X-ray spectroscopy in the microcalorimeter era 4: Optical depth effects on the soft X-rays studied with CLOUDY

TL;DR

This paper investigates how optical depth effects influence soft X-ray spectra in astrophysical environments, introducing a line modification factor to quantify suppression or enhancement of lines, and applies this to Chandra observations of V1223 Sgr.

Contribution

It presents a new framework for understanding optical depth effects on soft X-ray lines using the line modification factor and demonstrates its application to real astrophysical data.

Findings

Line suppression occurs in collisionally-ionized and photoionized systems.

Line enhancement occurs in photoionized environments.

Application to V1223 Sgr explains excess flux in specific X-ray lines.

Abstract

In this paper, we discuss atomic processes modifying the soft X-ray spectra in from optical depth effects like photoelectric absorption and electron scattering suppressing the soft X-ray lines. We also show the enhancement in soft X-ray line intensities in a photoionized environment via continuum pumping. We quantify the suppression/enhancement by introducing a "line modification factor ($f_{\rm mod}$)." If 0 $\leq$ $f_{\rm mod}$ $\leq$ 1, the line is suppressed, which could be the case in both collisionally-ionized and photoionized systems. If $f_{\rm mod}$ $\geq$ 1, the line is enhanced, which occurs in photoionized systems. Hybrid astrophysical sources are also very common, where the environment is partly photoionized and partly collisionally-ionized. Such a system is V1223 Sgr, an intermediate polar binary. We show the application of our theory by fitting the first-order Chandra MEG spectrum of V1223 Sgr with a combination of \textsc{Cloudy}-simulated additive cooling-flow and photoionized models. In particular, we account for the excess flux for O~VII, O~VIII, Ne~IX, Ne~X, and Mg~XI lines in the spectrum found in a recent study, which could not be explained with an absorbed cooling-flow model.