Radiative Acceleration and Transient, Radiation‐induced Electric Fields

The radiative acceleration of particles and the electrostatic potentialfields that arise in low density plasmas hit by radiation produced by atransient, compact source are investigated. We calculate the dynamicalevolution and asymptotic energy of the charged particles accelerated by thephotons and the radiation-induced electric double layer in the fullrelativistic, Klein-Nishina regime. For fluxes in excess of $10^{27}$ ${\rmerg} {\rm cm}^{-2} {\rm s}^{-1}$, the radiative force on a diluted plasma($n\la 10^{11}$ cm$^{-3}$) is so strong that electrons are accelerated rapidlyto relativistic speeds while ions lag behind owing to their larger inertia. Theions are later effectively accelerated by the strong radiation-induced doublelayer electric field up to Lorentz factors $\approx 100$, attainable in thecase of negligible Compton drag. The asymptotic energies achieved by both ionsand electrons are larger by a factor 2--4 with respect to what one couldnaively expect assuming that the electron-ion assembly is a rigidly coupledsystem. The regime we investigate may be relevant within the framework of giantflares from soft gamma-repeaters.Comment: 14 pages, 7 figures, ApJ, in press (tentatively scheduled for the v. 592, 2003 issue