Charge Exchange X-ray Emission of M82: K{\alpha} triplets of O VII, Ne IX, and Mg XI

TL;DR

This study analyzes X-ray emission lines in M82 to distinguish between thermal emission and charge exchange processes, revealing that CXE significantly contributes to observed triplets and affects abundance estimates.

Contribution

It provides the first quantification of charge exchange contributions to key X-ray triplets in a starburst galaxy, improving understanding of feedback mechanisms.

Findings

Charge exchange contributes ~50% to the total triplet flux.

CXE dominates the O VII triplet with 90% contribution.

Estimated elemental abundances are similar to solar ratios.

Abstract

Starburst galaxies are primary feedback sources of mechanical energy and metals, which are generally measured from associated X-ray emission lines assuming that they are from the thermal emission of the outflowing hot gas. Such line emission, however, can also arise from the charge exchange X-ray emission (CXE) between highly ionized ions and neutral species. To understand the feedback of energy and metals, it is crucial to determine the origin of the X-ray emission lines and to distinguish the contributions from the CXE and the thermal emission. The origin of the lines can be diagnosed by the K{\alpha} triplets of He-like ions, because the CXE favors the inter-combination and forbidden lines, while the thermal emission favors the resonance line. We analyze the triplets of O VII, Ne IX, and Mg XI observed in the XMM- Newton reflection grating spectra of the starburst galaxy M82. The flux contribution of the CXE is 90%, 50%, and 30% to the O VII, Ne IX, and Mg XI triplet, respectively. Averaged over all the three triplets, the contribution of the CXE is \sim 50% of the total observed triplet flux. To correctly understand the hot outflow of starburst galaxies, it is necessary to include the CXE. Based on the measured CXE contributions to the O VII, Ne IX, and Mg XI triplets, we estimate the relative abundances of O, Ne, and Mg of the outflow and find they are similar to the solar ratios.