Optical Continuum Reverberation in the Dwarf Seyfert Nucleus of NGC 4395

Abstract

The nearby dwarf spiral galaxy NGC 4395 contains a broad-lined active galactic nucleus (AGN) of exceptionally low luminosity powered by accretion onto a central black hole of very low mass ($\sim 10^4-10^5$ M${}_{\odot }$). In order to constrain the size of the optical continuum emission region through reverberation mapping, we carried out high-cadence photometric monitoring of NGC 4395 in the $griz$ filter bands on two consecutive nights in 2022 April using the four-channel MuSCAT3 camera on the Faulkes Telescope North at Haleakalā Observatory. Correlated variability across the $griz$ bands is clearly detected, and the $r$, $i$, and $z$ band light curves show lags of $8.4_{-1.1}^{+1.0}$, $14.2_{-1.4}^{+1.2}$, and $20.4_{-2.1}^{+2.0}$ minutes with respect to the $g$ band when measured using the full-duration light curves. When lags are measured for each night separately, the Night 2 data exhibit lower cross-correlation amplitudes and shorter lags than the Night 1 light curves. Using the full-duration lags, we find that the lag-wavelength relationship is consistent with the $\tau \propto {\lambda }^{4/3}$ dependence found for more luminous AGN. Combining our results with continuum lags measured for other objects, the lag between $g$ and $z$ band scales with optical continuum luminosity as ${\tau }_{gz}\propto L^{0.56\pm 0.05}$, similar to the scaling of broad-line region size with luminosity, reinforcing recent evidence that diffuse continuum emission from the broad-line region may contribute substantially to optical continuum variability and reverberation lags.

8 pages, 3 figures, 2 tables, submitted to ApJL. Comments welcome

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