2003ApJ...592..368Z

We investigate the radiative acceleration of particles and the electrostatic potential fields that arise in low-density plasmas hit by radiation produced by a transient, compact source. We calculate the dynamical evolution and asymptotic energy of the charged particles accelerated by the photons and the radiation-induced electric double layer in the full relativistic, Klein-Nishina regime. For fluxes in excess of 10²⁷ ergs/cm²/s, the radiative force on a diluted plasma (n≲10¹¹ cm^–3) is so strong that electrons are accelerated rapidly to relativistic speeds, while ions lag behind owing to their larger inertia. The ions are later effectively accelerated by the strong radiation-induced double-layer electric field up to Lorentz factors ~100, attainable in the case of negligible Compton drag. The asymptotic energies achieved by both ions and electrons are larger by a factor of 2-4 than what one could naively expect assuming that the electron-ion assembly is a rigidly coupled system. The regime we investigate may be relevant within the framework of giant flares from soft gamma repeaters.