2005ApJ...625...89K

We present the analysis of serendipitous sources in a deep, 500 ks, hard X-ray observation of the Coma Cluster region with the IBIS instrument on board INTEGRAL. In addition to the Coma Cluster, the final 20-50 keV image contains 12 serendipitous sources with statistical significance >4 σ. 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 1.0x10^–11 ergs/s/cm2 (≃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 A-4 instrument. The derived source counts are consistent with the Euclidean relation N(>f)∝f^–3/2. A large fraction of identified serendipitous sources are low-redshift, z<0.02, active galactic nuclei (AGNs), mostly of Seyfert 1 type. The surface density of hard X-ray sources is (1.4±0.5)x10^–2 deg^–2 above a flux threshold of 10^–11 ergs/s/cm2. These sources directly account for ∼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.