The Halo Occupation Distribution of X-ray-Bright Active Galactic Nuclei:
  A Comparison with Luminous Quasars
  We perform halo occupation distribution (HOD) modeling of the projected
two-point correlation function (2PCF) of high-redshift (z~1.2) X-ray-bright
active galactic nuclei (AGN) in the XMM-COSMOS field measured by Allevato et
al. The HOD parameterization is based on low-luminosity AGN in cosmological
simulations. At the median redshift of z~1.2, we derive a median mass of
(1.02+0.21/-0.23)x10^{13} Msun/h for halos hosting central AGN and an upper
limit of ~10% on the AGN satellite fraction. Our modeling results indicate (at
the 2.5-sigma level) that X-ray AGN reside in more massive halos compared to
more bolometrically luminous, optically-selected quasars at similar redshift.
The modeling also yields constraints on the duty cycle of the X-ray AGN, and we
find that at z~1.2 the average duration of the X-ray AGN phase is two orders of
magnitude longer than that of the quasar phase. Our inferred mean occupation
function of X-ray AGN is similar to recent empirical measurements with a group
catalog and suggests that AGN halo occupancy increases with increasing halo
mass. We project the XMM-COSMOS 2PCF measurements to forecast the required
survey parameters needed in future AGN clustering studies to enable higher
precision HOD constraints and determinations of key physical parameters like
the satellite fraction and duty cycle. We find that N^{2}/A~5x10^{6} deg^{-2}
(with N the number of AGN in a survey area of A deg^{2}) is sufficient to
constrain the HOD parameters at the 10% level, which is easily achievable by
upcoming and proposed X-ray surveys.
