Properties of warm absorbers in active galaxies: a systematic stability curve analysis

TL;DR

This study analyzes the stability and properties of warm absorbers in active galaxies using thermal equilibrium curves, revealing how ionizing continuum shape, density, and metallicity influence their existence and phase structure.

Contribution

It systematically investigates the conditions for stable warm absorbers, highlighting the roles of spectral index, density, metallicity, and elemental composition in their multiphase nature.

Findings

Warm absorbers exist only if spectral index > 0.2.

Most Seyfert 1 galaxies have a multiphase warm absorber at spectral index ~0.8.

High-density, soft-spectrum absorbers are sensitive to density and metallicity.

Abstract

Signatures of warm absorbers are seen in soft X-ray spectra of about half of all Seyfert1 galaxies observed and in some quasars and blazars. We use the thermal equilibrium curve to study the influence of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas on the existence and nature of the warm absorbers. We describe circumstances in which a stable warm absorber can exist as a multiphase medium or one with continuous variation in pressure. In particular we find the following results: i) the warm absorber exists only if the spectral index of the X-ray power-law ionizing continuum $\alpha > 0.2$ and has a multiphase nature if $\alpha \sim 0.8$, which interestingly is the spectral index for most of the observed Seyfert 1 galaxies; ii) thermal and ionization states of highly dense warm absorbers are sensitive to their density if the ionizing continuum is sufficiently soft, i.e. dominated by the ultraviolet iii) absorbing gas with super-Solar metallicity is more likely to have a multiphase nature; iv) the nature of the warm absorber is significantly influenced by the absence of iron and associated elements which are produced in the later stages of star formation history in supernovae of type Ia.