X-ray Transmission Through Photoionized Gas with Moderate Thomson Optical Depth

Abstract

We model the absorption of X-rays by gas obscuring the source and photoionized by it. We consider a broad range of column densities, including both Thomson-thin and Thomson-thick media. For the Thomson thin regime, we derive a simple criterion based on the source luminosity and spectrum, as well as the medium radius and column density, that distinguishes between the following cases: (i) The absorption can be modeled well by a neutral medium; (ii) The radiation ionizes its way through the medium, and no absorption is expected; and (iii) A detailed model is required because the column density inferred from modeling the absorption with a neutral gas is much lower than the actual column density, or because the absorption features cannot be fitted by a neutral absorber. We derive the criterion analytically using a toy model of hydrogen and oxygen and calibrate it for realistic compositions with metallicities in the range $Z/Z_{\odot}=0.01-50$, using \textsc{Cloudy}. We generalize the model to the Thomson-thick regime, where we consider, alongside photoabsorption, electron scattering, Compton heating, Comptonization, and photon degradation. In this case, the emergent spectrum depends on the boundary condition experienced by photons scattered back towards the source. We discuss the effect of a reflective boundary and a reprocessing boundary. We provide simple criteria for the expected absorption state and discuss additional effects that alter the spectrum. The main motivation for our modeling is X-ray emission from supernovae interacting with the circumstellar medium; however, we expect it to be useful for many other applications.