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It is well accepted that unabsorbed as well as absorbed AGN are needed toexplain the nature and the shape of the Cosmic X-ray background, even if thefraction of highly absorbed objects (dubbed Compton-thick sources)substantially still escapes detection. We derive and analyze the absorptiondistribution using a complete sample of AGN detected by Swift-BAT in the firstthree years of the survey. The fraction of Compton-thick AGN represents only4.6% of the total AGN population detected by Swift-BAT. However, we show thatonce corrected for the bias against the detection of very absorbed sources thereal intrinsic fraction of Compton-thick AGN is 20$^{+9}_{-6}$%. We proved forthe first time (also in the BAT band) that the anti-correlation of the fractionof absorbed AGN and luminosity it tightly connected to the different behaviorof the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This pointstowards a difference between the two subsamples of objects with absorbed AGNbeing, on average, intrinsically less luminous than unobscured ones. Moreoverthe XLFs show that the fraction of obscured AGN might also decrease at very lowluminosity. This can be successfully interpreted in the framework of a diskcloud outflow scenario as the disappearance of the obscuring region below acritical luminosity. Our results are discussed in the framework of populationsynthesis models and the origin of the Cosmic X-ray Background.

Three years Swift-BAT Survey of AGN: reconciling theory and observations?