MARDELS: A full-sky X-ray selected galaxy cluster catalog

We present the MARDELS catalog of 8,471 X-ray selected galaxy clusters over 25,000 deg^2 of extragalactic sky. The accumulation of deep, multiband optical imaging data, the development of the optical counterpart classification algorithm MCMF, and the release of the DESI Legacy Survey DR10 catalog covering the extragalactic sky makes it possible -- for the first time, more than 30 years after the launch of the ROSAT X-ray satellite -- to identify the majority of the galaxy clusters detected in the ROSAT All-Sky-Survey source catalog (2RXS). The resulting 90% pure MARDELS catalog is the largest ICM-selected cluster sample to date. MARDELS probes a large dynamic range in cluster mass spanning from galaxy groups to the most massive clusters in the Universe. The cluster redshift distribution peaks at z~0.1 and extends to redshifts z~1. Out to z~0.4, the MARDELS sample contains more clusters per redshift interval (dN/dz) than any other ICM-selected sample. In addition to the main sample, we present two subsamples with 6,930 and 5,522 clusters, exhibiting 95% and 99% purity, respectively. We forecast the utility of the sample for a cluster cosmological study, using realistic mock catalogs that incorporate most observational effects, including the X-ray exposure time and background variations, the existence likelihood selection adopted in 2RXS and the impact of the optical cleaning with MCMF. Using realistic priors on the observable--mass relation parameters from a DES-based weak lensing analysis, we estimate the constraining power of the MARDELSxDES sample to be of 0.026, 0.033 and 0.15 ($1\sigma$) on the parameters $\Omega_\mathrm{m}$, $\sigma_8$ and $w$, respectively.