High Forbidden-to-resonance Line Ratio of OVII Discovered in the Cygnus Loop

Abstract

Charge exchange (CX) is an important process in shock physics since it indicates an interaction between downstream ions and ambient neutral hydrogen, suggesting a presence of a collisionless shock. We present a high-resolution spectroscopy of an X-ray bright spot in a nearby supernova remnant (SNR), the Cygnus Loop, with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton. The target is a compact knotty structure called "southwestern knot (SW-K)" located at the outer edge of the shell, where the blast wave is likely interacting with dense surrounding materials. The RGS spectrum of the SW-K shows details of the line features below ~ 1 keV, where we discover a high forbidden-to-resonance line ratio of OVII He$\alpha$. The soft-band (10-35 Å) spectrum is well explained by a thermal component with a CX X-ray emission obscured by neutral and ionized absorbers. The presence of the CX X-ray emission will provide new insights into the shock physics of SNRs. The high-resolution spectroscopy also reveals that the CNO, Ne and Fe abundances are truly lower than the solar values (0.2-0.4 solar) at the SW-K region . Our result gives a clue to solving the previously known "low-abundance problem" reported from a number of evolved SNRs.

10 pages, including 4 figures and 2 tables, accepted for publication in ApJ

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• Charge Exchange X-ray Emission Detected in Multiple Shells of Supernova Remnant G296.1-0.5
• RGS Observations of Ejecta Knots in Tycho's Supernova Remnant
• Evidence for Resonance Scattering in the X-ray Grating Spectrum of the Supernova Remnant N49
• High resolution X-ray study of supernova remnant J0453.6$-$6829 with unusually high forbidden-to-resonance ratio
• Progenitor constraint with circumstellar material for the magnetar-hosting supernova remnant RCW 103
• Unusual X-ray Oxygen Line Ratios of SN 1987A Arising From the Absorption of Galactic Hot Interstellar Medium