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Detection of the OVII line in XMM-Newton RGS spectroscopy of Cygnus X-2. This feature is an important diagnostic for studying the interstellar medium, yet was undetected in previous Chandra observations of the identical target.

The hot interstellar medium (ISM) is an important component of the galactic ecosystem as it traces feedback from stars and may account for missing baryonic matter within the galactic halo. Its spatial distribution, temperature, chemical composition, and kinematic properties may be probed by X-ray absorption spectroscopy. This technique relies on strong absorption from H and He-like ions in the ISM and X-ray bright background sources

The X-ray binary Cygnus X-2 (comprising a neutron star accreting gas from a low-mass companion) represents such a source. Prior observations of the binary by Yao et al. (2009) did not reveal the expected absorption at 21.6 Angstroms due to OVII. This finding called into question the fidelity of Cygnus X-2 and similar sources for hot ISM studies. I reduced and analyzed high-resolution spectra (RGS) acquired by XMM-Newton of the same source (Cabot, Wang, & Yao, 2013) to understand potential variability in the OVII absorption. While the feature is detected in stacked spectra, I found that both OVII and OVI exhibited temporal variation, and even disappeared in some XMM-Newton exposures. The most likely reason is narrow line emission from the accretion disk surrounding the neutron star. It is crucial to model and account for these emission lines in similar X-ray studies since they manifest as variability in key ISM absorption lines. 

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