The extraordinary luminosities of AGNs are observed to vary. Variability studies provide crucial data concerning the timescales of variation, the nature of spectral variation, and the delays between the various spectral regions. Variability data are also essential for understanding the nature of accretion disks in AGNs because variability timescales allow us to probe the nuclear regions (< 1014 - 1018 cm). If characteristic timescales are found, these would allow us to investigate the physical mechanisms that drive the fluctuations. By comparing multi-wave-band light curves, the nature of propagation of a signal can be investigated.
3.5.1. Variability Timescales
Variability timescales range from hours to years depending on the luminosity of the AGN, and the observed wavelength range. The most commonly observed X-ray variations in Seyfert galaxies are about a few hours (Green, MacHardy, & Lehto 1993; Nandra et al. 1997a). The shortest timescale variation in the optical/UV observed so far is ~ 2 days for NGC 4151 (Crenshaw et al. 1996). Although ultrarapid variations are seen in the X-rays, there is as yet no detection of ultrarapid variability (i.e., variations on timescales of hours) in the optical and UV (Welsh et al. 1998). On average, the optical/UV continuum in Seyfert galaxies and lower luminosity quasars varies over timescales of weeks to months (Kaspi et al. 1996; Givon et al. 1998). The monitoring campaigns so far have found no convincing evidence for any characteristic timescales or periodicities (see, e.g., Netzer & Peterson 1997; Ulrich, Maraschi, & Urry 1996; Mushotzky, Done, & Pounds 1993 for excellent reviews).
3.5.2. Spectral Variations in the Optical/UV/X-Rays
The nature of the spectral variations allows us to characterize the emission mechanism and the factors that cause changes in the emission mechanism. Spectral variations are such that the optical/UV continuum hardens as the nucleus brightens (see Fig. 14). The shortest wavelengths vary the fastest (Clavel et al. 1992; Edelson et al. 1996; Peterson 1993; Nandra et al. 1998). In the X-rays, the power-law component often varies with no spectral variation (Ulrich et al. 1996; Nandra et al. 1998), but sometimes the spectrum does change, becoming softer as the object brightens (e.g., 3C 390.3, Leighly et al. 1997; NGC 5548, Magdziarz et al. 1998). Increased Compton cooling of the X-ray-emitting plasma, possibly because of a change in the disk geometry producing more UV photons (see Magdziarz et al. 1998; see Section 5 below), may be an explanation for this. We discuss the implications of spectral variations further in Section 4 below.
Figure 14. HST / FOS observations of NGC 3516. The observations were obtained over a period of a year and show a factor ~ 5 variation at 1360 Å and a factor of ~ 2 variation at 2200 Å. This type of variation, where the shortest wavelengths vary more than the longer wavelengths, is typical of AGNs. Thus the optical/UV continuum hardens as the nucleus brightens.
3.5.3. Correlations between the Optical/UV and X-Rays
One of the main characteristics of optically thick accretion disks is that different portions of the optical/UV spectrum arise predominately from spatially separated locations in the flow that have different overall size scales. Variability in the continuum might therefore have wavelength-dependent timescales and amplitudes, and one might also expect time lags owing to finite signal propagation speeds between different regions of the flow. For quite some time now, it has been known that variations in the optical/UV continuum of Seyfert galaxies and QSOs are highly correlated and imply signal propagations on faster timescales than the viscous or even sound-crossing times in the flow (Alloin et al. 1985; Cutri et al. 1985; Courvoisier & Clavel 1991). One possible explanation for this is that optical/UV variations are driven by reprocessing of UV or X-ray radiation from the inner parts of the disk (Krolik et al. 1991; Collin-Souffrin 1991). This is confirmed by simultaneous, multiwavelength monitoring campaigns of the Seyfert galaxies NGC 4151 and NGC 5548, which show correlated variability between optical/UV and X-ray energies (Edelson et al. 1996; Clavel et al. 1992).
NGC 7469 has been intensively monitored for the longest duration so far, and this campaign allowed time lags between the different continuum wave bands to be measured for the first time. In this source, optical/UV continuum variability is highly correlated and shows a monotonically increasing time lag with wavelength , which can be fitted with 4/3, the prediction for a Teff r-3/4 accretion disk (Wanders et al. 1997; Collier et al. 1998). The time delays ranged from 0.2 to 2 days, and their statistical significance is difficult to determine. A conservative analysis by Peterson et al. (1998) finds them to be marginally significant (> 2.2 ). Peterson et al. (1998) also showed that if the results for NGC 7469 are real and if we scale all the other AGNs with previous observations, the data from previous campaigns did not have the temporal resolution to detect lags if they were present.
More interestingly, variability in the X-rays and optical/UV is not correlated on such short timescales (Nandra et al. 1998). However, one can interpret the data in terms of an anticorrelation between the X-rays and UV, with the X-rays leading the UV by 4 days. Alternatively, the UV correlates with the X-rays, with the UV leading by 4 days. Neither of these statements provides a satisfactory representation of the light curves, which appear to show the UV leading the X-rays near the peak fluxes, while they both achieve approximate simultaneity near the minimum flux levels (see Fig. 15). The X-rays also show short timescale variability (50% in 1 day), which is not observed in the UV. While this last fact is consistent with the optical/UV radiation arising on larger scales from reprocessing of X-rays from the inner disk, the lack of short timescale correlations presents a severe problem for this simple picture. Models in which the UV drive the X-rays by providing the seed photons for Compton scattering are also too simple to explain the observations. As Nandra et al. (1998) point out, the observations require a more complex interaction between the UV- and X-ray-emitting regions. One possibility that they propose is that in bright states the source of UV seed photons lies 4 lt-days out from the X-ray source and drives the X-ray variability. This source of seed photons is lost in faint states, where closer sources of seed photons become important. It is not yet clear how to reconcile the UV/X-ray variations with a flow geometry.
Figure 15. X-ray and UV variability for NGC 7469. The UV variations are leading the X-rays near the peak fluxes, while they both achieve approximate simultaneity near the minimum flux levels. The X-rays also show short timescale variability (50% in 1 day), which is not observed in the UV. These observations indicate that there is complex interaction between the UV- and X-ray-emitting regions, and it is not clear how to reconcile the UV/X-ray variations with a flow geometry (courtesy K. Nandra).
Another indication from the NGC 7469 campaign that things are not so simple is the fact that, while the broad emission lines do tend to show correlated, lagged variability with the observed UV continuum, in this particular campaign they also showed long-term trends that were not present in the continuum, which suggests that either the observed UV photons are not in fact a good representation of the variability in the (unobservable) ionizing photon spectrum or that the broad line region is evolving over this period.