The unusually red delay spectrum of the low-mass black hole AGN NGC\,4051 as revealed by intensive continuum reverberation mapping with the Las Cumbres Observatory

TL;DR

This study uses intensive optical reverberation mapping of the low-mass black hole AGN NGC 4051, revealing unusual red spectra and larger-than-expected lags, challenging standard accretion disc models.

Contribution

It provides detailed reverberation mapping data for NGC 4051, highlighting deviations from standard models and proposing explanations for the red SED and lag anomalies.

Findings

Observed wavelength-dependent lags consistent with accretion disc scaling.

Detected significantly larger lags in i and z_s bands than expected.

The AGN's variable spectrum is markedly redder than canonical predictions.

Abstract

We present a two-year optical reverberation mapping campaign of NGC 4051, an active galactic nucleus (AGN) hosting a low-mass black hole ($8\times10^5 M_\odot$), 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, $\tau \propto \lambda^{4/3}$, and a pronounced u-band excess. However, the $i$ and $z_s$ 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, $F_\nu \propto \nu^{1/3}$, typically observed in more massive systems. We explore two scenarios to account for the red UV-optical SED and the anomalously large $i$ and $z_s$ lags: (a) SMC-like dust reddening ($E(B-V)\sim0.18$) 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.