On R-W1 as A Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-ray Surveys

• Published in: arXiv.org
• Main Authors: LaMassa, Stephanie M, Civano, Francesca, Brusa, Marcella, Stern, Daniel, Glikman, Eilat, Gallagher, Sarah, Urry, C Meg, Cales, Sabrina, Cappelluti, Nico, Cardamone, Carolin, Comastri, Andrea, Duncan, Farrah, Greene, Jenny E, Komossa, S, Merloni, Andrea, Mroczkowski, Tony, Natarajan, Priyamvada, Richards, Gordon, Salvato, Mara, Schawinski, Kevin, Treister, Ezequiel
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 29.12.2015
• Subjects: Active galactic nuclei; Color; Deposition; Diagnostic systems; Galaxies; Luminosity; Occultation; Parameters; Physics - Astrophysics of Galaxies; Physics - High Energy Astrophysical Phenomena; Sky surveys (astronomy)
• DOI: 10.48550/arxiv.1511.02883

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\(^{-1} < L_{\rm 0.5-10 keV} < 10^{44}\) erg s\(^{-1}\)) tend to have red colors while the highest luminosity AGN (\(L_{\rm 0.5-10 keV} > 10^{45}\) 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 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.