Radiation drag effects in black hole outflows from super-critical accretion disks via special relativistic radiation magnetohydrodynamics simulations

Abstract

By performing 2.5-dimensional special relativistic radiation magnetohydrodynamics simulations, we study super-critical accretion disks and the outflows launched via the radiation force. We find that the outflow is accelerated by the radiation flux force, but the radiation drag force prevents the outflow velocity from increasing. The outflow velocity saturates around 30%-40% of the light speed around the rotation axis, since then the flux force balances with the drag force. Our simulations show that the outflow velocity is kept nearly constant in the region of $\skew4\dot{M}-{\rm BH}\sim 10^{2-3}L-{\rm Edd}/c^{\,2}$, where $\skew4\dot{M}-{\rm BH}$ is the mass accretion rate, LEdd is the Eddington luminosity, and c is the light speed. Such a faster outflow is surrounded by a slower outflow of ∼ 0.1c. This velocity is also determined by the balance between the radiation flux force and the radiation drag. The radiation drag works to collimate the slower outflow in cooperation with the Lorentz force, although the faster outflow is mainly collimated by the Lorentz force. The kinetic energy is carried by the slower outflow rather than by the faster outflow. The total kinetic luminosity of the outflow as well as the photon luminosity is ∼ LEdd, almost independent of the mass accretion rate.