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

TL;DR

This study analyzes narrow UV absorption lines in the Seyfert galaxy Mrk 817 over 14 years, revealing their response to continuum variations and constraining their physical properties and location.

Contribution

It provides a detailed characterization of the narrow absorption outflow in Mrk 817, including its response to continuum changes and spatial constraints, using multi-epoch HST-COS observations.

Findings

The absorption outflow responds to UV continuum variations but its total column density remains constant.

The outflow is located within 38 parsecs of the central source with a density > 3000 cm^-3.

The outflow covers only the continuum source, not the broad emission line region.

Abstract

We observed the Seyfert 1 galaxy Mrk817 during an intensive multi-wavelength reverberation mapping campaign for 16 months. Here, we examine the behavior of narrow UV absorption lines seen in HST/COS spectra, both during the campaign and in other epochs extending over 14 years. We conclude that while the narrow absorption outflow system (at -3750 km/s with FWHM=177 km/s) responds to the variations of the UV continuum as modified by the X-ray obscurer, its total column density (logNH =19.5 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 UH =-1.0. The outflow is located at a distance smaller than 38 parsecs from the central source, which implies a hydrogen density of nH > 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 (< 1e16 cm), with potential cloud geometries ranging from spherical to elongated along the line of sight.