Using R-W1 to Uncover Obscured AGN in X-ray Surveys

X-ray surveys provide perhaps the most efficient method for discovering AGN across the Universe, though a major challenge is the time investment involved in following-up X-ray sources to identify them. To make the best use of limited resources, we explored the efficacy of m_r (optical) - m_3.4-micron (mid-infrared), hereafter R-W1, as a color diagnostic to hone in on signatures of obscured black hole growth in wide-area X-ray surveys which would otherwise be missed in optical and infrared surveys. We calibrated this color on the 16.5 deg^2 release of the Stripe 82X survey, finding that most X-ray emitting stars lie along a well-defined locus in R-W1 v. R-K color space and most AGN hosted in optically normal galaxies beyond z>0.5 have red R-W1 colors (R-W1 > 4 Vega). Our results will be useful for crafting target lists for current and future wide-area X-ray surveys to follow-up the most promising obscured AGN candidates