Improved Cosmological Constraints from SDSS redMaPPer Clusters via X-ray
  Follow-up of a Complete Subsample of Systems
  We improve upon the cosmological constraints derived from the abundance and
weak-lensing data of redMaPPer clusters detected in the Sloan Digital Sky
Survey (SDSS). Specifically, we derive gas mass data using Chandra X-ray
follow-up of a complete sample of the 30 richest SDSS redMaPPer clusters with
$z\in[0.1,0.3]$, and use these additional data to improve upon the original
analysis by Costanzi et al. (2019b). We simultaneously fit for the parameters
of the richness-mass relation, the cluster gas mass-mass relation, and
cosmology. By including our X-ray cluster sample in the SDSS cluster cosmology
analysis, we measure $\Omega_{\rm m} = 0.25 \pm 0.04$ and $\sigma_8 =
0.85^{+0.06}_{-0.08}$. These constraints represent a 25.5% and 29.8% reduction
in the size of the 68% confidence intervals of $\Omega_{\rm m}$ and $\sigma_8$
respectively, relative to the constraints published in Costanzi et al. (2019b).
Our cosmological constraints are in agreement with early universe results from
Planck. As a byproduct of our analysis, we also perform an independent
calibration of the amplitude of the $\langle M_{\rm gas}^{\rm true}|M_{\rm
500c}\rangle$ scaling relation. Our calibration is consistent with and of
comparable precision to that of Mantz et al. (2016b).
