X-ray observations of distant optically selected clusters

We have measured fluxes or flux limits for 31 of the 79 cluster candidates in the Palomar Distant Cluster Survey (PDCS) using archival ROSAT/PSPC pointed observations. Our X-ray survey reaches a flux limit of ≃ 3 × 10^-14 erg s^-1 cm^-2 (0.4-2.0 keV), which corresponds to luminosities of L_x ≃ 5 × 10⁴³ erg s^-1 (H₀ = 50 km s^-1 Mpc^-1, q₀ = 1/2), if we assume the PDCS estimated redshifts. Of the 31 cluster candidates, we detect six at a signal-to-noise greater than three. We estimate that 2.9^+3.3_-1.4 (90% confidence limits) of these six detections are a result of X-ray emission from objects unrelated to the PDCS cluster candidates. The net surface density of X-ray emitting cluster candidates in our survey, 1.71^+0.91_-2.19 clusters deg^-2, agrees with that of other, X-ray selected, surveys. It is possible, given the large error on our contamination rate, that we have not detected X-ray emission from any of our observed PDCS cluster candidates. We find no statistically significant difference between the X-ray luminosities of PDCS cluster candidates and those of Abell clusters of similar optical richness. This suggests that the PDCS contains objects at high redshift similar to the low redshift clusters in the Abell catalogs. We show that the PDCS cluster candidates are not bright X-ray sources; the average luminosity of the six detected candidates is only L̄_x=0.9 × 10⁴⁴ erg s^-1 (0.4-2.0 keV). This finding is in agreement with previous X-ray studies of high redshift, optically selected, rich clusters of galaxies.