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