Abstract We present a two-year optical reverberation mapping campaign of NGC 4051, an active galactic nucleus (AGN) hosting a low-mass black hole (8 × 105 M⊙), using daily observations in seven photometric bands from Las Cumbres Observatory augmented by archival data from Swift XRT and UVOT. The light curves show correlated variability with wavelength-dependent lags broadly consistent with the standard accretion disc scaling, τ∝λ4/3, and a pronounced u-band excess. However, the i and zs lags are significantly larger than expected and cannot be explained by a combination of disc emission and diffuse continuum (DC) from the broad-line region (BLR), making NGC 4051 a notable lag-luminosity outlier. The spectral energy distribution (SED) of the variable AGN component is markedly redder than the canonical accretion disc prediction, Fν∝ν1/3, typically observed in more massive systems. We explore two scenarios to account for the red UV-optical SED and the anomalously large i and zs lags: (a) SMC-like dust reddening (E(B − V) ~ 0.18) combined with optically thick emission from the inner edge of the dusty torus; and (b) a dominant diffuse continuum contribution. We discuss the implications of each scenario within a comprehensive multi-wavelength framework.
Marculewicz et al. (Thu,) studied this question.
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