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

TL;DR

This study presents ground-based optical photometry and lag measurements of Mrk 817, revealing wavelength-dependent reverberation lags, a disk size discrepancy, and variability linked to luminosity and obscuration changes.

Contribution

It provides detailed multi-epoch reverberation lag measurements across multiple wavelengths, highlighting the impact of luminosity and obscuration on AGN disk size estimates.

Findings

Lag measurements follow the expected λ^{4/3} dependence but are systematically shorter with other methods.

Observed disk sizes exceed thin-disk predictions by factors of 3-6, confirming the disk size discrepancy.

Lag variations correlate with changes in luminosity and obscuration, indicating dynamic accretion disk and broad-line region conditions.

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 years in \emph{uBgVriz} filters, with a median cadence of 0.4 days in \emph{g}. Reverberation lags are measured using three methods (ICCF, JAVELIN, and PyROA) with the Swift UVW2 band (1928 \AA) as the reference light curve. The ICCF centroid lags range from $3.0\pm0.8$ days for the $u$ band up to $7.9\pm1.5$ days for $z$, and are consistent with a $\tau\propto \lambda^{4/3}$ dependence, the relation expected for lamp-post reprocessing by a Shakura-Sunyaev disk. Lags measured with the other methods are systematically shorter, and deviate from a $\lambda^{4/3}$ power-law spectrum at long wavelengths. The lags exceed thin-disk reprocessing predictions by factors of $\sim$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 and/or obscuring outflow, although changes in obscuration between the central engine and broad-line region may also contribute to the lag variations.