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

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 ∝ 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 cut ), which are generally consistent with the lags ( τ 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 DCR – R BLR relation), as well as another correlation with the luminosity ( R DCR – L relation). These findings provide further evidence for a significant contribution of diffuse continuum emission from the BLR to the AGN continuum lags.