Evidence for variability timescale dependent UV/X-ray delay in Seyfert 1
  AGN NGC 7469
  Using a month-long X-ray lightcurve from ${\it RXTE}$/PCA and 1.5 month long
UV continuum lightcurves from ${\it IUE}$ spectra in 1220$-$1970 $\r{A}$, we
performed a detailed time-lag study of the Seyfert 1 galaxy NGC 7469. Our
cross-correlation analysis confirms previous results showing that the X-rays
are delayed relative to the UV continuum at 1315 $\r{A}$ by 3.49 $\pm$ 0.22
days which is possibly caused by either propagating fluctuation or variable
comptonisation. However, if variations slower than 5 days are removed from the
X-ray lightcurve, the UV variations then lag behind the X-rays variations by
0.37$\pm$0.14 day, consistent with reprocessing of the X-rays by a surrounding
accretion disc. A very similar reverberation delay is observed between ${\it
Swift}$/XRT X-ray and ${\it Swift}$/UVOT UVW2, U lightcurves. Continuum
lightcurves extracted from the ${\it Swift}$/GRISM spectra show delays with
respect to X-rays consistent with reverberation. Separating the UV continuum
variations faster and slower than 5 days, the slow variations at 1825 $\r{A}$
lag those at 1315 $\r{A}$ by $0.29\pm0.06$ day, while the fast variations are
coincident ($0.04\pm0.12$ day). The UV/optical continuum reverberation lag from
${\it IUE}$, ${\it Swift}$ and other optical telescopes at different
wavelengths are consistent with the relationship: $\tau \propto \lambda^{4/3}$,
predicted for the standard accretion disc theory while the best-fit X-ray delay
from ${\it RXTE}$ and ${\it Swift}$/XRT shows a negative X-ray offset of
$\sim$0.38 days from the standard disc delay prediction.
