POSSIBLE CHARGE-EXCHANGE X-RAY EMISSION IN THE CYGNUS LOOP DETECTED WITH<i>SUZAKU</i>

Abstract

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