The Quest for the Driving Force in Active Galactic Nuclei
Peterson, B.M., Collier, S., Horne, K., and Wanders, I. 1998, in Ultraviolet Astrophysics Beyond the IUE Final Archive, ed. R.A. Harris, ESA Conference Proceedings SP-413, in press.

Multiwavelength monitoring of continuum variability in active galactic nuclei (AGNs) can strongly constrain the emission mechanisms at work in these sources. For example, the first intensive monitoring program on the Seyfert 1 galaxy NGC 5548, undertaken by the International AGN Watch with IUE in 1989, showed that whatever mechanism was causing the continuum variations must propagate at close to the speed of light if the continuum was produced by a standard accretion disk. Further monitoring programs provided similar constraints. However, a continuous 49-day monitoring program on NGC 7469 carried out with IUE in its final year provides for the first time statistically significant evidence for time delays between continuum variations in different parts of the spectrum. Optical data confirm the trend seen in the UV. These interband time delays can be attributed to a temperature gradient in the continuum source combined with radiatively driven continuum variations. The time delays are consistent with the wavelength^4/3 dependence expected for a thin accretion disk. We will discuss the implications of this detection, as well as the absence of previous detections of wavelength-dependent continuum lags in AGNs.