Calibration of the galaxy cluster M500-Yx relation with XMM-Newton

The quantity Y-X, the product of the X-ray temperature T-X and gas mass M-g, has recently been proposed as a robust low- scatter mass indicator for galaxy clusters. Using precise measurements from XMM- Newton data of a sample of 10 relaxed nearby clusters, spanning a Y-X range of 10(13)-10(15) M-circle dot keV, we investigate the M500 - YX relation. The M-500-Y-X data exhibit a power law relation with slope a = 0.548 +/- 0.027, close to the self-similar value (3/5) and independent of the mass range considered. However, the normalisation is similar to 20% below the prediction from numerical simulations including cooling and galaxy feedback. We discuss two effects that could contribute to the normalisation offset: an underestimate of the true mass due to the hydrostatic equilibrium assumption used in X-ray mass estimates, and an underestimate of the hot gas mass fraction in the simulations. A comparison of the functional form and scatter of the relations between various observables and the mass suggest that Y-X may indeed be a better mass proxy than T-X or M-g500.