AGN STORM 2. XII. Ground-based Optical Photometry and Lag Measurements of Mrk 817

Authors

• John W. Montano, University of California Irvine
• Aaron J. Barth, University of California Irvine
• Keith Horne, University of St Andrews
• Edward M. Cackett, Wayne State University
• Gisella De Rosa, Space Telescope Science Institute
• Y. Homayouni, Space Telescope Science Institute
• Erin A. Kara, MIT Kavli Institute for Astrophysics and Space Research
• Gerard A. Kriss, Space Telescope Science Institute
• Hermine Landt, Durham University
• Gilvan G. Apolonio, Brigham Young University
• Nahum Arav, Virginia Polytechnic Institute and State University
• Benjamin D. Boizelle, Brigham Young University
• Elena Dalla Bontà, University of Padova
• Doron Chelouche, University of Haifa
• Maryam Dehghanian, University of KentuckyFollow
• Rick Edelson, Eureka Scientific Inc.
• Gary J. Ferland, University of KentuckyFollow
• Carina Fian, Osservatorio Astronomico di Trieste
• Catalina Sobrino Figaredo, University of Haifa
• Michael R. Goad, University of Leicester
• Diego H. Gonzalez-Buitrago, Universidad Nacional Autónoma de México
• Wei Jian Guo, CAS - National Astronomical Observatories
• Chen Hu, Chinese Academy of Sciences
• Dragana Ilić, University of Belgrade
• Michael D. Joner, Brigham Young University
• Shai Kaspi, Tel Aviv University
• Christopher S. Kochanek, Ohio State University
• Andjelka B. Kovačević, University of Belgrade
• Collin Lewin, MIT Kavli Institute for Astrophysics and Space Research
• Sha Sha Li, Yunnan Observatories
• Yan Rong Li, Chinese Academy of Sciences
• Jun Rong Liu, Chinese Academy of Sciences

Abstract

We present the ground-based imaging campaign and light curves of Markarian 817 as part of the multiwavelength monitoring program AGN STORM 2. Observations were carried out over 1.4 yr in the uBgVriz filters, with a median cadence of 0.4 day in the g band. Reverberation lags are measured using three methods (interpolated cross-correlation function (ICCF), Just Another Vehicle for Estimating Lags In Nuclei, and PyROA) with the Swift UVW2 band (1928 Å) as the reference light curve. The ICCF centroid lags range from 3.0 ± 0.8 days for the u band up to 7.9 ± 1.5 days for z, and are consistent with a τ ∝ λ^4/3 dependence, the relation expected for lamppost reprocessing by a Shakura–Sunyaev disk. Lags measured with the other methods are systematically shorter, and deviate from a λ^4/3 power-law spectrum at long wavelengths. The lags exceed thin-disk reprocessing predictions by factors of ∼3–6, similar to the “disk size discrepancy” seen in other Seyfert galaxies. We divide the campaign into three epochs with different levels of mean luminosity and X-ray obscuring column density and find that the lags vary by as much as a factor of 2 between epochs. The intrinsic spectral energy distribution is bluer and brighter during the first third of the campaign, and the longest continuum reverberation lags are obtained during that period. These results suggest that changes in ionizing luminosity can produce large variations in continuum lags on short timescales by altering the diffuse continuum luminosity emitted by the broad-line region (BLR) and/or obscuring outflow, although changes in obscuration between the central engine and BLR may also contribute to the lag variations.

Document Type

Article

Publication Date

6-1-2026

Notes/Citation Information

Publisher Copyright: © 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the https://creativecommons.org/licenses/by/4.0/. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Digital Object Identifier (DOI)

10.3847/1538-4357/ae6103

Archival?

Archival

Repository Citation

Montano, John W.; Barth, Aaron J.; Horne, Keith; Cackett, Edward M.; De Rosa, Gisella; Homayouni, Y.; Kara, Erin A.; Kriss, Gerard A.; Landt, Hermine; Apolonio, Gilvan G.; Arav, Nahum; Boizelle, Benjamin D.; Dalla Bontà, Elena; Chelouche, Doron; Dehghanian, Maryam; Edelson, Rick; Ferland, Gary J.; Fian, Carina; Sobrino Figaredo, Catalina; Goad, Michael R.; Gonzalez-Buitrago, Diego H.; Guo, Wei Jian; Hu, Chen; Ilić, Dragana; Joner, Michael D.; Kaspi, Shai; Kochanek, Christopher S.; Kovačević, Andjelka B.; Lewin, Collin; Li, Sha Sha; Li, Yan Rong; and Liu, Jun Rong, "AGN STORM 2. XII. Ground-based Optical Photometry and Lag Measurements of Mrk 817" (2026). Physics and Astronomy Faculty Publications. 770.
https://uknowledge.uky.edu/physastron_facpub/770