Numerical simulations of the line-force-driven winds from active galactic nuclei: The special relativistic effects

Abstract

Ultra-fast outflows (UFOs) with mildly relativistic velocities are frequently observed in active galactic nuclei (AGNs). The line-force-driving mechanism is often taken as a potential mechanism for driving UFOs. Due to the line-force-driven winds moving at mildly relativistic velocities, the special relativistic effects become important. There are two special relativistic effects: one is the influence of the disc rotation on the radiation field; the other is the radiation-drag effect. We wish to study the influence of the special relativistic effects on the line-force-driven winds, and we performed numerical simulations to investigate this. We find that the line-force-driven winds are significantly weakened when the special relativistic effects are considered. Compared with the case without special relativistic effects, when special relativistic effects are considered the winds are closer to the disc surface, the maximum speed of winds is reduced by $\sim$20 percent--70 percent, and the mass outflow rate and the kinetic power is significantly reduced.