X-ray spectroscopy in the microcalorimeter era I: Effects of Fe XXIV Resonance Auger Destruction on Fe XXV K$\alpha$ spectra

TL;DR

This paper investigates how Resonance Auger Destruction (RAD) affects Fe XXV Kα spectra in astrophysical environments, revealing significant photon destruction in dense, optically thick clouds and implications for spectral interpretation.

Contribution

It extends the understanding of RAD effects on Fe XXV Kα lines across a wide range of column densities using CLOUDY simulations, highlighting the impact on line intensities.

Findings

Approximately 32% of x photons are destroyed at high column densities due to RAD.

The y line intensity decreases by up to 2% because of RAD effects.

z line remains unaffected by RAD but may be slightly enhanced through photon conversion.

Abstract

We discuss the importance of Fe$^{23+}$ in determining the line intensities of the Fe XXV K$\alpha$ complex in an optically thick cloud, and investigate the prediction of Liedahl (2005) on Resonance Auger Destruction (RAD) with CLOUDY. Although initially motivated by the Perseus cluster, our calculations are extended to the wide range of column densities encountered in astronomy. A Fe XXV line photon can change/lose its identity upon absorption by three-electron iron as a result of "line interlocking". This may lead to the autoionization of the absorbing ion, ultimately destroying the Fe XXV K$\alpha$ photon by RAD. Out of the four members in the Fe XXV K$\alpha$ complex, a significant fraction of the x line photons is absorbed by Fe$^{23+}$ and destroyed, causing the x line intensity to decrease. For example, at a hydrogen column density of 10$^{25}$ cm$^{-2}$, $\sim$ 32\% of x photons are destroyed due to RAD while w is mostly unaffected. The line intensity of y is slightly ($\leq$2\%) reduced. z is not directly affected by RAD, but the contrasting behavior between z and x line intensities points towards the possible conversion of a tiny fraction ($\sim$ 2\%) of x photons into z photons. The change in line intensities due to Electron Scattering Escape (ESE) off fast thermal electrons is also discussed.