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

Abstract

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 $\log (N) \hbox{--} \log(S)$ for a flux limit of $F_{\mathrm{X}} \geq 1 \times 10^{-14} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1}$ was calculated and compared with that of the Lockman Hole. A $\sim 3\,\sigma$ 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 $10^{39.6} \leq L_{\mathrm{X}} \leq 10^{41.4} \,\mathrm{erg} \,\mathrm{s}^{-1}$, 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 $L_{\mathrm{X}} \geq 10^{40.5} \,\mathrm{erg} \,\mathrm{s}^{-1}$, 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 ($L_{\mathrm{X}} / L_{\mathrm{B}}$), the observed overdensity is ascribed to AGN fueling by its infall into cluster environment for $L_{\mathrm{X}} \leq 10^{40.5} \,\mathrm{erg} \,\mathrm{s}^{-1}$ 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.