Is the low-energy tail of shock-accelerated protons responsible for over-ionized plasma in supernova remnants?

TL;DR

This study investigates whether low-energy shock-accelerated protons can cause over-ionized plasma in supernova remnants, concluding that the required proton abundance is too high to be feasible based on current acceleration models.

Contribution

The paper provides the first detailed calculation of proton impact ionization rates on heavy-element ions in SNR ejecta and assesses the viability of the proton ionization scenario for over-ionization.

Findings

Proton impact ionization can contribute to over-ionization but requires an unrealistically high proton abundance.

The necessary proton-to-electron ratio exceeds typical acceleration efficiencies, making the scenario unlikely.

The results challenge the hypothesis that low-energy protons are responsible for over-ionized plasma in SNRs.

Abstract

Over-ionized, recombining plasma is an emerging class of X-ray bright supernova remnants (SNRs). This unique thermal state where the ionization temperature ($T_{\rm z}$) is significantly higher than the electron temperature ($T_{\rm e}$) is not expected from the standard evolution model assuming a point explosion in a uniform interstellar medium, requiring a new scenario for the dynamical and thermal evolution. A recently proposed idea attributes the over-ionization state to additional ionization contribution from the low-energy tail of shock-accelerated protons. However, this new scenario has been left untested, especially from the atomic physics point of view. We report calculation results of the proton impact ionization rates of heavy-element ions in ejecta of SNRs. We conservatively estimate the requirement for accelerated protons, and find that their relative number density to thermal electrons needs to be higher than $5~(T_{\rm e}/{\rm 1~keV})\%$ in order to explain the observed over-ionization degree at $T_{\rm z}/T_{\rm e} \ge 2$ for K-shell emission. We conclude that the proton ionization scenario is not feasible because such a high abundance of accelerated protons is prohibited by the injection fraction from thermal to non-thermal energies, which is expected to be $\sim 1\%$ at largest.