Reconciling Planck cluster counts and cosmology? Chandra/XMM instrumental calibration and hydrostatic mass bias.

The mass of galaxy clusters can be inferred from the temperature of their X-ray-emitting gas, TX. Their masses may be underestimated if it is assumed that the gas is in hydrostatic equilibrium, by an amount bhyd ∼ (20 ± 10) per cent suggested by simulations. We have previously found consistency between a sample of observed Chandra X-ray masses and independent weak lensing measurements. Unfortunately, uncertainties in the instrumental calibration of Chandra and XMM–Newton observatories mean that they measure different temperatures for the same gas. In this paper, we translate that relative instrumental bias into mass bias, and infer that XMM–Newton masses of ∼1014 M⊙ ( ≳ 5 × 1014 M⊙) clusters are unbiased (∼35 per cent lower) compared to weak lensing masses. For massive clusters, Chandra's calibration may thus be more accurate. The opposite appears to be true at the low-mass end. We observe the mass bias to increase with cluster mass, but presence of Eddington bias precludes firm conclusions at this stage. Nevertheless, the systematic Chandra − XMM–Newton difference is important because Planck's detections of massive clusters via the Sunyaev–Zeldovich (SZ) effect are calibrated via XMM–Newton observations. The number of detected SZ clusters are inconsistent with Planck's cosmological measurements of the primary cosmic microwave background. Given the Planck cluster masses, if an (unlikely) uncorrected ∼20 per cent calibration bias existed, this tension would be eased, but not resolved