An XMM-Newton Study of Hard X-ray Sources
  We briefly report on an on-going spectroscopic study of hard X-ray sources selected serendipitously in 12 XMM-Newton fields. Results for the analysis of the 41 sources from the first seven EPIC observations have been discussed in a previous paper (Piconcelli et al. 2002, Paper I) where we found an absolute fraction of X-ray absorbed sources (~30%) lower than expected (~50%) by the predictions of popular CXB synthesis models at F(2-10)~5x10**(-14) erg cm**-2 s**-1. We present here the preliminary results concerning the whole sample including five new deeper XMM-Newton measurements increasing the sample to 90 sources. Even if still on-going, the present study appears to confirm and extend down to F(2-10)~10**(-14) erg cm**-2 s**-1 the above mismatch between observational data and theoretical expectations regarding the fraction of absorbed sources. Furthermore the sample average spectral index of 1.5-1.6 is steeper than the CXB slope indicating that the majority of obscured sources making the bulk of the CXB resides at even lower hard X-ray fluxes. 
E. Piconcelli (1), L. Bassani (1), M. Cappi (1), F. Fiore (2), G. Di   Cocco (1), J.B. Stephen (1) ((1) IASF-CNR Bologna, (2...