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Continuum Variability of Active Galactic Nuclei in the Optical-UV Range

The variability of the continuum spectral energy distribution has been analyzed for a complete magnitude-limited sample of quasars in Selected Area 57, observed at two epochs in the photographic U, B J , F and N bands with the Mayall 4m telescope at Kitt Peak. Changes δα of the spectral slope α appear correlated with brightness variations δlogf ν indicating an average hardening of the spectrum in the bright phases. This confirms that the correlation of variability with redshift, found in a single observing band, is due to intrinsic spectral changes. The average observed δα − δlogf ν relation is consistent with the spectral change due to temperature variation of a black body of about 2.5 · 10 4 K.