Evidence for Nonlinear X-ray Variability from the Broad-line Radio Galaxy 3C 390.3

We present analysis of the light curve from the ROSAT HRI monitoring observations of the broad-line radio galaxy 3C 390.3. Observed every three days for about 9 months, this is the first well sampled X-ray light curve on these time scales. The flares and quiescent periods in the light curve suggest that the variability is nonlinear, and a statistical test yields a detection with >6 sigma confidence. The structure function has a steep slope ~0.7, while the periodogram is much steeper with a slope ~2.6, with the difference partially due to a linear trend in the data. The non-stationary character of the light curve could be evidence that the variability power spectrum has not turned over to low frequencies, or it could be an essential part of the nonlinear process. Evidence for X-ray reprocessing suggests that the X-ray emission is not from the compact radio jet, and the reduced variability before and after flares suggests there cannot be two components contributing to the X-ray short term variability. Thus, these results cannot be explained easily by simple models for AGN variability, including shot noise which may be associated with flares in disk-corona models or active regions on a rotating disk, because in those models the events are independent and the variability is therefore linear. The character of the variability is similar to that seen in Cygnus X-1, which has been explained by a reservoir or self-organized criticality model. Inherently nonlinear, this model can reproduce the reduced variability before and after large flares and the steep PDS seen generally from AGN. The 3C 390.3 light curve presented here is the first support for such models to explain AGN variability on intermediate time scales from a few days to months