https://orcid.org/0000-0003-2008-6887
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Makoto Sawada, Liyi Gu, Ryo Yamazaki, Is the low-energy tail of shock-accelerated protons responsible for over-ionized plasma in supernova remnants?, Publications of the Astronomical Society of Japan, Volume 76, Issue 6, December 2024, Pages 1158–1172, https://doi.org/10.1093/pasj/psae077
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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 that assumes a point explosion in a uniform interstellar medium, thus 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 |$5\ (k T_{\rm e}/1\:\mbox{keV})\%$| 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 most.
