Using the Bright Ultrahard XMM-Newton survey to define an IR selection of luminous AGN based on WISE colours

We present a highly complete and reliable mid-infrared (MIR) colour selection of luminous active galactic nucleus (AGN) candidates using the 3.4, 4.6 and 12 μm bands of the Wide-field Infrared Survey Explorer (WISE) survey. The MIR colour wedge was defined using the wide-angle Bright Ultrahard XMM-Newton survey (BUXS), one of the largest complete flux-limited samples of bright (f4.5-10 keV >6×10-14 erg s-1 cm -2) 'ultrahard' (4.5-10 keV) X-ray-selected AGN to date. The BUXS includes 258 objects detected over a total sky area of 44.43 deg² of which 251 are spectroscopically identified and classified, with 145 being type 1 AGN and 106 type 2 AGN. Our technique is designed to select objects with red MIR power-law spectral energy distributions (SEDs) in the three shortest bands of WISE and properly accounts for the errors in the photometry and deviations of the MIR SEDs from a pure power-law. The completeness of the MIR selection is a strong function of luminosity. At L_{2-10 keV} >10⁴⁴ erg s^-1, where the AGN is expected to dominate the MIR emission, 97.1-4.8+2.2 and 76.5-18.4+13.3 per cent of the BUXS type 1 and type 2 AGN, respectively, meet the selection. Our technique shows one of the highest reliability and efficiency of detection of the X-ray-selected luminous AGN population with WISE amongst those in the literature. In the area covered by BUXS our selection identifies 2755 AGN candidates detected with signal-to-noise ratio ≥5 in the three shorter wavelength bands of WISE with 38.5 per cent having a detection at 2-10 keV X-ray energies. We also analysed the possibility of including the 22 μm WISE band to select AGN candidates, but neither the completeness nor the reliability of the selection improves. This is likely due to both the significantly shallower depth at 22 μm compared with the first three bands of WISE and star formation contributing to the 22 μm emission at the WISE 22 μm sensitivity.