Departures from Standard Disk Predictions in Intensive Ground-Based Monitoring of Three AGN

Abstract

We present ground-based, multi-band light curves of the AGN Mrk~509, NGC\,4151, and NGC\,4593 obtained contemporaneously with \sw\, monitoring. We measure cross-correlation lags relative to \sw\, UVW2 (1928~\AA) and test the standard prediction for disk reprocessing, which assumes a geometrically thin, optically thick accretion disk where continuum interband delays follow the relation \( \tau(\lambda) \propto \lambda^{4/3} \). For Mrk~509 the 273-d \sw\, campaign gives well-defined lags that increase with wavelength as $\tau(\lambda)\propto\lambda^{2.17\pm0.2}$, steeper than the thin-disk prediction, and the optical lags are a factor of $\sim5$ longer than expected for a simple disk-reprocessing model. This ``disk-size discrepancy'' as well as excess lags in the $u$ and $r$ bands (which include the Balmer continuum and H$\alpha$, respectively) suggest a mix of short lags from the disk and longer lags from nebular continuum originating in the broad-line region. The shorter \sw\, campaigns, 69~d on NGC\,4151 and 22~d on NGC\,4593, yield less well-defined, shorter lags $<2$~d. The NGC\,4593 lags are consistent with $\tau(\lambda) \propto \lambda^{4/3}$ but with uncertainties too large for a strong test. For NGC\,4151 the \sw\, lags match $\tau(\lambda) \propto \lambda^{4/3}$, with a small $U$-band excess, but the ground-based lags in the $r$, $i$, and $z$ bands are significantly shorter than the $B$ and $g$ lags, and also shorter than expected from the thin-disk prediction. The interpretation of this unusual lag spectrum is unclear. Overall these results indicate significant diversity in the $\tau-\lambda$ relation across the optical/UV/NIR, which differs from the more homogeneous behavior seen in the \sw\, bands.