New X‐Ray Clusters in theEinsteinExtended Medium‐Sensitivity Survey. I. Modifications to the X‐Ray Luminosity Function

Abstract

The complete ensemble of Einstein Imaging Proportional Counter (IPC) X-ray images has been reprocessed and reanalyzed using a multiaperture source detection algorithm. A catalog of 772 new source candidates detected within the 38' diameter central regions of the 1435 IPC fields comprising the Extended Medium-Sensitivity Survey (EMSS) has been compiled. By comparison, 478 EMSS sources fall within the same area of sky. A randomly selected subsample of 133 fields was examined; 73 sources were detected and compared with 49 original EMSS sources in the same region of sky. We expect, on the basis of confusion statistics, that most of these sources either are the summation of two or more lower count-rate point sources that fall within the larger detection apertures or are single point sources. An optical imaging study discovered one possible cluster of galaxies among 43 identified sources, suggesting that ≤2.3% of the full catalog of sources are extrapolated to be actual distant (z ≥ 0.14) clusters whose extended X-ray structure kept them from being detected in the EMSS despite having sufficient total flux. We have constructed other subsamples specifically selected to contain those X-ray sources most likely to be clusters based upon additional X-ray and optical criteria. Both a database search and an optical imaging study of these subsamples have found several new distant clusters, setting a firm lower limit on the number of new clusters in the entire catalog. Given both the numbers of new EMSS clusters and their spectroscopic or photometric redshifts, we estimate that the original EMSS cluster sample is 72%-83% complete. We update the Henry et al. EMSS distant (z ≥ 0.14) cluster sample with more recent information and use the redshifts and X-ray luminosities for these new EMSS clusters to compute revised X-ray luminosity functions (XLFs) in the three redshift shells defined by Henry et al. The addition of these new high-z, high-LX clusters to the EMSS is sufficient to remove the requirement for negative evolution at high LX out to z ~ 0.5. Although the best estimate of the EMSS XLF at z = 0.3-0.6 and log LX = 44.9-45.2 ergs s-1 falls 1 σ below the low-z (<0.3) XLF, the optical identification of the full 772 source catalog remains incomplete. We conclude that the EMSS has systematically missed clusters of low surface brightness. Since all X-ray cluster surveys are less sensitive to low surface brightness emission, they may also be affected.