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Astrophysics and Space Science Library
The spectrum of the hard X-ray background records the history of accretion processes integrated over the cosmic time. Several pieces of observational and theoretical evidence indicate that a significant fraction of the energy density is obscured by large columns of gas and dust. The absorbing matter is often very thick, with column densities exceeding N_H > 1.5 10^24 cm-2, the value corresponding to unity optical depth for Compton scattering. These sources are called "Compton thick" and appeardoi:10.1007/978-1-4020-2471-9_8 fatcat:vl3awxzkc5gvxbyxjwkatdn3yq