AGN STORM 2. VIII. Investigating the Narrow Absorption Lines in Mrk 817 Using HST-COS Observations

We observed the Seyfert 1 galaxy Mrk 817 during an intensive multiwavelength reverberation mapping campaign for 16 months. Here, we examine the behavior of narrow UV absorption lines seen in the Hubble Space Telescope/Cosmic Origins Spectrograph spectra, both during the campaign and in other epochs extending over 14 yr. We conclude that, while the narrow absorption outflow system (at −3750 km s ^−1 with FWHM = 177 km s ^−1 ) responds to the variations of the UV continuum as modified by the X-ray obscurer, its total column density (log N _H = 19.5 ${}_{-0.13}^{+0.61}$ cm ^−2 ) did not change across all epochs. The adjusted ionization parameter (scaled with respect to the variations in the hydrogen-ionizing continuum flux) is log U _H = −1.0 ${}_{-0.3}^{+0.1}$ . The outflow is located at a distance smaller than 38 pc from the central source, which implies a hydrogen density of n _H > 3000 cm ^−3 . The absorption outflow system only covers the continuum emission source and not the broad emission line region, which suggests that its transverse size is small (< 10 ^16 cm), with potential cloud geometries ranging from spherical to elongated along the line of sight.