| Contributors: |
Gonzalez-Buitrago, D.; Barth, A. J.; Edelson, R.; Santisteban, J. V. H.; Horne, K.; Schmidt, T.; Li, Y. -R.; Guo, H.; Joner, M. D.; Cackett, E.; Gelbord, J.; Bentz, M. C.; Brandt, W. N.; Goad, M.; Korista, K.; Vestergaard, M.; Villforth, C.; Breeveld, A.; Brink, T. G.; Corsini, E. M.; Dalla Bonta, E.; Ferland, G. J.; Filippenko, A. V.; Garcia-Diaz, M. T.; Hallum, M.; Horst, J. C.; Kim, M.; Krongold, Y.; Kruger, J.; Kuhn, B.; Kumar, S.; Mehdipour, M.; Morelli, L.; Mathur, S.; Netzer, H.; Ochner, P.; Pagotto, I.; Pizzella, A.; Sand, D. J.; Siviero, A.; Spencer, M.; Sung, H.; Vaughan, S.; Winkler, H.; Zheng, W. |
| Description: |
We present ground-based multiband light curves of the AGN Mrk 509, NGC 4151, and NGC 4593 obtained contemporaneously with Swift monitoring. We measure cross-correlation lags relative to Swift UVW2 (1928 Å) and test the standard prediction for disc reprocessing, which assumes a geometrically thin optically thick accretion disc where continuum interband delays follow the relation tau(lambda)~lambda^4/3 . For Mrk 509 the 273-d Swift campaign gives well-defined lags that increase with wavelength as tau(lambda)~λ^2.17+/-0.2 , steeper than the thin-disc prediction, and the optical lags are a factor of ~5 longer than expected for a simple disc-reprocessing model. This 'disc-size discrepancy' as well as excess lags in the u and r bands (which include the Balmer continuum and Halpha , respectively) suggest a mix of short lags from the disc and longer lags from nebular continuum originating in the broad-line region. The shorter Swift 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)~lambda^4/3 but with uncertainties too large for a strong test. For NGC 4151 the Swift lags match tau(lambda)~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-disc 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 behaviour seen in the Swift bands. |