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

TL;DR

This study extends continuum reverberation mapping of AGNs using Swift data, revealing that diffuse continuum emission from the broad-line region significantly influences observed continuum lags, challenging standard accretion disk models.

Contribution

It provides new systematic analysis of AGN continuum lags with an enlarged sample, confirming the role of BLR diffuse emission and establishing correlations with BLR size and luminosity.

Findings

Interband lags follow τ ∝ λ^{4/3} but are larger than standard model predictions.

U band lag excess attributed to diffuse BLR emission.

Positive correlation between diffuse continuum region size and BLR size/luminosity.

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 $\tau \propto \lambda^{4/3}$, 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 ($\tau_{\rm cut}$), which are generally consistent with the lags ($\tau_{\rm jav}$) 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.