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

TL;DR

This study reveals that the UV emission line responses in Mrk 817 vary over time, influenced by obscuring outflows that alter the ionizing continuum, leading to different observed time lags in the reverberation mapping data.

Contribution

It demonstrates the temporal variability of emission line responses and links these changes to obscuration events affecting the ionizing continuum in Mrk 817.

Findings

Emission line response varies over 60 days.

Time lags range from 2 to 13 days.

Obscuration correlates with lag changes.

Abstract

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