POSSIBLE CHARGE-EXCHANGE X-RAY EMISSION IN THE CYGNUS LOOP DETECTED WITHSUZAKU

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 ∼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 ∼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 ∼0.7 keV. This feature is likely a complex of He-like O K(γ + δ + ǫ), although other possibilities cannot be fully excluded. The intensity of this emission relative to He-like O Kα 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.

X-ray emission from charge exchange in the Cygnus Loop SNR

“…Radially both this analysis and that of Katsuda et al (2011) agree with the theoretical calcuations done by Lallement (2004), at least qualitatively. In that work, the author shows that CX effects are generally negligible for an SNR in the Sedov phase, except in the narrow rim region where line of sight effects amplify the fraction of emission from CX.…”

“…In that work, the author shows that CX effects are generally negligible for an SNR in the Sedov phase, except in the narrow rim region where line of sight effects amplify the fraction of emission from CX. In Figure 3 of that work, it is shown that CX can contribute over 50% of the X-ray emission immediately behind the shock and approximately 30% in the region contained within the shock radius(Rs) and r=0.98Rs (Katsuda et al 2011), thus detailing the confinement of CX to the rim. However, azimuthally we find that the regions with OVII Kα line centroid shifts do not match those with the emission enhancement at ∼ 0.7 keV, as studied by Katsuda et al (2011).…”

“…Since this cometary epiphany, CX has been proposed and studied in a wide range of X-ray sources, from planetary atmospheres (Dennerl 2002;Dennerl et al 2006;Kharchenko et al 2008) through individual star-forming complexes (Townsley et al 2011) and star-forming galaxies (Tsuru et al 2007;Liu et al 2011Liu et al , 2012 to the cores of clusters (Fabian et al 2011). More recently, the first evidence for X-ray emitting CX in a supernova remnant (SNR) has been suggested (Katsuda et al 2011).…”

Four new X-ray-selected supernova remnants in the Large Magellanic Cloud

“…There could be charge exchange (CX) emission contribution to that line complex, instead of a purely thermal emission. CX X-ray emission in SNRs has been detected in the Cygnus Loop and Puppis A remnants (Katsuda et al 2011(Katsuda et al , 2012. The fraction of CX to the X-ray emission can be enhanced if the hot gas is interacting with denser, neutral gas, as we suggest for MCSNR J0517−6759.…”

A distance estimate to the Cygnus Loop based on the distances to two stars located within the remnant

“…In addition, X-ray emissions from certain regions along northeast and southwest limbs have relatively high metal abundances also suggesting a high-mass, core-collapse supernova (Tsunemi et al 2007;Katsuda et al 2008;Kimura et al 2009;Uchida et al 2009;Fujimoto et al 2011;Katsuda et al 2011). Hubble (1937) of the bright eastern and western nebulae away from the centre of expansion.…”