We present an analysis of the deepest hard X-ray survey to date of about 2500 deg ^ { 2 } performed by the IBIS instrument on board INTEGRAL in the 20–60 keV band , with a total exposure time of 4 Ms. We find 34 candidate sources , for which we try to find counterparts at other wavelengths .
The ratio of Seyfert 1 to Seyfert 2 is significantly more than the ratio found in the optical .
This effect may be explained in the framework of the receding-torus model , but could also be due to absorption columns large enough to affect the 20–60 keV band .
None of the predicted Compton-thick objects with 10 ^ { 24 } < N _ { \mathrm { H } } < 10 ^ { 25 } cm ^ { -2 } is detected unambiguously ; when taking lower limits on N _ { \mathrm { H } } into account , the fraction of these objects is found to be lower than 24 % .
We do not see , but can not exclude , a relationship between absorption and luminosity similar to what is seen in the 2–10 keV band .
Our data suggests the possibility of a lack of objects with 10 ^ { 21 } \leq N _ { \mathrm { H } } \leq 10 ^ { 22 } cm ^ { -2 } , which could be expected if absorption has two origins , for instance a torus-like structure and the host galaxy .
We find that the Log N –Log S diagram of our sources is compatible with those obtained in other surveys in hard X-rays .
Compared to models of the AGN population selected in the 2–10 keV band , the Log N –Log S diagram is generally in good agreement , but the N _ { \mathrm { H } } distribution is significantly different , with significantly less unabsorbed sources ( N _ { \mathrm { H } } < 10 ^ { 22 } cm ^ { -2 } ) at a given flux limit compared to the models .
In this survey , we resolve about 2.5 % of the cosmic X-ray background in the 20–60 keV band .
We also study the local hard X-ray luminosity function , which is compatible with what is found in other recent hard X-ray surveys .
The characteristic luminosity Log L ^ { * } _ { 20 - 60 \mathrm { keV } } = 43.66 is found to be a factor about 5 lower than the value observed in the 2–10 keV band .
We find a space density of 10 ^ { -3 } AGN with L _ { 20 - 60 \mathrm { keV } } > 10 ^ { 41 } per Mpc ^ { 3 } and a corresponding luminosity density of 0.9 10 ^ { 39 } erg s ^ { -1 } Mpc ^ { -3 } .