X-ray Emission from Charge Exchange in the Cygnus Loop SNR

TL;DR

This study uses X-ray spectral analysis to identify charge exchange contributions in the Cygnus Loop SNR, highlighting the importance of including CX in models to accurately determine the remnant's properties.

Contribution

It provides observational evidence of charge exchange effects in the Cygnus Loop SNR using OVII triplet diagnostics, emphasizing the need for CX modeling in SNR studies.

Findings

Enhanced forbidden line emission correlates with non-radiative Hα filaments.

Charge exchange effects are present throughout much of the remnant's rim.

Alternative explanations for the observed line enhancements are ruled out.

Abstract

The Cygnus Loop has been the focus of substantial debate concerning the contribution of charge exchange (CX) to supernova remnant (SNR) X-ray emission. We take advantage of a distinct feature of CX, enhanced K{\alpha} forbidden line emission, and employ the energy centroid of the OVII K{\alpha} triplet as a diagnostic. Based on X-ray spectra extracted from an extensive set of Suzaku observations, we measure the energy centroid shifts of the triplet on and off the shock rim of the remnant. We find that enhanced forbidden to resonance line emission exists throughout much of the rim and this enhancement azimuthally correlates with non-radiative H{\alpha} filaments, a tracer of strong neutral-plasma interaction in the optical. We also show that alternative mechanisms cannot explain the enhancement observed. These results demonstrate the need to model the CX contribution to the X-ray emission of SNRs, particularly for shocks propagating in a partially neutral medium. Such modeling may be critically important to the correct measurements of the ionization, thermal, and chemical properties of SNRs.