X-ray spectral analysis of optically faint sources in the Chandra Deep   Fields
  We present the results of a detailed spectral analysis of optically faint hard X-ray sources in the Chandra deep fields selected on the basis of their high X-ray to optical flux ratio (X/O). The stacked spectra of high X/O sources in both Chandra deep fields, fitted with a single power-law model, are much harder than the spectrum of the X-ray background (XRB). The average slope is also insensitive to the 2-8 keV flux, being approximately constant around Gamma~1 over more than two decades, strongly indicating that high X/O sources represent the most obscured component of the XRB. For about half of the sample, a redshift estimate (in most of the cases a photometric redshift) is available from the literature. Individual fits of a few of the brightest objects and of stacked spectra in different redshift bins imply column densities in the range 10^{22-23.5} cm^{-2}. A trend of increasing absorption towards higher redshifts is suggested. 
Francesca Civano (1,2), Andrea Comastri (2), Marcella Brusa (3,2) ((1)   Dipartimento di Astronomia, Universita' di Bologna;...