2019ApJ...871..234U

Charge exchange (CX) is an important process in shock physics because it indicates an interaction between downstream ions and ambient neutral hydrogen, suggesting the 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) on board XMM-Newton. The target is a compact knotty structure called the "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 O VII Heα. 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.