Reddening and the Shape of the Variable Component of the Continua of Active Galactic Nuclei From the Optical to the Far Ultraviolet. I

Abstract: We analyze the photometric variability of 4,611 active galactic nuclei (AGNs) from the Sloan Digital Sky Survey Stripe 82. We recover the spectral energy distribution (SED) of the variable flux as a function of wavelength. For rest wavelengths longer than $\sim \lambda2200$ we find that the SED of the variable component of the bluest AGNs is consistent with the $F_{\nu} \propto \nu^{+1/3}$ spectrum predicted for an externally-illuminated accretion disc. We confirm there is some residual variable emission corresponding to the "small blue bump" and other broad-line region variability. We interpret steeper optical spectra of the variable component as being due to intrinsic reddening. This is supported by the correlation of the Balmer decrement with the colour excess of the variable component. We find the median internal reddening of SDSS AGNs in Stripe 82 with $z < 0.4$ to be $E(B-V) \thickapprox 0.10$ in agreement with the reddening derived from the Balmer decrement. Individual AGNs in the sample can have $E(B-V) > 0.4$