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The hot interstellar medium is an important part of the Galactic ecosystemand can be effectively characterized through X-ray absorption linespectroscopy. However, in a study of the hot medium using the accreting neutronstar X-ray binary, Cyg X-2, as a background light source, a mystery came aboutwhen the putatively strong OVII Kalpha line was not detected in Chandra gratingobservations, while other normally weaker lines such as OVII Kbeta as well asOVI and OVIII Kalpha are clearly present (Yao et al. 2009). We haveinvestigated the grating spectra of Cyg X-2 from 10 XMM-Newton observations, insearch of the missing line. We detect it consistently in nine of theseobservations, but the line is absent in the remaining one observation or isinconsistent with the detection in others at a 4sigma confidence level. Thisabsence of the line resembles that seen in the Chandra observations. Similarly,the OVI Kalpha line is found to disappear occasionally, but not in concert withthe variation of the OVII Kalpha line. All these variations are most likely dueto 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. Are-examination of the Chandra grating data indeed shows evidence for a narrowemission line slightly off the OVI Kalpha absorption line. We further show thatnarrow NV emission lines with varying centroids and fluxes are present infar-UV spectra from the Cosmic Origins Spectrograph aboard the Hubble SpaceTelescope. These results provide new constraints on the accretion around theneutron star and on the X-ray-heating of the stellar companion. Theunderstanding of these physical processes is also important to the fidelity ofusing such local X-ray binaries for interstellar absorption line spectroscopy.

XMM-Newton/RGS Detection of the Missing Interstellar OVII Kalpha Absorption Line in the Spectrum of Cyg X-2