Frequency-resolved lags in UV/optical continuum reverberation mapping

Abstract

In recent years, continuum reverberation mapping involving high cadence UV/optical monitoring campaigns of nearby Active Galactic Nuclei has been used to infer the size of their accretion disks. One of the main results from these campaigns has been that in many cases the accretion disks appear too large, by a factor of 2 -3, compared to standard models. Part of this may be due to diffuse continuum emission from the broad line region (BLR), which is indicated by excess lags around the Balmer jump. Standard cross correlation lag analysis techniques are usually used to just recover the peak or centroid lag and can not easily distinguish between reprocessing from the disk and BLR. However, frequency-resolved lag analysis, where the lag is determined at each Fourier frequency, has the potential to separate out reprocessing on different size scales. Here we present simulations to demonstrate the potential of this method and then apply a maximum likelihood approach to determine frequency-resolved lags in NGC 5548. We find that the lags in NGC 5548 generally decrease smoothly with increasing frequency, and are not easily described by accretion disk reprocessing alone. The standard cross correlation lags are consistent with lags at frequencies lower than 0.1 day −1 , indicating they are dominated from reprocessing at size scales greater than ∼10 light days. A combination of a more distant reprocessor, consistent with the BLR, along with a standard-sized accretion disk is more consistent with the observed lags than a larger disk alone.

The luminosity-dependent contribution from the broad-line region to the wavelength-dependent lags in Mrk 110

“…It has also been proposed that the UV/optical lags may arise entirely from the BLR (see e.g. Chelouche et al 2019;Netzer 2021) or at least be dominated by it (Cackett et al 2021a). In this case, therefore, the light travel time from the inner accretion flow to the BLR inner radius would correspond to the X-ray to UVW2 lag and not the UVW2 to U-band.…”

“…Moreover, the X-ray vs UVW2 lags seem to show a larger-than-expected amplitude when compared to standard accretion disc models (see also Li et al 2021). This led to the development of several models attempting to reproduce the lag spectrum with reprocessing driven in a lamp-post model with a relatively high corona (Kammoun et al 2021a), an extended reprocessor (Gardner & Done 2015), or even Vincentelli et al with the BLR (Korista & Goad 2001, 2019Lawther et al 2018;Chelouche et al 2019;Netzer 2020Netzer , 2021Cackett et al 2021a).…”

The Super-Massive Black Hole close environment in Active Galactic Nuclei

“…This is currently an active area of research, which will provide insight into the inner accretion flow (e.g. [216,219]). Fig.…”

The Paschen Jump as a Diagnostic of the Diffuse Nebular Continuum Emission in Active Galactic Nuclei

“…Continuum reverberation mapping campaigns have identified a distinct excess in lag in the U -band spectral region in several objects that has been identified with this DC emission (e.g., Edelson et al 2015;Fausnaugh et al 2016;Cackett et al 2018Cackett et al , 2020Hernández Santisteban et al 2020). If the observed U -band excess lags do arise from DC emission from the BLR, this implies that the continuum lags across all wavelengths are also affected by the DC component, and other AGN monitoring results have suggested a substantial or even dominant contribution of DC emission to AGN optical variability (Chelouche et al 2019;Cackett et al 2021b;Netzer 2021).…”