Evidence for variability timescale dependent UV/X-ray delay in Seyfert 1 AGN NGC 7469

Abstract

Using a month-long X-ray lightcurve from $RXTE$/PCA and 1.5 month long UV continuum lightcurves from $IUE$ spectra in 1220$-$1970 $\tilde{A}$, 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 $\tilde{A}$ 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 $Swift$/XRT X-ray and $Swift$/UVOT UVW2, U lightcurves. Continuum lightcurves extracted from the $Swift$/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 $\tilde{A}$ lag those at 1315 $\tilde{A}$ by $0.29\pm 0.06$ day, while the fast variations are coincident ($0.04\pm 0.12$ day). The UV/optical continuum reverberation lag from $IUE$, $Swift$ and other optical telescopes at different wavelengths are consistent with the relationship: $\ddot{I}„\propto \hat{I}{»}^{4/3}$, predicted for the standard accretion disc theory while the best-fit X-ray delay from $RXTE$ and $Swift$/XRT shows a negative X-ray offset of $\sim$0.38 days from the standard disc delay prediction.

13 pages, 9 figures, 1 table, accepted for publication in MNRAS