Continuum Variability of Active Galactic Nuclei in the Optical-UV 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 $U$, $B_J$, $F$ and $N$ bands with the Mayall 4m telescope at Kitt Peak. Changes $\delta \alpha$ of the spectral slope $\alpha$ appear correlated with brightness variations $\delta log{f}_{\nu }$ 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 $\delta \alpha -\delta log{f}_{\nu }$ relation is consistent with the spectral change due to temperature variation of a black body of about $2.5\cdot 10^{4}K$.

30 pages, 6 figures, to appear in ApJ (2001)

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