Contrasting the Ultraviolet and X‐Ray OviColumn Density Inferred for the Outflow in NGC 5548

We compare X-ray and UV spectroscopic observations of NGC 5548. Both datasets show O VI absorption troughs associated with the AGN outflow from thisgalaxy. We find that the robust lower limit on the column density of the O VIX-ray trough is seven times larger than the column density found in a study ofthe O VI UV troughs. This discrepancy suggests that column densities inferredfor UV troughs of Seyfert outflows are often severely underestimated. Weidentify the physical limitations of the UV Gaussian modeling as the probableexplanation of the O VI column density discrepancy. Specifically, Gaussianmodeling cannot account for a velocity dependent covering fraction, and it is apoor representation for absorption associated with a dynamical outflow.Analysis techniques that use a single covering fraction value for eachabsorption component suffer from similar limitations. We conclude by suggestingways to improve the UV analysis.Comment: 16 pages, 1 figure, accepted for publication in Ap