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 {\it WISE}, and the 35\% and 50\% sky coverage from SDSS and Pan-STARRS, respectively, we explore the efficacy of $m_R$ (optical) - $m_{3.4\mu m}$ (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 $\sim$16.5 deg${}^2$ Stripe 82 X-ray survey data as a test-bed to compare $R-W1$ with $R-K$, an oft-used obscured AGN selection criterion, and examine where different classes of objects lie in this parameter space. Most stars follow a well-defined path in $R-K$ vs. $R-W1$ space. We demonstrate that optically normal galaxies hosting X-ray AGN at redshifts $0.5<z<1$ can be recovered with an $R-W1>4$ color-cut, while they typically are not selected as AGN based on their $W1-W2$ colors. Additionally, different observed X-ray luminosity bins favor different regions in $R-W1$ parameter space: moderate luminosity AGN ($10^{43}$ erg s${}^{-1}<L_{0.5-10keV}<10^{44}$ erg s${}^{-1}$) tend to have red colors while the highest luminosity AGN ($L_{0.5-10keV}>10^{45}$ 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 AGN 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 {\it eROSITA} mission.

accepted for publication to ApJ; 17 pages (emulateapj style), 21 figures, 3 tables

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• Luminous WISE-selected Obscured, Unobscured, and Red Quasars in Stripe 82
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• Galaxy properties of type 1 and 2 X-ray selected AGN and comparison among different classification criteria
• An obscured AGN population hidden in the VIPERS galaxies: identification through spectral energy distribution decomposition
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