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The recent survey of the core-collapse supernova remnant Cassiopeia A (CasA)with the MIRI instrument on board the James Webb Space Telescope (JWST)revealed a large structure in the interior region, referred to as the "GreenMonster". Although its location suggests that it is an ejecta structure, theinfrared properties of the "Green Monster" hint at a circumstellar medium (CSM)origin. In this companion paper to the JWST Cas A paper, we investigate thefilamentary X-ray structures associated with the "Green Monster" using ChandraX-ray Observatory data. We extracted spectra along the "Green Monster" as wellas from shocked CSM regions. Both the extracted spectra and a principalcomponent analysis show that the "Green Monster" emission properties aresimilar to those of the shocked CSM. The spectra are well-fit by a modelconsisting of a combination of a non-equilibrium ionization model and apower-law component, modified by Galactic absorption. All the "Green Monster"spectra show a blueshift of around ~2500 km/s, suggesting that the structure ison the near side of Cas A. The ionization age is around $n_{e}t$ = $1.4 \times10^{11}$ cm$^{-3}$s. This translates into a pre-shock density of ~11 cm$^{-3}$,higher than previous estimates of the unshocked CSM. The relatively high netand relatively low radial velocity suggest that this structure has a relativelyhigh density compared to other shocked CSM plasma. This analysis provides yetanother piece of evidence that the CSM around Cas A's progenitor was not thatof a smooth steady wind profile.

X-ray diagnostics of Cassiopeia A's "Green Monster": evidence for dense shocked circumstellar plasma