Poynting-Robertson effect on black-hole-driven winds

¹ Department of Physics, Tor Vergata University of Rome, Via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail: mattia.marzi@libero.it
² Department of Astronomy, University of Maryland, 4296 Stadium Dr College, College Park, MD, 20742 USA
³ NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD, 20771 USA
⁴ INFN – Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
⁶ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
⁷ Department of Physics and Astronomy, James Madison University, 800 South Main Street, Harrisonburg, VA, 22807 USA

Received: 28 July 2022
Accepted: 1 January 2023

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 = 10r_S (where r_S 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.