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The ionization structures of AGNs are determined by the specific ionization mechanisms and the shapes of the ionizing continua. The technique of using emission-line intensity ratios to classify objects and investigate their ionization mechanisms is enhanced significantly by combining high-quality UV and optical spectrophotometry. The HST archive is an excellent source of such data.
The FOS spectra are recalibrated using the latest ‘average inverse sensitivity’ (AIS) calibration. This new method for flux calibrating FOS data (1) normalizes count data from all apertures to the 4”.3 aperture to account for changes of aperture throughput as a function of the Optical Telescope Assembly focus; (2) corrects the data for time-dependent detector sensitivity degradation; and (3) scales the data to the white-dwarf reference scale. The AIS calibration method has been developed over a period of several years, with improvements applied progressively based on observed deficiencies and/or discrepancies of the existing calibrations. Indeed, the final AIS calibration incorporates corrections in the wavelength overlap regions of adjacent gratings derived based on inconsistencies discovered as a result of this study of over 1000 FOS spectra.
Significant improvements in our understanding of the physical conditions in active galaxies requires the development of sophisticated models that can predict in detail the emission from the nucleus. Accurate measurements of both the UV continua and the many weak diagnostic UV lines provide critical calibrations for such models.
Rigorous comparisons of the HST FOS and IUE UV flux calibrations have so far been performed using spectrophotometric standard star spectra. These calibration observations have precision acquisition sequences and the spectra have very high S/N. Such data are not representative of typical FOS observations of an AGN, which often cannot be acquired with the same precision as standard stars, especially for pre-COSTAR data. Furthermore, the UV flux from a typical AGN is several magnitudes fainter than from a spectrophotometric standard star, and is more representative of typical non-stellar fluxes. Flux-dependent effects (in either FOS or IUE) may need to be considered at these lower flux levels.
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