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 Swiftmonitoring. We measure cross-correlation lags relative to SwiftUVW2 (1928 Å) and test the standard prediction for disk reprocessing, which assumes a geometrically thin, optically thick accretion disk where continuum interband delays follow the relation τ(λ)∝λ4/3. For Mrk 509 the 273-d Swiftcampaign gives well-defined lags that increase with wavelength as τ(λ)∝λ2.17 ± 0.2, steeper than the thin-disk prediction, and the optical lags are a factor of ∼5 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 Hα, respectively) suggest a mix of short lags from the disk and longer lags from nebular continuum originating in the broad-line region. The shorter Swiftcampaigns, 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 τ(λ)∝λ4/3 but with uncertainties too large for a strong test. For NGC 4151 the Swiftlags match τ(λ)∝λ4/3, 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 τ − λ relation across the optical/UV/NIR, which differs from the more homogeneous behavior seen in the Swiftbands.