The nature of the Compton-thick X-ray reprocessor in NGC 4945

Abstract

We present an exhaustive methodology for fitting Compton-thick X-ray reprocessor models to obscured AGNs and for interpreting the results. We focus on the MYTORUS model but also utilize other models. We apply the techniques to Suzaku, BeppoSAX, and Swift BAT spectra of the Sy 2 galaxy NGC 4945, but the methods are applicable to other AGNs including Compton-thin sources. The models overcome a major restriction of disk-reflection models, namely the assumption of an infinite column density. Finite column-density models produce a richer variety of spectral shapes and characteristics, even for Compton-thin AGNs. Although NGC 4945 is one of the brightest AGNs above 10 keV, the models span nearly a factor of 3 in column density (~2 to 6 x 10^{24} cm^{-2}) and 2 orders of magnitude in the intrinsic 2-195 keV luminosity. Models in which the continuum above 10 keV is dominated by the direct (unscattered) continuum or Compton-scattered continuum give the highest and lowest intrinsic luminosities respectively. Variability properties favor solutions in which the unscattered continuum dominates above 10 keV. The data require that the Compton-scattered continuum and Fe Kalpha line emission come predominantly from the illuminated surfaces of the X-ray reprocessor, implying a clumpy medium with a global covering factor that is small enough that the Compton-scattered continuum does not dominate the spectrum above 10 keV. This can be identified with the ~30 pc region spatially resolved by Chandra. The implied intrinsic bolometric luminosity is close to, or greater than, the Eddington luminosity. However, a strongly beamed AGN embedded in a shell of Compton-thick (but clumpy) matter requires less fine-tuning of the covering factor. Beaming is consistent with recent radio and Fermi results. Such beamed Compton-thick AGNs would be preferentially selected in surveys over unbeamed Compton-thick AGNs.