2024ApJ...972..141D

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 (< 10¹⁶ cm), with potential cloud geometries ranging from spherical to elongated along the line of sight.