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

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) – (mid-infrared), hereafter , as a color diagnostic to identify obscured supermassive black hole accretion in wide-area X-ray surveys. We use the ∼16.5 deg 2 Stripe 82 X-ray survey data as a test bed to compare 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 space. We demonstrate that optically normal galaxies hosting X-ray AGNs at redshifts can be recovered with an color cut, while they typically are not selected as AGNs based on their colors. Additionally, different observed X-ray luminosity bins favor different regions in parameter space: moderate-luminosity AGNs (10 43 erg erg s −1 ) tend to have red colors, while the highest-luminosity AGNs ( 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.