The UV/optical Continuum Reverberation Mapping of Eight Active Galactic Nuclei with Swift: Further Evidence for the Outer Component

TL;DR

This work expands continuum reverberation mapping to eight additional low-redshift AGNs using Swift UVOT data and confirms that interband lags largely follow $\tau \propto \lambda^{4/3}$ but are systematically larger than standard thin-disk predictions, with a pronounced excess in the $U$ band. The authors isolate a diffuse continuum component from BLR emission using the ICCF-Cut method and validate it with the JAVELIN Pmap model, finding strong correlations between the diffuse-continuum region size and both BLR size and optical luminosity, i.e., $R_{DCR}-R_{BLR}$ and $R_{DCR}-L$. They show that the diffuse BLR continuum can account for a substantial part of the observed lags, extending the interpretation of AGN variability beyond a pure disk reprocessing scenario. The results support a significant BLR contribution to the UV/optical continuum lags and establish robust $R_{DCR}-R_{BLR}$ and $R_{DCR}-L_{5100}$ relations, with practical implications for accretion-disk size estimates and BLR structure. Overall, the study strengthens the case that BLR diffuse continuum reprocessing plays a key role in shaping AGN continuum variability on multi-band timescales.

Abstract

In our previous work, we applied the ICCF-Cut method to the continuum reverberation mapping (CRM) of six active galactic nuclei (AGNs) based on the published Swift data. Extending this work, we perform a systematic AGN CRM study utilizing the Swift archive. We enlarge our sample with eight additional AGNs at $z<0.05$ with high-cadence ($<3$ days) and multiband photometric observations. Time series analysis of these light curves shows two main results: (1) The interband lags are broadly consistent with $τ\propto λ^{4/3}$, while the average interband lags are larger than those predicted by the standard thin accretion disk model. (2) For most targets, there exists a $U$ band lag excess, which is probably due to the diffuse continuum emission from the broad-line region (BLR). We employ the ICCF-Cut method to extract the possible diffuse continuum component from the $U$ band light curves and calculate the diffuse continuum lags ($τ_{\rm cut}$), which are generally consistent with the lags ($τ_{\rm jav}$) derived by the JAVELIN Photometric Reverberation Mapping Model. Further analysis with our sample indicates a positive correlation between the diffuse continuum region size and the BLR size ($R_{\rm DCR}-R_{\rm BLR}$ relation), as well as another correlation with the luminosity ($R_{\rm DCR}-L$ relation). These findings provide further evidence for a significant contribution of diffuse continuum emission from the BLR to the AGN continuum lags.

Figures (16)

• Figure 1: Left: Swift UVOT light curves of Fairall 9. Data are ordered by wavelength from the shortest (top) to the longest (bottom). Right: CCFs (in black) and FR/RSS centroid distributions (in color) for each band relative to the $UVW2$ band. A positive value means the band lags behind $UVW2$ band.
• Figure 2: Left panel: $UVW2$ band light curves for Fairall 9. Right panel: $UVW2$ band light curves for NGC 6814. Both panels show the original light curve (blue solid), the long-term trend fit by a second-order (orange dashed), and the detrended light curve (green dotted).
• Figure 3: The same as Figure \ref{['fig:Fairall9_lc']}, but for the first part detrended light curves of Fairall 9.
• Figure 4: The same as Figure \ref{['fig:Fairall9_lc']}, but for the second part light curves of Fairall 9.
• Figure 5: Plots of measured median ICCF centroid lags (black circles) as a function of wavelength for each target. All lags are measured with respect to the $UVW2$ band. The best fits of $\tau_0$ with $\alpha=4/3, \lambda=1928$ Å to the function $\tau=\tau_0[(\lambda/\lambda_0)^{\alpha}-1]$ are shown by the black solid lines. The predictions from the standard thin disk model (Equation \ref{['eq:lag-wave-thin-disk']}) are shown by the black dashed lines.