Stacking clusters in the ROSAT All-Sky Survey

Ongoing and planned wide-area surveys at optical and infrared wavelengths should detect a few times 10(5) galaxy clusters, roughly 10% of which are expected to be at redshifts greater than or similar to0.8. We investigate what can be learned about the X-ray emission of these clusters from the ROSAT All-Sky Survey. While individual clusters at redshifts greater than or similar to0.5 contribute at most a few photons to the survey, a significant measurement of the mean flux of cluster subsamples can be obtained by stacking cluster fields. We show that the mean X-ray luminosity of clusters with mass Mgreater than or similar to2x10(14) h(-1) M-circle dot selected from the Sloan Digital Sky Survey should be measurable out to redshift unity with signal-to-noise greater than or similar to10, even if clusters are binned with Deltaz=0.1 and Deltaln Msimilar to0.3. For such bins, a suitably chosen hardness ratio allows the mean temperature of clusters to be determined out to zsimilar to0.7 with a relative accuracy of DeltaT/Tless than or similar to0.15 for M>10(14) h(-1) M-circle dot.