The hot interstellar medium is an important part of the Galactic ecosystem and can be effectively characterized through X-ray absorption line spectroscopy. However, in a study of the hot medium using the accreting neutron star X-ray binary, Cyg X-2, as a background light source, a mystery came about when the putatively strong OVII Kalpha line was not detected in Chandra grating observations, while other normally weaker lines such as OVII Kbeta as well as OVI and OVIII Kalpha are clearly present (Yao et al. 2009). We have investigated the grating spectra of Cyg X-2 from 10 XMM-Newton observations, in search of the missing line. We detect it consistently in nine of these observations, but the line is absent in the remaining one observation or is inconsistent with the detection in others at a 4sigma confidence level. This absence of the line resembles that seen in the Chandra observations. Similarly, the OVI Kalpha line is found to disappear occasionally, but not in concert with the variation of the OVII Kalpha line. All these variations are most likely due to the presence of changing OVII and OVI Kalpha emission lines of Cyg X-2, which are blurred together with the absorption ones in the X-ray spectra. A re-examination of the Chandra grating data indeed shows evidence for a narrow emission line slightly off the OVI Kalpha absorption line. We further show that narrow NV emission lines with varying centroids and fluxes are present in far-UV spectra from the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. These results provide new constraints on the accretion around the neutron star and on the X-ray-heating of the stellar companion. The understanding of these physical processes is also important to the fidelity of using such local X-ray binaries for interstellar absorption line spectroscopy.
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