Experimental electron impact ionization cross sections of Fe XXV and Maxwellian-averaged rate coefficients

¹ Deutsches Elektronen Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
² Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
³ Associate, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
⁴ Center for Astrophysics | Harvard and Smithsonian, Cambridge, MA 02138, USA
⁵ National Institute of Standards and Technology, Boulder, CO 80305, USA
⁶ Northern Arizona University, Flagstaff, AZ 86011, USA
⁷ National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
⁸ Department of Physics, University of Colorado, Boulder, CO 80309, USA

^★ Corresponding author: etakacs@clemson.edu

Received: 28 February 2025
Accepted: 15 July 2025

Abstract

Context. Collisionally ionized plasmas produce X-ray spectra that depend on electron energy and atomic processes, requiring accurate atomic rate coefficients for modeling. While most data are theoretical, experimental measurements – such as those for Fe XXV – are critical for providing important benchmarks and quantifying uncertainties, thereby improving astrophysical spectral analysis, especially with high-resolution observatories like XRISM.

Aims. This study aimed to experimentally measure the electron impact ionization (EII) cross section of Fe XXV, a key ion in astrophysical plasma diagnostics, and to evaluate the impact of these measurements on the interpretation of X-ray spectra from high-temperature plasma environments.

Methods. The EII cross sections were measured using X-ray spectroscopy with an electron beam ion trap (EBIT) at various electron beam energies. The intensity ratios of the w resonance line in Fe XXV and the Lyα_1,2 lines in Fe XXVI were used to derive the cross sections. Level-resolved population calculations were performed using the NOMAD collisional-radiative code to account for processes affecting the emission ratios and ionization balance in the non-Maxwellian EBIT plasma.

Results. The measured EII cross sections show good agreement with relativistic convergent close-coupling calculations. The uncertainties in the measurements, ranging from 8% to 12%, are discussed in terms of their impact on the accuracy of temperature diagnostics for astrophysical plasma environments.