Low-scatter galaxy cluster mass proxies for the eROSITA all-sky survey
  The on-going X-ray all-sky survey with the eROSITA instrument will yield
large galaxy cluster samples, which will bring strong constraints on
cosmological parameters. In particular, the survey holds great promise to
investigate the tension between CMB and low-redshift measurements. The current
bottleneck preventing the full exploitation of the survey data is the
systematics associated with the relation between survey observable and halo
mass. Numerous recent studies have shown that gas mass and core-excised X-ray
luminosity exhibit very low scatter at fixed mass. We propose a new method to
reconstruct these quantities from low photon count data and validate the method
using extensive eROSITA-like simulations. We find that even near the detection
threshold of ~50 counts the core-excised luminosity and the gas mass can be
recovered with 20-30% precision, which is substantially less than the scatter
of the full integrated X-ray luminosity at fixed mass. When combined with an
accurate calibration of the absolute mass scale (e.g. through weak
gravitational lensing), our technique reduces the systematics on cosmological
parameters induced by the mass calibration.
