The soft X - ray Cluster - AGN spatial cross - correlation function in the ROSAT - NEP survey

Context. X-ray surveys facilitate investigations of the environment of AGNs. Deep Chandra observations revealed that the AGNs source surface density rises near clusters of galaxies. The natural extension of these works is the measurement of spatial clustering of AGNs around clusters and the investigation of relative biasing between active galactic nuclei and galaxies near clusters. Aims. The major aims of this work are to obtain a measurement of the correlation length of AGNs around clusters and a measure of the averaged clustering properties of a complete sample of AGNs in dense environments. Methods. We present the first measurement of the soft X-ray cluster-AGN cross-correlation function in redshift space using the data of the ROSAT-NEP survey. The survey covers 9×9 deg2 around the North Ecliptic Pole where 442 X-ray sources were detected and almost completely spectroscopically identified. Results. We detected a >3σ significant clustering signal on scales s ≤50 h−1 70 Mpc. We performed a classical maximum-likelihood power-law fit to the data and obtained a correlation length s0=8.7 −0.3 h −1 70 Mpc and a slope γ=1.7 +0.2 −0.7 ( 1σ errors). Conclusions. This is a strong evidence that AGNs are good tracers of the large scale structure of the Universe. Our data were compared to the results obtained by cross-correlating X-ray clusters and galaxies. We observe, with a large uncertainty, a similar behaviour of the AGNs clustering around clusters similar to the clustering of galaxies around clusters.