A New RASS Galaxy Cluster Catalog with Low Contamination Extending to
  $z\sim1$ in the DES Overlap Region
  We present the MARD-Y3 catalog of between 1086 and 2171 galaxy clusters (52\%
and 65\% new) produced using multi-component matched filter (MCMF) followup in
5000\,deg$^2$ of DES-Y3 optical data of the $\sim$20000 overlapping 2RXS X-ray
sources. Optical counterparts are identified as peaks in galaxy richness as a
function of redshift along the line of sight toward each 2RXS source within a
search region informed by an X-ray prior. All peaks are assigned a probability
\fcont\ of being a random superposition. The clusters lie at $0.02<z<1.1$ with
more than 100 clusters at $z>0.5$. Residual contamination is 2.6\% and 9.6\%
for the cuts adopted here. For each cluster we present the optical center,
redshift, rest frame X-ray luminosity, $M_{500}$ mass, coincidence with NWAY
infrared sources and estimators of dynamical state. About 2\% of MARD-Y3
clusters have multiple possible counterparts, the photo-z's are high quality
with $\sigma_{\Delta z/(1+z)}=0.0046$, and $\sim$1\% of clusters exhibit
evidence of X-ray luminosity boosting from emission by cluster AGN. Comparison
with other catalogs (MCXC, RM, SPT-SZ, Planck) is performed to test consistency
of richness, luminosity and mass estimates. We measure the MARD-Y3 X-ray
luminosity function and compare it to the expectation from a fiducial cosmology
and externally calibrated luminosity- and richness-mass relations. Agreement is
good, providing evidence that MARD-Y3 has low contamination and can be
understood as a simple two step selection-- X-ray and then optical-- of an
underlying cluster population described by the halo mass function.
