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

Chi-Zhuo Wang,Xuebing Wu,Yuanzhe Jiang,Qinchun Ma,Huapeng Gu,Yuhan Wen

Published 2025 in Astrophysical Journal

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 ∝ λ4/3, 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 (τcut), which are generally consistent with the lags (τjav) 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 (RDCR–RBLR relation), as well as another correlation with the luminosity (RDCR–L relation). These findings provide further evidence for a significant contribution of diffuse continuum emission from the BLR to the AGN continuum lags.

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