Poynting-Robertson effect on black-hole-driven winds

Abstract

Context. Layers of ionized plasma in the form of winds ejected from the accretion disk of supermassive black holes (SMBHs) are frequently observed in active galactic nuclei (AGNs). Winds with a velocity often exceeding 0.1c are called ultrafast outflows (UFOs) and thanks to their high power they can play a key role in the co-evolution between the SMBH and the host galaxy. In order to construct a realistic model of the properties of these winds, it is necessary to consider special relativistic corrections due to their very high velocities. Aims. We present a derivation of the Poynting–Robertson effect (P–R effect) and apply it to the description of the dynamics of UFOs. The P–R effect is a special relativistic correction that breaks the isotropy of the radiation emitted by a moving particle, funneling the radiation in the direction of motion. As a result of the conservation of the four-momentum, the emitting particles are subjected to a drag force and decelerate. Methods. We provide a derivation of the drag force caused by the P–R effect starting from general Lorentz transformations and assuming isotropic emission in the gas reference frame. We then derive the equations to easily implement this drag force in future simulations. Finally, we apply them in a simple case in which we assume that the gas can be described by a toy model in which the gas particles move radially under the influence of the gravitation force, the force caused by radiation pressure, and the drag force due to the P–R effect. Results. The P–R effect plays an important role in determining the velocity profile of the wind. For a wind launched from r0 = 10rS (where rS stands for the Schwarzschild radius), the asymptotic velocity reached by the wind is between 10% and 24% lower than if we neglect the P–R effect. This result demonstrates that, in order to obtain proper values of the mass and energy outflow rates, the P–R effect should be taken into account when studying the dynamics of high-velocity, photoionized outflows in general.