Constraining the fraction of Compton-thick AGN in the Universe by modelling the diffuse X-ray background spectrum. (arXiv:1209.5398v1 [astro-ph.HE])

Constraining the fraction of Compton-thick AGN in the Universe by modelling the diffuse X-ray background spectrum. (arXiv:1209.5398v1 [astro-ph.HE]):
This paper investigates what constraints can be placed on the fraction of
Compton-thick (CT) AGN in the Universe from the modeling of the spectrum of the
diffuse X-ray background (XRB). We present a model for the synthesis of the XRB
that uses as input a library of AGN X-ray spectra generated by the Monte Carlo
simulations described by Brightman & Nandra. This is essential to account for
the Compton scattering of X-ray photons in a dense medium and the impact of
that process on the spectra of obscured AGN. We identify a small number of
input parameters to the XRB synthesis code which encapsulate the minimum level
of uncertainty in reconstructing the XRB spectrum. These are the power-law
index and high energy cutoff of the intrinsic X-ray spectra of AGN, the level
of the reflection component in AGN spectra and the fraction of CT AGN in the
Universe. We then map the volume of the space allowed to these parameters by
current observations of the XRB spectrum in the range 3-100 keV. One of the
least constrained parameters is the fraction of CT AGN. Statistically
acceptable fits to the XRB spectrum at the 68% confidence level can be obtained
for CT fractions in the range 5-50%. This is because of degeneracies among
input parameters to the XRB synthesis code and uncertainties in the modeling of
AGN spectra (e.g. reflection). The most promising route for constraining the
fraction of CT AGN in the Universe is via the direct detection of those sources
in high energy (>10keV) surveys. It is shown that the observed fraction of CT
sources identified in the SWIFT/BAT survey, limits the intrinsic fraction of CT
AGN, at least at low redshift, to 10-20% (68% confidence level). We also make
predictions on the number density of CT sources that current and future X-ray
missions are expected to discover. Testing those predictions will constrain the
intrinsic fraction of CT AGN as a function of redshift.