2016ApJ...818...88L

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_3.4µm (mid-infrared), hereafter R - W1, 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 - W1 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 - W1 space. We demonstrate that optically normal galaxies hosting X-ray AGNs at redshifts 0.5 < z < 1 can be recovered with an R - W1 > 4 color cut, while they typically are not selected as AGNs based on their W1 - W2 colors. Additionally, different observed X-ray luminosity bins favor different regions in R - W1 parameter space: moderate-luminosity AGNs (10⁴³ erg erg/s < L_0.5–10kev < 10⁴⁴ erg/s) tend to have red colors, while the highest-luminosity AGNs ( L_0.5–10kev > 10⁴⁵ erg/s) 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.