4.6. Very High Energies

The detection of luminous E > 10 keV emission from several nearby, apparently low-luminosity objects (e.g., NGC4945; Done et al. 2003) shows that even the 2 - 8 keV band can suffer from selection effects. The BeppoSAX survey of Seyfert 2 galaxies (Risaliti, Maiolino, & Salvati 1999) shows that the distribution of column densities is flat in log space from 10²¹ to 3 × 10²⁴ atoms cm^-2. While very high column density objects may be absent from the Chandra samples, the argument put forward by Fabian (2000) that three-fifths of the nearest AGN have very high column densities has, in my opinion, strong merit. The absence of a large solid angle sensitive E > 10 keV survey means that the statistics of such objects are hard to constrain, but it certainly seems possible that the number of low to moderate-luminosity objects at z < 0.5 with very large column densities may be similar to that of less absorbed objects. The fundamental question is whether "absorbed" accretion is a major component to the total AGN luminosity. It is not clear how, absent a new X-ray mission ⁸, to proceed to answer this question.

⁸ It seems as if INTEGRAL is not sensitive enough to perform a survey of AGN in the 10 - 30 keV band; the sensitivity of SWIFT is not yet clear, but it may provide the best hard X-ray survey yet performed. Back.