We present the analysis of serendipitous sources in a deep , 500 ksec , hard X-ray observation of the Coma cluster region with the IBIS instrument onboard INTEGRAL .
In addition to the Coma cluster , the final 20–50 keV image contains 12 serendipitous sources with statistical significance > 4 \sigma .
We use these data ( after correcting for expected number of false detections ) to extend the extragalactic source counts in the 20–50 keV energy band down to a limiting flux of 1.0 \times 10 ^ { -11 } erg s ^ { -1 } cm ^ { -2 } ( \simeq 1 mCrab ) .
This is a more than a factor of 10 improvement in sensitivity compared to the previous results in this energy band obtained with the HEAO-1 A4 instrument .
The derived source counts are consistent with the Euclidean relation , N ( > f ) \propto f ^ { -3 / 2 } .
A large fraction of identified serendipitous sources are low-redshift , z < 0.02 AGNs , mostly of Seyfert 1 type .
The surface density of hard X-ray sources is ( 1.4 \pm 0.5 ) \times 10 ^ { -2 } per square degree above a flux threshold of 10 ^ { -11 } erg s ^ { -1 } cm ^ { -2 } .
These sources directly account for \sim 3 \% of the cosmic X-ray background in the 20–50 keV energy band .
Given the low redshift depth of our sample , we expect that similar sources at higher redshifts account for a significant fraction of the hard X-ray background .
Our field covers only 3 % of the sky ; a systematic analysis of other extragalactic INTEGRAL observations can produce much larger source samples and is , therefore , critically important .