Survival of the obscuring torus in the most powerful active galactic
  nuclei
  Dedicated searches generally find a decreasing fraction of obscured Active
Galactic Nuclei (AGN) with increasing AGN luminosity. This has often been
interpreted as evidence for a decrease of the covering factor of the AGN torus
with increasing luminosity, the so-called receding torus models. Using a
complete flux-limited X-ray selected sample of 199 AGN, from the Bright
Ultra-hard XMM-Newton Survey, we determine the intrinsic fraction of optical
type-2 AGN at 0.05$\leq$z$\leq$1 as a function of rest-frame 2-10 keV X-ray
luminosity from 10$^{42}$ to 10$^{45}$ erg/s. We use the distributions of
covering factors of AGN tori derived from CLUMPY torus models. Since these
distributions combined over the total AGN population need to match the
intrinsic type-2 AGN fraction, we reveal a population of X-ray undetected
objects with high-covering factor tori, which are increasingly numerous at
higher AGN luminosities. When these "missing" objects are included, we find
that Compton-thick AGN account at most for 37$_{-10}^{+9}$% of the total
population. The intrinsic type-2 AGN fraction is 58$\pm$4% and has a weak,
non-significant (less than 2$\sigma$) luminosity dependence. This contradicts
the results generally reported by AGN surveys, and the expectations from
receding torus models. Our findings imply that the majority of luminous
rapidly-accreting supermassive black holes at z<1 reside in highly-obscured
nuclear environments but most of them are so deeply embedded that they have so
far escaped detection in X-rays in <10 keV wide-area surveys.
