2005A&A...430...39C

The first systematic study of the serendipitous X-ray source density around 10 high z (0.24<z<1.2) clusters has been performed with Chandra. A factor ∼2 overdensity has been found in 4 cluster fields, increasing to 11 the number of known source overdensities around high z clusters. The result is statistically highly significant (at >2σ per field, and >5σ overall) only on scales 8'x8' and peculiar to cluster fields. The blank field fluctuations, i.e. the most sensitive measurements to date of the X-ray cosmic variance on this scale, indicate 1σ variations within ∼15-25% around the average value. The first marginal evidence that these overdensities increase with cluster redshift is also presented. We speculate that the most likely explanation for what has been observed is that the X-ray sources are AGNs which trace filaments connected to the clusters. Nevertheless other possible explanations such as an ``X-ray Butcher-Oemler'' effect are not ruled out by the present results. If the association of the overdensities with large-scale structures and their positive relation with cluster redshift is confirmed, these studies could represent a new, direct, way of mapping the cosmic web and its parameters at high z.