High-precision multi-band measurements of the angular clustering of X-ray sources

Aims: We aim to study the large-scale structure of an extragalactic serendipitous X-ray survey with unprecedented accuracy thanks to the large statistics involved, and provide insight into the environment of AGN at the epochs when their space density declines (z ∼ 1{-}2).
Methods: In this paper we present the two-point angular correlation function of the X-ray source population of 1063 XMM-Newton observations at high Galactic latitudes, comprising up to 30 000 sources over a sky area of 125.5 deg², in three energy bands: 0.5-2 (soft), 2-10 (hard), and 4.5-10 (ultrahard) keV. This is the largest survey of serendipitous X-ray sources ever used for clustering analysis.
Results: We have measured the angular clustering of our survey and find significant positive clustering signals in the soft and hard bands ( 10σ and 5σ, respectively), and a marginal clustering detection in the ultrahard band (<1σ). We find dependency of the clustering strength on the flux limit and no significant differences in the clustering properties between sources with high hardness ratios (and therefore likely to be obscured AGN) and those with low hardness ratios. We deprojected the angular clustering parameters via Limber's equation to compute their typical spatial lengths. From that we have inferred that AGN at redshifts of 1 are embedded in dark matter haloes with typical masses of < log M_DMH> ≃ 12.60±0.34 h^-1 M_⊙ and lifetimes of t_AGN=3.1{-}4.5 × 10⁸ yr.
Conclusions: Our results show that obscured and unobscured objects share similar clustering properties and therefore they both reside in similar environments, in agreement with the unified model of AGN. The short AGN lifetimes derived suggest that AGN activity might be a transient phase that can be experienced several times by a large fraction of galaxies throughout their lives.