Energy distribution of individual quasars from far-ultraviolet to X-rays.: I.: Intrinsic ultraviolet hardness and dust opacities

Using Chandra and HST archival data, we have studied the individual SED of 11 quasars at redshifts 0.3 < z < 1. 8. All UV spectra show a spectral break around 1100 angstrom. Five X-ray spectra showed the presence of a soft excess,'' and seven spectra showed an intrinsic absorption. We found that for most quasars a simple extrapolation of the far-UV power law into the X-ray domain generally lies below the X-ray data and that the big blue bump and the soft X-ray excess do not share a common physical origin. We explore the issue of whether the observed SED might be dust absorbed in the far-and near-UV. We fit the UV break, assuming a power law that is absorbed by cubic nanodiamond dust grains. We then explore the possibility of a universal SED (with a unique spectral index) by including further absorption from SMC-like extinction. Using this approach, satisfactory fits to the spectra can be obtained. The hydrogen column densities required by either nanodiamonds or amorphous dust models are all consistent, except for one object, with the columns deduced by our X-ray analysis, provided that the C depletion is similar to 0.6. Because dust absorption implies a flux recovery in the EUV (< 700 8), our modeling opens the possibility that the intrinsic quasar SED is much harder and more luminous in the EUV than inferred from the near-UV data, as required by photoionization models of the broad emission line region. We conclude that the intrinsic UV SED must undergo a sharp turnover before the X-ray domain.