Continuum Variability of Active Galactic Nuclei in the Optical–Ultraviolet Range

Abstract

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, BJ, F, and N bands with the Mayall 4 m telescope at Kitt Peak National Observatory. Changes δα of the spectral slope α appear correlated with brightness variations δlogfν, 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-δlogfν relation is consistent with the spectral change due to temperature variation of a blackbody of about 2.5×104 K.