The long term X-ray time-lags of NGC 4051
  We present the long term, frequency-dependent, X-ray time lags of the Seyfert
galaxy NGC 4051. We used Rossi X-ray Timing Explorer light curves in the 2-4,
4-7 and 7-10 keV bands and we measured the time-lags at $10^{-7}-10^{-6}$ Hz.
This is the lowest frequency range that AGN time-lags have ever been measured.
The variations in the higher energy bands are delayed with respect to the
variations we observe at 2-4 keV, in agreement with the time-lags at high
frequencies. When we combine our results with those from the model fitting of
the time lags at higher frequencies we find that that the X-ray hard lags in
NGC 4051 follow a power law of slope $\sim -1$ over a broad frequency range,
from $\sim 10^{-7}$ to $\sim 10^{-3}$ Hz. This is consistent with the time-lags
of Cyg X-1, a result which further supports the analogy between active galaxies
and Galactic X-ray black hole binaries.
