AGN STORM 2. IV. Swift X-ray and ultraviolet/optical monitoring of Mrk 817

TL;DR

This study presents a detailed multiwavelength reverberation mapping of Mrk 817, revealing continuum lags consistent with accretion disk models and highlighting the impact of absorption on observed variability.

Contribution

It provides the first detailed UV/optical/X-ray monitoring of Mrk 817, measuring continuum lags and analyzing absorption effects on variability.

Findings

Continuum lags increase with wavelength as τ(λ) ∝ λ^{4/3}

X-ray flux was significantly fainter with a brief flare

Absorption affects the continuum response and emission line variability

Abstract

The AGN STORM 2 campaign is a large, multiwavelength reverberation mapping project designed to trace out the structure of Mrk 817 from the inner accretion disk to the broad emission line region and out to the dusty torus. As part of this campaign, Swift performed daily monitoring of Mrk 817 for approximately 15 months, obtaining observations in X-rays and six UV/optical filters. The X-ray monitoring shows that Mrk 817 was in a significantly fainter state than in previous observations, with only a brief flare where it reached prior flux levels. The X-ray spectrum is heavily obscured. The UV/optical light curves show significant variability throughout the campaign and are well correlated with one another, but uncorrelated with the X-rays. Combining the Swift UV/optical light curves with Hubble UV continuum light curves, we measure interband continuum lags, $\tau(\lambda)$, that increase with increasing wavelength roughly following $\tau(\lambda) \propto \lambda^{4/3}$, the dependence expected for a geometrically thin, optically thick, centrally illuminated disk. Modeling of the light curves reveals a period at the beginning of the campaign where the response of the continuum is suppressed compared to later in the light curve - the light curves are not simple shifted and scaled versions of each other. The interval of suppressed response corresponds to a period of high UV line and X-ray absorption, and reduced emission line variability amplitudes. We suggest that this indicates a significant contribution to the continuum from the broad line region gas that sees an absorbed ionizing continuum.