The NH distribution of Seyfert 2 galaxies and the X-ray background Original Research Article

Past hard X-ray studies of type 2 Seyferts were biased towards X-ray bright objects, hence they were generally biased also against heavily obscured objects. We present the results of a hard X-ray survey of weak Sy2 galaxies, performed by means of BeppoSAX, aimed at reducing these biases. The X-ray spectra of these objects indicate that all of them are heavily obscured (NH > 1025cm−2 in most cases). We combine our data with other hard X-ray archive data to create a three-dimensional diagnostic diagram that allows the identification of other heavily obscured Sy2s. By using these joint data we derive an estimate for the distribution of the absorbing column density in Sy2s. The average absorbing NH results to be much higher than that estimated in former studies. We use this distribution of column density to obtain a synthesis of the hard X-ray background without free parameters and that is in good agreement with the observational data.