Continuum variability of active galactic nuclei in the optical-ultraviolet 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 4 m telescope at Kitt Peak National Observatory. Changes delta alpha of the spectral slope alpha appear correlated with brightness variations delta log f(v), 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 delta alpha-delta log f(v) relation is consistent with the spectral change due to temperature variation of a blackbody of about 2.5 x 10(4) K.