The mass of galaxy clusters can be inferred from the temperature of their X-ray emitting gas , T _ { \mathrm { X } } .
Their masses may be underestimated if it is assumed that the gas is in hydrostatic equilibrium , by an amount b ^ { \mathrm { hyd } } \sim ( 20 \pm 10 ) % 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 \sim 10 ^ { 14 } \mbox { M } _ { \odot } ( \ga 5 \cdot 10 ^ { 14 } \mbox { M } _ { \odot } ) clusters are unbiased ( \sim 35 % lower ) compared to WL 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 ( CMB ) .
Given the Planck cluster masses , if an ( unlikely ) uncorrected \sim 20 % calibration bias existed , this tension would be eased , but not resolved .