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

Abstract

Using a month-long X-ray light curve from RXTE/PCA and 1.5 month-long UV continuum light curves from IUE spectra in 1220–1970 Å, 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 Å by 3.49 ± 0.22 d, which is possibly caused by either propagating fluctuation or variable Comptonization. However, if variations slower than 5 d are removed from the X-ray light curve, the UV variations then lag behind the X-ray variations by 0.37 ± 0.14 d, 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 light curves. Continuum light curves 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 d, the slow variations at 1825 Å lag those at 1315 Å by 0.29 ± 0.06 d, while the fast variations are coincident (0.04 ± 0.12 d). The UV/optical continuum reverberation lag from IUE, Swift, and other optical telescopes at different wavelengths are consistent with the relationship: τ ∝ λ4/3, predicted for the standard accretion disc theory while the best-fitting X-ray delay from RXTE and Swift/XRT shows a negative X-ray offset of ∼0.38 d from the standard disc delay prediction.

Complex optical/UV and X-ray variability in Seyfert 1 galaxy Mrk 509

“…On filtering out the longer timescale variabilities from the light curves, the excess lags are found to decrease significantly, making the X-ray and UV/optical correlation stronger. This behaviour has been found in NGC 5548 (Edelson et al 2015), NGC 4593 (McHardy et al, 2018), NGC 7469 (Pahari et al 2020).…”

“…Therefore, the long-term variations should be filtered out from the light curves to determine short time information. Such filtering has been carried out in recent studies in a few AGNs, e.g., NGC 5548 (McHardy et al 2014), NGC 4593 (McHardy et al 2018), NGC 7469 (Pahari et al 2020), to eliminate the effect of long-term variations. We used the locally weighted scatter smoothing (LOWESS) method for the filtering process, which is based on non-parametric and non-linear least square regression method (Cleveland & Devlin 1988).…”

A physical model for the broadband energy spectrum of X-ray illuminated accretion discs: fitting the spectral energy distribution of NGC 5548

“…The observational effort has significantly intensified the last 10 years. A few AGN have been observed continuously and intensively for many months, over a wide range of spectral bands, from optical to X-rays (e.g., McHardy et al 2014;Fausnaugh et al 2016;Cackett et al 2018;Edelson et al 2019;Cackett et al 2020;Pahari et al 2020;Hernández Santisteban et al 2020;Vincentelli et al 2021;Kara et al 2021). In all cases, the UV and optical variations are well correlated, but with a delay, which increases from the UV to the optical bands, as expected in the case of X-ray illumination of the disc.…”

Reverberation mapping of active galactic nuclei: From X-ray corona to dusty torus

“…Other AGNs show no correlation between X-ray and UV/optical variability at all ( Buisson et al., 2018 ; Morales et al., 2019 ). This lack of a clear connection between the X-ray and UV/optical is puzzling and a problem for the simple disk reprocessing picture and therefore may indicate more complex geometries or absorption ( Gardner and Done, 2017 ) or that different variability processes are taking place on different timescales ( Pahari et al., 2020 ). In the future, it will be important to connect the implied geometry from X-ray reverberation to the UV/optical continuum geometry.…”