Thermal irradiation induced wind outflow in a geometrically thin accretion disk: A hydrodynamic study

TL;DR

This study models wind outflows in thin accretion disks caused by irradiation effects, revealing that winds mainly originate from outer disk regions and are influenced by ejection height and radius, with implications for X-ray binary observations.

Contribution

It introduces a hydrodynamic model of irradiation-driven wind outflow in geometrically thin disks, highlighting the role of irradiation and disk geometry in wind launching.

Findings

Wind outflow occurs mainly from outer disk regions.

Wind density decreases with increasing launching radius.

Wind power diminishes with higher ejection heights.

Abstract

Many astrophysical sources, e.g., cataclysmic variables, X-ray binaries, active galactic nuclei, exhibit a wind outflow, when they reveal a multicolor blackbody spectrum, hence harboring a geometrically thin Keplerian accretion disk. Unlike an advective disk, in the thin disk, the physical environment, like, emission line, external heating, is expected to play a key role to drive the wind outflow. We show the wind outflow in a thin disk attributing a disk irradiation effect, probably from the inner to outer disks. We solve the set of steady, axisymmetric disk model equations in cylindrical coordinates along the vertical direction for a given launching radius $(r)$ from the midplane, introducing irradiation as a parameter. We obtain an acceleration solution, for a finite irradiation in the presence of a fixed but tiny initial vertical velocity (hence thin disk properties practically do not alter) at the midplane, upto a maximum height ($z^{max}$). We find that wind outflow mainly occurs from the outer region of the disk and its density decreases with increasing launching radius, and for a given launching radius with increasing ejection height. Wind power decreases with increasing ejection height. For $z^{max} < 2r$, wind outflow is ejected tangentially (or parallel to the disk midplane) in all directions with the fluid speed same as the azimuthal speed. This confirms mainly, for low mass X-ray binaries, (a) wind outflow should be preferentially observed in high-inclination sources, (b) the expectation of red and blue shifted absorption lines.