AGN STORM 2: I. First results: A Change in the Weather of Mrk 817

TL;DR

This study reports on multi-wavelength monitoring of Mrk 817, revealing variable obscuration, UV and optical reverberation lags consistent with accretion disk models, and a temporary decoupling of continuum and emission line variability during extreme obscuration phases.

Contribution

First detailed multi-wavelength reverberation mapping of Mrk 817 showing variable obscuration and its effects on emission line and continuum variability.

Findings

Detection of highly absorbed X-ray spectrum and new UV absorption lines.

Reverberation lags consistent with a standard accretion disk and broad line region.

Temporary decoupling of UV continuum and emission line variability during extreme obscuration.

Abstract

We present the first results from the ongoing, intensive, multi-wavelength monitoring program of the luminous Seyfert 1 galaxy Mrk 817. While this AGN was, in part, selected for its historically unobscured nature, we discovered that the X-ray spectrum is highly absorbed, and there are new blueshifted, broad and narrow UV absorption lines, which suggest that a dust-free, ionized obscurer located at the inner broad line region partially covers the central source. Despite the obscuration, we measure UV and optical continuum reverberation lags consistent with a centrally illuminated Shakura-Sunyaev thin accretion disk, and measure reverberation lags associated with the optical broad line region, as expected. However, in the first 55 days of the campaign, when the obscuration was becoming most extreme, we observe a de-coupling of the UV continuum and the UV broad emission line variability. The correlation recovers in the next 42 days of the campaign, as Mrk 817 enters a less obscured state. The short CIV and Ly alpha lags suggest that the accretion disk extends beyond the UV broad line region.