Do X-ray dark or underluminous galaxy clusters exist?
  [Abridged] We study the X-ray properties of a color-selected sample of
clusters at 0.1<z<0.3, to quantify the real aboundance of the population of
X-ray dark or underluminous clusters and at the same time the spurious
detection contamination level of color-selected cluster catalogs. A careful
selection allowed us to have an X-ray-unbiased sample of 33 clusters to measure
the Lx-richness relation. Swift 1.4 Ms X-ray observations show that at least
89% of the color-detected clusters are real objects with a potential well deep
enough to heat and retain an intracluster medium. The percentage rises to 94 %
when one includes the single spectroscopically confirmed color-selected cluster
whose X-ray emission is not secured. Looking at our results from the opposite
perspective, the percentage of X-ray dark clusters among color-selected
clusters is very low: at most about 11 per cent (at 90 % confidence).
Supplementing our data with those from literature, we conclude that X-ray- and
color- cluster surveys sample the same population and consequently that in this
regard we can safely use clusters selected with any of the two methods for
cosmological purposes. This is an essential and promising piece of information
for upcoming surveys in both the optical/IR (DES, EUCLID) and X-ray (eRosita).
Richness correlates with X-ray luminosity with a large scatter, 0.51+/-0.08
(0.44+/-0.07) dex in lgLx at a given richness, when Lx is measured in a 500
(1070) kpc aperture. We release data and software to estimate the X-ray flux,
or its upper limit, of a source with over-Poisson background fluctuations
(found in this work to be ~20% on cluster angular scales) and to fit X-ray
luminosity vs richness if there is an intrinsic scatter. These Bayesian
applications rigorously account for boundaries (e.g. the X-ray luminosity and
the richness cannot be negative).
