2025/08/12 by D. González–Buitrago, Aaron J. Barth, Gonzalez-Buitrago, Diego +78 · 1 citation
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #Astronomy and Astrophysical Research
paper · doi:10.48550/arxiv.2508.08720
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~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 \sw campaign 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 \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 τ(λ) ∝ λ4/3 but with uncertainties too large for a strong test. For NGC 4151 the \sw lags 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 \sw bands.