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 \hbox { \raise 2.15 pt \hbox { $ > \lower 4.558 pt \hbox { $ \kern - 10.7 pt { \sim } $ } $ } } 6 \sigma confidence .
The structure function has a steep slope \sim 0.7 , while the periodogram is much steeper with a slope \sim 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 can not be two components contributing to the X-ray short term variability .
Thus , these results can not 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 .