Particle acceleration by magnetic field-aligned electric fields in active galactic nuclei

Weshow that the formation ofmagnetic field-aligned electric fields E‖ explains the existence of relativistic leptons with Lorentz factors less than about 2000 in active galactic nuclei. This Lorentz factor is the minimum value for Fermi processes to accelerate leptons. This is also known as the injection problem: particles with γ < 2000 cannot be accelerated by shock waves or MHD turbulence, also known as Fermi I and II mechanisms. E‖ can accelerate charged particles very efficiently on very short time scales. The appearance of such magnetic field-aligned electric fields is an implicit property of magnetized, turbulent plasmas in which the electrical conductivity is not infinitely high. We describe the appearance of these fields within an analytical kinematic approach as well as on the grounds of a numerical dynamical model. Due to the small spatial extension of such layers the acceleration time is much shorter than the energy loss time due to inverseCompton scattering of the UV radiation of the underlying accretion disk. Thus, relativistic leptons can be effectively produced in the corona of an accretion disk even in the presence of an intense radiation bath. This is an important result for γ-raymodels, which require a high initial density of leptons with Lorentz factors of about 103.