NuSTAR observations of heavily obscured Swift/BAT AGN: constraints on
  the Compton-thick AGN fraction
  The all-sky hard X-ray survey performed by Swift/BAT allowed the detection of
many heavily obscured Compton-thick AGN. In our previous work, we have
identified more than 50 candidate Compton-thick AGN in the local Universe,
corresponding to an observed fraction of about 7% of the total AGN population.
This number can be converted to the intrinsic Compton-thick AGN number density,
only if we know the form of the Compton-thick AGN spectrum, that is the energy
of their absorption turnover, photon-index and its cut-off energy at high
energies, as well as the strength of the reflection component on the matter
surrounding the nucleus. In order to constrain their number density, we analyse
the spectra of 19 Compton-thick AGN which have been detected with Swift/BAT and
have been subsequently observed with NuSTAR in the 3-80 keV band. We analyse
their X-ray spectra using the MYTORUS models of Murphy and Yaqoob which
properly take into account the Compton scattering effects. These are combined
with physically motivated Comptonisation models which accurately describe the
primary coronal X-ray emission. We derive absorbing column densities which are
consistent with those derived by the previous Swift/BAT analyses. We estimate
the coronal temperatures to be roughly between 25 and 80 keV corresponding to
high energy cut-offs roughly between 75 and 250 keV. We find that the majority
of our AGN lacks a strong reflection component in the 20-40 keV band placing
tighter constraints on the intrinsic Compton-thick AGN fraction. Combining
these results with our X-ray background synthesis models, we estimate a
Compton-thick AGN fraction in the local Universe of ~20 +/-3 % relative to the
type-II AGN population.
