TPHO: a time-dependent photoionisation model for AGN outflows

TL;DR

The paper introduces TPHO, a novel time-dependent photoionisation model that enables the measurement of plasma density in AGN outflows through time-resolved X-ray spectroscopy, improving understanding of their role in AGN feedback.

Contribution

The development of a self-consistent, time-dependent photoionisation model that accurately reproduces the spectra of variable AGN outflows and measures their density and location.

Findings

Standard models fail for non-equilibrium ionised gas spectra.

Time-changing warm absorbers may be mistaken for multiple components.

TPHO enables constraints on outflow density and location.

Abstract

Outflows in active galactic nuclei (AGN) are considered a promising candidate for driving AGN feedback at large scales. However, without information on the density of these outflows, we cannot determine how much kinetic power they are imparting to the surrounding medium. Monitoring the response of the ionisation state of the absorbing outflows to changes in the ionising continuum provides the recombination timescale of the outflow, which is a function of the electron density. We have developed a new self-consistent time-dependent photoionisation model, TPHO, enabling the measurement of the plasma density through time-resolved X-ray spectroscopy. The algorithm solves the full-time-dependent energy and ionisation balance equations in a self-consistent fashion for all the ionic species. The model can therefore reproduce the time-dependent absorption spectrum of ionized outflows responding to changes in the ionizing radiation of the AGN. We find that when the ionised gas is in a non-equilibrium state its transmitted spectra are not accurately reproduced by standard photoionisation models. Our simulations with the current X-ray grating observations show that the spectral features identified as a multiple-components warm absorber, might be in fact features of a time-changing warm absorber and not distinctive components. The TPHO model facilitates accurate photoionisation modelling in the presence of a variable ionising source, thus providing constraints on the density and in turn the location of the AGN outflows. Ascertaining these two parameters will provide important insight into the role and impact of ionised outflows in AGN feedback.