Measuring density of X-ray detected AGN outflows with free-free heating

Efficiently accreting radiative Active Galactic Nuclei (AGN) drive powerful wide-angle outflows, the most energetic (massive) of which imprint absorption features onto the unresolved black hole X-ray radiation. These winds can have velocities from hundreds to tens of thousands of kilometers per second, potentially carrying away large amounts of mass, momentum, and energy, and thus affecting black hole accretion and star formation of the host galaxy. However, to actually measure this impact of AGN on galaxy evolution, we must first constrain the ionization, velocity, and density structure of the highly ionized winds. Unfortunately, density, the critical property of the outflowing gas, remains virtually unconstrained, preventing us from measuring mass loss rates and wind energetics, and therefore understanding wind-mode AGN feedback.

In this talk we will introduce a novel approach to measuring the density of the outflows detected in absorption with the Chandra and XMM-Newton gratings. We will demonstrate, with examples from recent observations, how by properly accounting for free-free heating and infrared radiation, we can precisely measure density of dense outflows with statistical precision of up to few per cent, and provide meaningful upper limits for low density gas. Critically, we will show how failing to account for these effects can dramatically bias the derived properties of X-ray detected outflows, such as the ionization parameter and velocity. We will also discuss systematic effects impacting our measurements, and how to implement our approach with leading photoionization codes (Cloudy, XSTAR, pion/SPEX). The potential of our method will be fully realized with the next generation of X-ray gratings and microcalorimeters, such as XRISM, Athena, LEM, and Arcus.