The Spatial Clustering of ROSAT All-Sky Survey AGN: I. The
  cross-correlation function with SDSS Luminous Red Galaxies
  We investigate the clustering properties of ~1550 broad-line active galactic
nuclei (AGNs) at <z>=0.25 detected in the ROSAT All-Sky Survey (RASS) through
their measured cross-correlation function with ~46,000 Luminous Red Galaxies
(LRGs) in the Sloan Digital Sky Survey. By measuring the cross-correlation of
our AGN sample with a larger tracer set of LRGs, we both minimize shot noise
errors due to the relatively small AGN sample size and avoid systematic errors
due to the spatially varying Galactic absorption that would affect direct
measurements of the auto-correlation function (ACF) of the AGN sample.
  The measured ACF correlation length for the total RASS-AGN sample
(<L_(0.1-2.4 keV)>=1.5 x 10^(44) erg/s) is r_0=4.3^{+0.4}_{-0.5} h^(-1) Mpc and
the slope \gamma=1.7^{+0.1}_{-0.1}. Splitting the sample into low and high L_X
samples at L_(0.5-10 keV)=10^(44) erg/s, we detect an X-ray luminosity
dependence of the clustering amplitude at the ~2.5 \sigma level. The low L_X
sample has r_0=3.3^{+0.6}_{-0.8} h^(-1) Mpc (\gamma=1.7^{+0.4}_{-0.3}), which
is similar to the correlation length of blue star-forming galaxies at low
redshift. The high L_X sample has r_0=5.4^{+0.7}_{-1.0} h^(-1) Mpc
(\gamma=1.9^{+0.2}_{-0.2}), which is consistent with the clustering of red
galaxies. From the observed clustering amplitude, we infer that the typical
dark matter halo (DMH) mass harboring RASS-AGN with broad optical emission
lines is log (M_DMH/(h^(-1) M_SUN)) =12.6^{+0.2}_{-0.3}, 11.8^{+0.6}_{-\infty},
13.1^{+0.2}_{-0.4} for the total, low L_X, and high L_X RASS-AGN samples,
respectively.
