  We report the detection of a long-timescale negative lag, where the blue
bands lag the red bands, in the nearby Seyfert 1 galaxy Fairall 9. Active
Galactic Nuclei (AGN) light curves show variability over a wide range of
timescales. By measuring time lags between different wavelengths, the otherwise
inaccessible structure and kinematics of the accretion disk can be studied. One
common approach, reverberation mapping, quantifies the continuum and line lags
moving outwards through the disk at the light-travel time, revealing the size
and temperature profile of the disk. Inspired by numerical simulations, we
expect longer lags to exist in AGN light curves that travel inward on longer
timescales, tracing the accretion process itself. By analyzing AGN light curves
in both temporal and frequency space, we report the detection of long-timescale
lags ($\sim -70$ days) in Fairall 9 which propagate in the opposite direction
to the reverberation lag. The short continuum lag ($<10$ days) is also detected
and is consistent with reverberation lags reported in the literature. When
fitting the longer lag as a function of frequency with a model motivated by the
thin disk model, we find that the disk scale height likely increases outward in
the disk. This detection raises the exciting prospect of mapping accretion disk
structures across a wide range of AGN parameters.
