Overdensity of X-Ray Sources in the Field of Two Nearby Clusters of Galaxies: XMM-Newton View of A 194 and A 1060

Abstract

Two nearby clusters of galaxies, A194 (z=0.018) and A1060 (z=0.0114), have been analyzed for their X-ray point-source properties with XMM-Newton EPIC-PN data. A multi-band source detection technique was applied to both of the clusters, resulting in 46 sources from the A194 field and 32 sources from the A1060 field, respectively. The cumulative log(N)-log(S) for a flux limit of F_X ≥ 1 × 10^-14erg cm^-2 s^-1 was calculated and compared with that of the Lockman Hole. A ∼ 3σ excess of X-ray sources was found for the cluster regions. Considering the higher fraction observed in optical studies from the clusters, we estimate that the cluster source density is 6-times higher than the blank-field source density, and 15-times higher than the local group. Our X-ray selected sources have luminosity values of 10^39.6 ≤ L_X ≤ 10^41.4 erg s^-1, in which X-ray emission from LMXBs, hot halos and starburst galaxies becomes noticeable. The significance of the source-density excess gradually vanishes for sources with L_X ≥ 10^40.5erg s^-1, at which point the source density becomes comparable to that of the blank-field level. Considering this confined low-luminosity range and the X-ray to optical luminosity ratios (L_X/L_B), the observed overdensity is ascribed to AGN fueling by its infall into cluster environment for L_X ≤ 10^40.5 erg s^-1 in the X-ray luminosity function. However, the quenching of AGN activity by the deep cluster potential explains why the excess of the source density vanishes for brighter sources.