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

TLDR

The 22 µm WISE band is too shallow and contaminated by star formation, limiting its usefulness for AGN selection. The study develops a highly complete and reliable mid‑infrared colour selection of luminous AGN using WISE 3.4, 4.6, and 12 µm bands. Using the Bright Ultra‑Hard XMM‑Newton Survey, the authors defined a MIR colour wedge that selects red power‑law SEDs in the three shortest WISE bands, accounts for photometric errors, and found that adding the 22 µm band does not improve completeness or reliability. The selection is 97.1 % complete for type‑1 and 76.5 % for type‑2 AGN at L(2–10 keV)>10^44 erg s⁻¹, achieves high reliability and efficiency, and identifies 2 755 candidates in the BUXS area, 38.5 % of which have 2–10 keV detections.

Abstract

We present a highly complete and reliable mid-infrared (MIR) colour selection of luminous AGN candidates using the 3.4, 4.6, and 12 um bands of the WISE survey. The MIR colour wedge was defined using the wide-angle Bright Ultra-Hard XMM-Newton Survey (BUXS), one of the largest complete flux-limited samples of bright (f(4.5-10 keV)>6x10^{-14} erg cm^-2 s^-1) "ultra-hard" (4.5-10 keV) X-ray selected AGN to date. BUXS includes 258 objects detected over a total sky area of 44.43 deg^2 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 (SED) 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)>10^{44} 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}% of the BUXS type-1 and type-2 AGN 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 the BUXS survey our selection identifies 2755 AGN candidates detected with SNR>5 in the three shorter wavelength bands of WISE with 38.5% having a detection at 2-10 keV X-ray energies. We also analyzed the possibility of including the 22um 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 22um compared with the first three bands of WISE and star-formation contributing to the 22um emission at the WISE 22um sensitivity.