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S. Paltani (Harvard-Smithsonian Center for Astrophysics)
Intensive monitoring campaigns on Seyfert 1 galaxies provide access to observational properties of the blue-bumps that go far beyond the measurement of their spectra. Among these observational properties, the existence of a very short lag between ultraviolet and optical light curves, which has already been observed in two objects, is a quite strong indication that blue-bump emission in Seyfert 1 galaxies is a consequence of reprocessing of hard X-ray emission on cold material.
Under the assumption that reprocessing is the primary origin of the blue-bump, we study the effect of the geometry on the observable properties that can be derived from blue-bump and X-ray observations. We explore several classes of models, involving different irradiation geometries and different cold matter distributions, like a central extended X-ray source, a point source located above the accretion disk, and several disk-corona geometries. For each model we derive "observational" parameters that can be compared to those measured in individual Seyfert 1 galaxies. Among these properties are: the delay between the different light curves, the reflection fraction, the spectrum and luminosity. But we stress here the importance of a parameter that is most often neglected in studies of the blue-bump: the spectral variability.
We find that the combination of the observational constraints is very difficult to satisfy. In particular, most objects in our sample of Seyfert 1 galaxies show evidence of discontinuities in the temperature distribution of reprocessing matters. Simple geometries, like standard geometrically-thin accretion disks are therefore ruled-out.