  Inspired by the discovery of the Phoenix cluster by the South Pole Telescope
team, we initiated a search for other massive clusters of galaxies missing from
the standard X-ray catalogs. We began by identifying 25 cluster candidates not
included in the Meta-Catalog of X-ray Clusters of galaxies cluster compilation
through cross-identification of the central galaxies of optically identified
clusters in the Sloan Digital Sky Survey GMBCG catalog with bright X-ray
sources in the ROSAT Bright Source Catalog. Those candidates were mostly
unidentified or previously classified as X-ray active galactic nucleus (AGN).
We analyzed brief Chandra X-ray Observatory observations of 14 of these X-ray
sources and found that eight are X-ray luminous clusters of galaxies, only one
showing evidence for a central X-ray point source. The remaining six candidates
turned out to be point-source dominated, with faint detections or upper limits
on any extended emission. We were not able to rule out the presence of extended
X-ray emission from any of the point sources. The levels of extended emission
around the six point sources are consistent with expectations based on optical
richness, but could also be contaminated by scattered X-ray light from the
central point source or extended nonthermal emission from possible radio lobes.
We characterize the extended components of each of the well-detected cluster
sources, finding that six of the eight X-ray clusters are consistent with being
compact cool-core clusters. One of the newly identified low-luminosity X-ray
clusters may have had an X-ray-luminous AGN 20 yr prior to the recent Chandra
observations, based on the 4{\sigma} difference between its Chandra and ROSAT
fluxes.
