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, T_X. Their masses may be underestimated if it is assumed that the gas is in hydrostatic equilibrium, by an amount b^hyd ∼ (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.