher energy electrons are discussed briefly. It is concluded that the large-angle Coulomb collisional electron re-acceleration process may be quite important for pair cascade models of AGN spectra, as it can produce a significant fraction of the total electron injection luminosity.