Abstract
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.
Highlights
Emission-line reverberation mapping (Blandford and McKee 1982; Peterson 2014), based on measured lags between continuum and emission-line bands and the width of the emission line, is a very successful technique for determining AGN broad line region (BLR) size and black hole virial mass
An flux randomisation (FR)+random subset selection (RSS) cross-correlation between filtered lightcurves along with the cross correlation function, shown in the right panel of Figure 3, clearly demonstrates that on a timescale faster than 5 days, the UV continuum lags the X-rays by 0.37±0.14 days
Such a lag timescale is consistent with the accretion disc reverberation delay observed from other AGN
Summary
Emission-line reverberation mapping (Blandford and McKee 1982; Peterson 2014), based on measured lags between continuum and emission-line bands and the width of the emission line, is a very successful technique for determining AGN broad line region (BLR) size and black hole virial mass. The Swift observations provide evidence of reprocessing of high energy emission from a larger reprocessor than just the accretion disc, probably the broad line region (BLR) clouds (Korista and Goad 2019; Chelouche et al 2019; Lawther et al 2018; McHardy et al 2018; Cackett et al 2018; Sun et al 2018; Pal and Naik 2018) This evidence is in the form of an excess lag in the U-band (Edelson et al 2017; Fausnaugh et al 2016; Edelson et al 2015), which contains the Balmer continuum, (Kotov et al 2001) and an excess lag at 3634 ̊A (known as the Balmer jump), and in the fact that the reprocessing function required to explain the optical emission as reprocessing of X-ray emission, has a tail to long delays (a few days) as well as a sharp peak at short timescales (∼hours) from the disc. Just within the IUE band, Wanders et al (1997) estimated the delay of Lyα, C iv, N v, Si iv and He ii emission lines with respect to the UV continuum as 2.3-3.1 days, ∼2.7 days, 1.9-2.4 days, 1.7-1.8 days and 0.7-1 day respectively which is broadly consistent with observations
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