On determining the cluster abundance normalization

Accepted...; Received...; in original form... Different determinations currently suggest scattered values for the power spectrum normalization on the scale of galaxy clusters, σ8. Here we concentrate on the constraints coming from the X-ray temperature and luminosity functions (XTF and XLF), and investigate several possible sources of discrepancies in the results. We conclude that the main source of error in both methods is the scaling relation involved, in particular the way its intrinsic scatter and systematic normalization are treated. For temperature derived constraints, we use a sample adapted from HIFLUGCS, and test for several sources of systematic error. We parameterize the mass-temperature relation with an overall factor T∗, which varies between about 1.5 and 1.9 in the literature, with simulations typically giving lower results than empirically derived estimates. After marginalising over this range of T∗, we obtain a 68 per cent confidence range of σ8 = 0.77 +0.05 −0.04