Possible Charge-Exchange X-Ray Emission in the Cygnus Loop Detected with Suzaku

TL;DR

This study investigates anomalous X-ray emission features in the Cygnus Loop supernova remnant, proposing charge-exchange processes as a cause, which could explain apparent abundance enhancements observed in certain rim regions.

Contribution

It introduces the hypothesis that charge-exchange emission explains localized abundance anomalies, challenging previous thermal emission interpretations.

Findings

Detection of a 0.7 keV emission feature in rim regions.

Charge-exchange processes may cause apparent abundance enhancements.

Uniform low abundances could be true if charge-exchange is considered.

Abstract

X-ray spectroscopic measurements of the Cygnus Loop supernova remnant indicate that metal abundances throughout most of the remnant's rim are depleted to about 0.2 times the solar value. However, recent X-ray studies have revealed in some narrow regions along the outermost rim anomalously "enhanced" abundances (up to about 1 solar). The reason for these anomalous abundances is not understood. Here, we examine X-ray spectra in annular sectors covering nearly the entire rim of the Cygnus Loop using Suzaku (21 pointings) and XMM-Newton (1 pointing). We find that spectra in the "enhanced" abundance regions commonly show a strong emission feature at about 0.7 keV. This feature is likely a complex of He-like O K(gamma + delta + epsilon), although other possibilities cannot be fully excluded. The intensity of this emission relative to He-like O Kalpha appears to be too high to be explained as thermal emission. This fact, as well as the spatial concentration of the anomalous abundances in the outermost rim, leads us to propose an origin from charge-exchange processes between neutrals and H-like O. We show that the presence of charge-exchange emission could lead to the inference of apparently "enhanced" metal abundances using pure thermal emission models. Accounting for charge-exchange emission, the actual abundances could be uniformly low throughout the rim. The overall abundance depletion remains an open question.