Radiation drag effects on magnetically dominated outflows around compact objects

Abstract

The effects of radiation drag force on the structure of relativistic electron-positron and electron-proton outflows are considered within the one-fluid approximation for quasi-monopole cold outflow. It is shown that for a Poynting-dominated case the drag force does not change the particle energy inside the fast magnetosonic surface. In this region the action of the drag results in a diminishing of the Poynting flux, not the particle flux. Outside the fast magnetosonic surface, for intermediate photon density the drag force may result in additional acceleration of the plasma. This acceleration is a result of the disturbance of magnetic surfaces under the action of the drag. At even larger distances particles are not frozen into the magnetic field and the drag force decelerates them efficiently. In the case of extreme photon densities, the disturbance of magnetic surfaces becomes large and the drag force changes the total energy flux significantly, the particles becoming nonrelativistic. We find that for Active Galactic Nuclei the photon density is too low to disturb the parameters of an ideal MHD outflow. The drag action may result in additional acceleration of outgoing plasma only for central engines with very high luminosities. For cosmological gamma-ray bursts the drag force can strongly affect the process of formation of a Poynting-dominated outflow.