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<SUB>R</SUB> (optical) - {m}<SUB>3.4μ {{m</SUB>}} (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 deg<SUP>2</SUP> 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\lt z\lt 1 can be recovered with an R-W1\gt 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 (10<SUP>43</SUP> erg {{{s}}}<SUP>-1</SUP>\lt {L}<SUB>0.5-10{keV</SUB>}\lt {10}<SUP>44</SUP> erg s<SUP>-1</SUP>) tend to have red colors, while the highest-luminosity AGNs ({L}<SUB>0.5-10{keV</SUB>}\gt {10}<SUP>45</SUP> erg s<SUP>-1</SUP>) 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.