The influence of soft spectral components on the structure and stability of warm absorbers in AGN

TL;DR

This paper models how the unobservable extreme ultraviolet continuum shape, including a soft excess, influences the thermal, ionization, and absorption features of warm absorbers in AGN, aiding in indirect continuum characterization.

Contribution

It predicts the impact of different unobservable continuum shapes on warm absorber properties, highlighting the role of soft excess and disk temperature in shaping absorption features.

Findings

Warm absorber ionization states depend on the unobservable continuum shape.

Soft excess and disk temperature significantly influence absorption line strengths.

Results may enable indirect inference of the unobservable AGN continuum.

Abstract

The radiation from the central regions of active galactic nuclei, including that from the accretion disk surrounding the black hole, is likely to peak in the extreme ultraviolet $\sim 13 -100$ eV. However, due to Galactic absorption, we are limited to constrain the physical properties, i.e. the black hole mass and the accretion rate, from what observations we have below $\sim 10$ eV or above $\sim 100$ eV. In this paper we predict the thermal and ionization states of warm absorbers as a function of the shape of the unobservable continuum. In particular we model an accretion disk at $kT_{in} \sim 10$ eV and a {\it soft excess} at $kT_{se} \sim 150$ eV. The warm absorber, which is the highly ionized gas along the line of sight to the continuum, shows signatures in the $\sim 0.3 - 2$ keV energy range consisting of numerous absorption lines and edges of various ions, some of the prominent ones being H- and He-like oxygen, neon, magnesium and silicon. We find that the properties of the warm absorber are significantly influenced by the changes in the temperature of the accretion disk, as well as by the strength of the {\it soft excess}, as they affect the optical depth particularly for iron and oxygen. These trends may help develop a method of characterising the shape of the unobservable continuum and the occurrence of warm absorbers.