XMM Observations of the Hardest ASCA Sources

We propose XMM observations of 4 Xray (210 keV) selected sources with a veryhard spectra. The proposed objects are probably the brightest and most extremeexamples of a population of sources that is showing up at faint fluxes and thus ideal targets for a detailed study with XMM. They could represent a newpopulation of very hard serendipitous sources or, alternatively, a population ofvery absorbed sources as expected from the CXB synthesis models based on the AGNUnification Scheme. Combining these data with those of 4 other objects, whichare contained in XMM GTO fields, we should be able to assemble a small, butstatistically complete, sample. This sample will have a fundamental and crucialimportance to test our understanding of the CXB and the AGN Unification Scheme. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]