Departures from standard disc predictions in intensive ground-based monitoring of three AGNs

ABSTRACT 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 $\tau (\lambda) \propto \lambda ^{4/3}$. For Mrk 509 the 273-d Swift campaign gives well-defined lags that increase with wavelength as $\tau (\lambda)\propto \lambda ^{2.17\pm 0.2}$, steeper than the thin-disc prediction, and the optical lags are a factor of $\sim 5$ 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 H $\alpha$, 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 $\tau (\lambda) \propto \lambda ^{4/3}$ but with uncertainties too large for a strong test. For NGC 4151 the Swift 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-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.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-08-12
DOI
https://doi.org/10.1093/mnras/staf1334
Citations
2
Primary Topic
GNSS positioning and interference
Type
article
Field-Weighted Citation Impact
9.99
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article

Departures from standard disc predictions in intensive ground-based monitoring of three AGNs

J V Hernández Santisteban, Bertram Kühn, E. M. Corsini, P. Ochner et al.
2 citations
Monthly Notices of the Royal Astronomical Society
GNSS positioning and interference
9.99
article

Departures from standard disc predictions in intensive ground-based monitoring of three AGNs

J V Hernández Santisteban, Bertram Kühn, E. M. Corsini, P. Ochner, J. M. Gelbord, Misty C. Bentz, W. N. Brandt, M. Spencer, Aaron J. Barth, M. D. Joner, Y. Krongold, L. Morelli, K. T. Korista, M. Mehdipour, Edward M. Cackett, M. R. Goad, D. González–Buitrago, Hartmut Winkler, M Kim, M. K. Hallum, M. Vestergaard, A. V. Filippenko, Thomas Schmidt, Smita Mathur, G. J. Ferland, A. Siviero, Hengxiao Guo, Rick Edelson, David J. Sand, A. A. Breeveld, Ilaria Pagotto, J. Chuck Horst, E. Dalla Bontà, C. Villforth, A. Pizzella, Thomas G. Brink, Sahana Kumar, Hyuna Sung, H. Netzer, Ma. T. García-Díaz, Wei Zheng, Keith Horne, Yan-Rong Li, J. Krüger, S. Vaughan
article en
2 citations

Abstract

ABSTRACT 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 $\tau (\lambda) \propto \lambda ^{4/3}$. For Mrk 509 the 273-d Swift campaign gives well-defined lags that increase with wavelength as $\tau (\lambda)\propto \lambda ^{2.17\pm 0.2}$, steeper than the thin-disc prediction, and the optical lags are a factor of $\sim 5$ 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 H $\alpha$, 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 $\tau (\lambda) \propto \lambda ^{4/3}$ but with uncertainties too large for a strong test. For NGC 4151 the Swift 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-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.

Monthly Notices of the Royal Astronomical SocietyVol. 542(3)
Brigham Young University (US), Andrews University (US), Boston University (US), University of Copenhagen (DK), Space Telescope Science Institute (US), Pennsylvania State University (US), University of Arizona (US), Western Michigan University (US), University of Kentucky (US), University of Leicester (GB), Tel Aviv University (IL), University of St Andrews (GB), Spectral Sciences (United States) (US), Georgia State University (US), Wayne State University (US), University of Lancashire (GB), Chinese Academy of Sciences (CN), University of Atacama (CL), University of California, Irvine (US), University of Johannesburg (ZA), San Diego State University (US), Shanghai Astronomical Observatory (CN), Korea Astronomy and Space Science Institute (KR), Kitt Peak National Observatory (US), Kyungpook National University (KR), Department of Space (IN), Université Bourgogne Franche-Comté (FR), Institute of High Energy Physics (CN), Eureka Scientific (US), Osservatorio Astronomico di Padova (IT), University College London (GB), University of Virginia (US), University of Bath (GB), The Ohio State University (US), Universidad Nacional Autónoma de México (MX), University of California, Berkeley (US)
Openalex Percentile: Top 2%
GNSS positioning and interference
9.99
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