The 4 th Palermo Swift-BAT catalogue: 7 years of survey of the hard X-ray sky

The 4th Palermo Swift-BAT hard X-raycatalogue is obtained by analyzing data acquired in the first 7 years of the Swiftmission. The survey covers 50% of the sky up to a 15-150 keV flux limitof 7.0× 10 −12 erg cm −2 s −1 . We used a source detection algorithm that optimizes the energy band and the time interval allowing to achieve the maximum signal to noise ratio for each pixel of the all-sky map. We obtain a list of ∼ 1700 source candidates with a negligible number of spurious detections. The identification of the source counterparts i n softer energy bands is pursued both through the analysis of soft X-ray field observations and thr ough the cross-correlation with source databases. The 4th Palermo Swift-BAT catalogue includes ∼ 20% of Galactic sources, ∼ 60% of extragalactic sources, ∼ 10% of sources with a counterpart at softer energies whose nature has not been determined yet while ∼ 10% still lack any associated counterpart.