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The densities in the outer regions of clusters of galaxies are very low, andthe collisional timescales are very long. As a result, heavy elements will beunder-ionized after they have passed through the accretion shock. We havestudied systematically the effects of non-equilibrium ionization for relaxedclusters in the LambdaCDM cosmology using one-dimensional hydrodynamicsimulations. We found that non-equilibrium ionization effects do not depend oncluster mass but depend strongly on redshift which can be understood byself-similar scaling arguments. The effects are stronger for clusters at lowerredshifts. We present X-ray signatures such as surface brightness profiles andemission lines in detail for a massive cluster at low redshift. In general,soft emission (0.3-1.0 keV) is enhanced significantly by under-ionization, andthe enhancement can be nearly an order of magnitude near the shock radius. Themost prominent non-equilibrium ionization signature we found is the O VII and OVIII line ratio. The ratios for non-equilibrium ionization and collisionalionization equilibrium models are different by more than an order of magnitudeat radii beyond half of the shock radius. These non-equilibrium ionizationsignatures are equally strong for models with different non-adiabatic shockelectron heating efficiencies. We have also calculated the detectability of theO VII and O VIII lines with the future International X-ray Observatory (IXO).Depending on the line ratio measured, we conclude that an exposure of ~130-380ksec on a moderate-redshift, massive regular cluster with the X-rayMicrocalorimeter Spectrometer (XMS) on the IXO will be sufficient to provide astrong test for the non-equilibrium ionization model.

X-ray Signatures of Non-Equilibrium Ionization Effects in Galaxy Cluster Accretion Shock Regions