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An intensive reverberation mapping campaign on the Seyfert 1 galaxy Mrk817using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST)revealed significant variations in the response of the broad UV emission linesto fluctuations in the continuum emission. The response of the prominent UVemission lines changes over a $\sim$60-day duration, resulting in distinctlydifferent time lags in the various segments of the light curve over the 14months observing campaign. One-dimensional echo-mapping models fit thesevariations if a slowly varying background is included for each emission line.These variations are more evident in the CIV light curve, which is the lineleast affected by intrinsic absorption in Mrk817 and least blended withneighboring emission lines. We identify five temporal windows with distinctemission line response, and measure their corresponding time delays, whichrange from 2 to 13 days. These temporal windows are plausibly linked to changesin the UV and X-ray obscuration occurring during these same intervals. Theshortest time lags occur during periods with diminishing obscuration, whereasthe longest lags occur during periods with rising obscuration. We propose thatthe obscuring outflow shields the ultraviolet broad lines from the ionizingcontinuum. The resulting change in the spectral energy distribution of theionizing continuum, as seen by clouds at a range of distances from the nucleus,is responsible for the changes in the line response.

AGN STORM 2: V. Anomalous Behavior of the CIV Light Curve in Mrk 817