Multiwavelength selection of obscured agn and contributions to the X-ray background

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University of Alabama Libraries

Obscuration in AGN is a crucial component to understanding the observed spectrum of the X-ray Background. We tested numerous AGN selection techniques in X-ray, mid-infrared, and optical to test for multiwavelength correlations and to help establish selection criteria for obscured AGN. With AGN sources dominating background X-ray sources, we selected medium-depth archival Chandra observations covering 5.6 deg^2 of sky and generating a large sample of serendipitous X-ray sources (greater than 10,000). The mid-infrared component came from archival Spitzer data, with ~3,500 sources being detected in at least two IRAC bands and 1,485 in all 4 bands. For the optical component, greater than 70% of the Chandra observations also had full coverage within the SDSS Data Release 7, >2,300 optical counterparts and 125 spectra. In analyzing the sample, we have identified the parameter spaces in the X-ray/mid-infrared/optical that are optimized for containing members of the elusive class of obscured AGN, and provide a candidate list. We cross-check our X-ray number counts and source densities with contributions to the X-ray background, and find that we resolve approximately 90% of the X-ray Background in the 0.5-8.0 keV range. Testing populations divided on X-ray hardness and flux-level confirms that the unresolved hard X-ray background will be dominated by large populations of increasingly fainter and harder sources.

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