2025ApJ...985...30M

We present a continuum lag analysis for a sample of 37 relatively high-luminosity active galactic nuclei (AGNs) from the Seoul National University AGN Monitoring Project (SAMP), utilizing the light-curve data in B and V bands from SAMP and in g, r, i bands from the Zwicky Transient Facility. We find that the interband lags (τ) increase with the wavelength (i.e., τ ∝ λ^∼4/3) as prescribed by the standard disk model (Shakura-Sunyaev disk, hereafter SSD), suggesting consistency with the “lamp-post” reprocessing model. We report that the size of the continuum emitting region (CER) normalized at 2500 Å (R₂₅₀₀) is a factor of ∼5 (i.e., 0.69 ± 0.04 dex) larger than predicted by SSD. By combining our new measurements with the remeasurements of the literature sample, we report a correlation between R₂₅₀₀ and AGN continuum luminosity as ${R}_{2500}\,\propto \,{L}_{5100}^{0.58\pm 0.03}$, which suggests that the observed continuum could be composed of both the disk emission and the diffuse emission from the broad-line region (BLR). The size of CER shows a tight relation with the size of Hβ BLR with a sublinear slope (i.e., ${R}_{\,\rm{BLR}\,}\,\propto \,{R}_{\,\rm{2500}\,}^{0.87\pm 0.07}$) and a scatter of 0.29 dex. This empirical relation offers a promising method for estimating single-epoch black hole masses, once established over a large dynamic range of AGN luminosity.