Dynamics and emission of wind-powered afterglows of gamma-ray bursts: flares, plateaus and steep decays

We develop a model of early X-ray afterglows of gamma-ray bursts originating from the reverse shock (RS) propagating through ultra-relativistic, highly magnetized pulsar-like winds produced by long-lasting central engines. We first perform fluid and MHD numerical simulations of relativistic double explosions. We demonstrate that even for constant properties of the wind a variety of temporal behaviors can be produced, depending on the energy of the initial explosion and the wind power, the delay time for the switch-on of the wind, and magnetization of the wind. X-ray emission of the highly magnetized RS occurs in the fast cooling regime - this ensures high radiative efficiency and allows fast intensity variations. We demonstrate that: (i) RS emission naturally produces light curves showing power-law temporal evolution with various temporal indices; (ii) mild wind power, of the order of $\sim 10^{46}$ erg s$^{-1}$ (equivalent isotropic), can reproduce the afterglows' plateau phase; (iii) termination of the wind can produce sudden steep decays; (iv) short-duration afterglow flares are due to mild variations in the wind luminosity, with small total injected energy.