Dynamic Monte Carlo simulations of radiatively accelerated GRB fireballs

We present a novel Dynamic Monte Carlo code (DynaMo code) that self-consistently simulates the Compton-scattering-driven dynamic evolution of a plasma. We use the DynaMo code to investigate the time-dependent expansion and acceleration of dissipationless gamma-ray burst fireballs by varying their initial opacities and baryonic content. We study the opacity and energy density evolution of an initially optically thick, radiation-dominated fireball across its entire phase space - in particular during the R-ph < R-sat regime. Our results reveal new phases of fireball evolution: a transition phase with a radial extent of several orders of magnitude - the fireball transitions from Gamma proportional to R to Gamma proportional to R-0, a post-photospheric acceleration phase-where fireballs accelerate beyond the photosphere and a Thomson-dominated acceleration phase - characterized by slow acceleration of optically thick, matter-dominated fireballs due to Thomson scattering. We quantify the new phases by providing analytical expressions of Lorentz factor evolution, which will be useful for deriving jet parameters.