Abstract

Long time scale ultraviolet light curves of blazars and Seyfert 1s both show very strong continuum variations, but this similarity vanishes when short time scales, spectral variability and correlations between variability and luminosity are studied. For instance, blazars show much more rapid variations than Seyfert 1s. Also, the spectra of Seyfert 1s harden as the source brightens, while blazars show little spectral variability. Third, the most luminous blazars tend to be the most strongly variable, while for Seyfert 1s, the strongest variations are seen in the least luminous sources. These differences suggest that in spite of some overall similarities, the observed emission from blazars and Seyfert 1s have different physical origins. These results are consistent with models which hold that the ultraviolet emission from blazars is incoherent synchrotron emission from a jet, while that from Seyfert 1s is dominated by thermal emission from an accretion disk.

Background

In the 13 years since its launch, IUE has obtained over 5000 ultraviolet spectra of active galactic nuclei (AGN). In this paper, we use ∼2500 spectra of 16 objects to survey the ultraviolet variability properties of Seyfert 1s (defined to include quasars as well; [2]) and blazars (BL Lacs and OVV quasars; [6]).

Long and Short Time Scale Variability

Although it is a common prejudice that blazars are more strongly variable than Seyfert 1s, the long term variability properties of the two types of object are actually very difficult to distinguish.