X-ray Signatures of Non-Equilibrium Ionization Effects in Galaxy Cluster
  Accretion Shock Regions
  The densities in the outer regions of clusters of galaxies are very low, and
the collisional timescales are very long. As a result, heavy elements will be
under-ionized after they have passed through the accretion shock. We have
studied systematically the effects of non-equilibrium ionization for relaxed
clusters in the LambdaCDM cosmology using one-dimensional hydrodynamic
simulations. We found that non-equilibrium ionization effects do not depend on
cluster mass but depend strongly on redshift which can be understood by
self-similar scaling arguments. The effects are stronger for clusters at lower
redshifts. We present X-ray signatures such as surface brightness profiles and
emission 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, and
the enhancement can be nearly an order of magnitude near the shock radius. The
most prominent non-equilibrium ionization signature we found is the O VII and O
VIII line ratio. The ratios for non-equilibrium ionization and collisional
ionization equilibrium models are different by more than an order of magnitude
at radii beyond half of the shock radius. These non-equilibrium ionization
signatures are equally strong for models with different non-adiabatic shock
electron heating efficiencies. We have also calculated the detectability of the
O 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 ~130-380
ksec on a moderate-redshift, massive regular cluster with the X-ray
Microcalorimeter Spectrometer (XMS) on the IXO will be sufficient to provide a
strong test for the non-equilibrium ionization model.
