NuSTAR observations of heavily obscured Swift/BAT AGN: constraints on the Compton-thick AGN fraction

TL;DR

This study uses NuSTAR observations to analyze the spectra of heavily obscured Compton-thick AGN detected by Swift/BAT, constraining their spectral properties and estimating their fraction in the local universe.

Contribution

It provides new spectral constraints on Compton-thick AGN and refines the estimate of their fraction in the local universe using combined NuSTAR and Swift/BAT data.

Findings

Coronal temperatures between 25 and 80 keV.

Most AGN lack a strong reflection component.

Estimated Compton-thick AGN fraction is ~20% in the local universe.

Abstract

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.