2004ApJ...617..192M

We present a detailed description of the first direct measurement of the spatial correlation function of X-ray-selected active galactic nuclei (AGNs). This result is based on an X-ray flux-limited sample of 219 AGNs discovered in the contiguous 80.7 deg² region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at the 4 σ level at comoving scales in the interval r=5-60h^–1 Mpc. Fitting the data with a power law of slope γ=1.8, we find a correlation length of r₀=7.4^+1.8_–1.9h^–1 Mpc (Ω_M=0.3, Ω_Λ=0.7). The median redshift of the AGN contributing to the signal is z_ξ=0.22. This clustering amplitude implies that X-ray-selected AGNs are spatially distributed in a manner similar to that of optically selected AGNs. Furthermore, the ROSAT NEP determination establishes the local behavior of AGN clustering, a regime that is poorly sampled in general. Combined with high-redshift measures from optical studies, the ROSAT NEP results argue that the AGN correlation strength essentially does not evolve with redshift, at least out to z∼2.2. In the local universe, X-ray-selected AGNs appear to be unbiased relative to galaxies, and the inferred X-ray bias parameter is near unity, b_X∼1. Hence, X-ray-selected AGNs closely trace the underlying mass distribution. The ROSAT NEP AGN catalog, presented here, features complete optical identifications and spectroscopic redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41, f_X=1.1x10^–13 ergs/cm2/s, and L_X=9.2x10⁴³h^–2₇₀ergs/s (0.5-2.0 keV), respectively. Unobscured, type I AGNs are the dominant constituents (90%) of this soft X-ray-selected sample of AGNs.