ON R − W1 AS A DIAGNOSTIC TO DISCOVER OBSCURED ACTIVE GALACTIC NUCLEI IN WIDE-AREA X-RAY SURVEYS

Abstract

ABSTRACT Capitalizing on the all-sky coverage of WISE and the 35% and 50% sky coverage from Sloan Digital Sky Survey and Pan-STARRS, respectively, we explore the efficacy of m R (optical) – m 3.4 &mgr; m ?> (mid-infrared), hereafter R − W 1 ?> , as a color diagnostic to identify obscured supermassive black hole accretion in wide-area X-ray surveys. We use the ∼16.5 deg2 Stripe 82 X-ray survey data as a test bed to compare R − W 1 ?> with R − K, an oft-used obscured active galactic nucleus (AGN) selection criterion, and examine where different classes of objects lie in this parameter space. Most stars follow a well-defined path in R − K versus R − W 1 ?> space. We demonstrate that optically normal galaxies hosting X-ray AGNs at redshifts 0.5 < z < 1 ?> can be recovered with an R − W 1 > 4 ?> color cut, while they typically are not selected as AGNs based on their W 1 − W 2 ?> colors. Additionally, different observed X-ray luminosity bins favor different regions in R − W 1 ?> parameter space: moderate-luminosity AGNs (1043 erg s − 1 < L 0.5 − 10 keV < 10 44 ?> erg s−1) tend to have red colors, while the highest-luminosity AGNs ( L 0.5 − 10 keV > 10 45 ?> erg s−1) have bluer colors; higher spectroscopic completeness of the Stripe 82X sample is needed to determine whether this is a selection effect or an intrinsic property. Finally, we parameterize X-ray obscuration of Stripe 82X AGNs by calculating their hardness ratios (HRs) and find no clear trends between HR and optical reddening. Our results will help inform best-effort practices in following up obscured AGN candidates in current and future wide-area, shallow X-ray surveys, including the all-sky eROSITA mission.