8th HEAD Meeting, 8-11 September, 2004
Session 35 Surveys and the Cosmic X-ray Background
Oral, Saturday, September 11, 2004, 4:00-5:38pm

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[35.06] Finding the Obscured AGN with ChAMP

T. Aldcroft, P. Green, J. Silverman, R. Cameron, D. Kim, B. Wilkes (CfA), ChaMP Collaboration

X-ray surveys with \textit{Chandra} and \textit{XMM} provide the most complete census of accretion powered luminosity in the universe. Coupled with recent advances suggesting an intimate connection between galaxy and supermassive black hole evolution, X-ray AGN surveys have emerged as a powerful tool for investigating topics from galaxies to large scale cosmology. One empirical input which plays a key role in modeling is the distribution of obscuration in AGN. Obscuration changes the AGN spectral contribution to the hard Cosmic X-ray Background (CXRB), the apparent luminosity of AGN, and can even make them entirely "disappear", so understanding this phenomenon is important. Using X-ray and optical imaging and spectroscopic data from the \textit{Chandra} Multiwavelength Project (ChaMP), we have assembled a sample of ~900~AGN, covering 3.8 square degrees to a uniform flux limit of 2.7\times 10-15~ergs\,cm-2\,sec-1. Using direct spectral fitting with \textit{CIAO/Sherpa} we estimate the intrinsic absorbing column NH in each source. Because of the large sample size, we are able to determine the NH distribution within each of six flux bins, confirming the trend of increasing obscuration at lower fluxes and providing the strongest constraints to date on the obscured fraction. We discuss the implications for modeling of the CXRB and AGN luminosity function.

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.