Position dependent radiation fields near accretion disks

TL;DR

This paper investigates how the anisotropic radiation field near accretion disks in AGN varies with position, affecting ionization and radiation forces, which are crucial for understanding disk wind dynamics.

Contribution

It provides the first quantitative analysis of position-dependent radiation fields near accretion disks, highlighting their impact on ionization and radiation force calculations.

Findings

Radiation field properties vary significantly with position.

Mean intensity is softer than radial flux, reducing overionization.

Radiation force is not aligned with radiation flux in winds.

Abstract

In disk wind models for active galactic nuclei (AGN) outflows, high-energy radiation poses a significant problem wherein the gas can become overionized, effectively disabling what is often inferred to be the largest force acting on the gas: the radiation force due to spectral line opacity. Calculations of this radiation force depend on the magnitude of ionizing radiation, which can strongly depend on the position above a disk where the radiation is anisotropic. As our first step to quantify the position and direction dependence of the radiation field, we assumed free streaming of photons and computed energy distributions of the mean intensity and components of flux as well as energy-integrated quantities such as mean photon energy. We find a significant dependence of radiation field properties on position, but this dependence is not necessarily the same for different field quantities. A key example is that the mean intensity is much softer than the radial flux at many points near the disk. Because the mean intensity largely controls ionization, this softening decreases the severity of the overionization problem. The position dependence of mean intensity implies the position dependence of gas opacity, which we illustrate by computing the radiation force a fluid element feels in an accelerating wind. We find that in a vertical accelerating flow, the force due to radiation is not parallel to the radiation flux. This misalignment is due to the force's geometric weighting by both the velocity field's directionality and the position dependence of the mean intensity.