Does the AGN Continuum Shape Change with Luminosity?

Abstract

We test the general wisdom that AGN continua vary with larger amplitude at shorter wavelengths, through a re-analysis of historical data from the optical and UV (IUE) monitoring campaign on the Seyfert 1 galaxy NCG 5548 by the AGN Watch Consortium. We assume that the only non-varying component of the optical continuum is the integrated light from the host galaxy, which does not affect the UV continuum. Therefore, we expect any curvature in a linear plot of the UV continuum flux versus the simultaneous optical flux to represent a true change in the spectral index alpha; the y-intercept provides an estimate of the host galaxy contribution in the optical region. We measured the continuum fluxes at 1350A (rest frame) from the NEWSIPS-extracted spectra and adopted the optical continuum determinations of Wanders & Peterson (1996). The data confirm that the curvature in the F_{lambda}(5100) -- F_{lambda}(1350) plot is real, and that the spectral index is a function of luminosity. Hence for NGC 5548, the continuum does get harder as it gets brighter. However, the relationship between the optical and UV continuum flux is more complicated than the models we tested would indicate.