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X-ray spectroscopic measurements of the Cygnus Loop supernova remnantindicate that metal abundances throughout most of the remnant's rim aredepleted to about 0.2 times the solar value. However, recent X-ray studies haverevealed in some narrow regions along the outermost rim anomalously "enhanced"abundances (up to about 1 solar). The reason for these anomalous abundances isnot understood. Here, we examine X-ray spectra in annular sectors coveringnearly the entire rim of the Cygnus Loop using Suzaku (21 pointings) andXMM-Newton (1 pointing). We find that spectra in the "enhanced" abundanceregions commonly show a strong emission feature at about 0.7 keV. This featureis likely a complex of He-like O K(gamma + delta + epsilon), although otherpossibilities cannot be fully excluded. The intensity of this emission relativeto 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 inthe outermost rim, leads us to propose an origin from charge-exchange processesbetween neutrals and H-like O. We show that the presence of charge-exchangeemission could lead to the inference of apparently "enhanced" metal abundancesusing pure thermal emission models. Accounting for charge-exchange emission,the actual abundances could be uniformly low throughout the rim. The overallabundance depletion remains an open question.

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