On the Observability of Optically Thin Coronal Hyperfine Structure Lines

TL;DR

This paper models coronal hyperfine lines' intensity in various environments, providing estimates for their observability and implications for interpreting spectra near bright AGN, with potential for future radio and sub-mm observations.

Contribution

It offers detailed Cloudy calculations of hyperfine line emissivity, considering indirect transitions and environmental effects, advancing understanding of their observability.

Findings

Hyperfine lines can be enhanced by radiation fields of AGN.

The 57Fe XXIV line is about 20 μK, much fainter than previously thought.

Hyperfine lines in the Orion Nebula are weak due to sightline effects.

Abstract

We present Cloudy calculations for the intensity of coronal hyperfine lines in various environments. We model indirect collisional and radiative transitions, and quantify the collisionally-excited line emissivity in the density-temperature phase-space. As an observational aid, we also express the emissivity in units of the continuum in the 0.4--0.7 keV band. For most hyperfine lines, knowledge of the X-ray surface brightness and the plasma temperature is sufficient for rough estimates. We find that the radiation fields of both Perseus A and Virgo A can enhance the populations of highly ionized species within 1 kpc. They can also enhance line emissivity within the cluster core. This could have implications for the interpretation of spectra around bright AGN. We find the intensity of the $^{57}$Fe XXIV {\lambda}3.068 mm to be about two orders of magnitude fainter than previously thought, at about 20 {\mu}K. Comparably bright lines may be found in the infrared. Finally, we find the intensity of hyperfine lines in the Extended Orion Nebula to be low, due to the shallow sightline. Observations of coronal hyperfine lines will likely be feasible with the next generation of radio and sub-mm telescopes.