Thermalization by pair production in astrophysical radiative plasma turbulence*

Collisionless relativistic magnetized turbulence is thought to be a prominent nonthermal particle accelerator in astrophysics. In some sources, including blazar jets and black hole accretion disk coronae, such turbulence is likely illuminated by an intense background of low-energy photons. Accelerated plasma particles can inverse Compton (IC) scatter these photons to energies above pair-production threshold with the background, triggering in situ electron-positron pair creation. In this work, we demonstrate – using 3D particle-in-cell simulations of driven magnetized turbulence with IC radiation and pair production – that such a regime cannot be sustained. Though nonthermal particle acceleration and pair injection are initially efficient, pairs eventually build up and weigh down the plasma magnetization. This inhibits further nonthermal particle acceleration and eventually thermalizes the particle energy distribution at a temperature below pair-production threshold. We show that such behavior can be explained via a simplified Fokker-Planck model in which particle acceleration is diffusive, and we use this model to characterize the temperature, magnetization, and newborn pair density in the inevitable thermal state. Finally, we suggest that plasma turbulence of the kind studied here could be a novel pair source in blazar jets far away from their black hole central engines.