2019ApJ...879..102P

NGC 4051 is one of the most variable and X-ray bright Seyfert galaxies. During 2009, NGC 4051 was caught at its lowest state ever observed by XMM-Newton. The low source continuum emission allows a clear measurement of the He-like emission lines of N⁺⁵ and O⁺⁶. The exceptionally high intercombination line flux is a direct measurement of either a high density or a UV irradiated photoionized plasma. Either way, this provides an unambiguous distance diagnostic. We find that the line emitting region has a density of ∼3 x 10¹⁰ cm^–3 and a distance of a few 10¹⁵ cm from the continuum source, placing it within the UV-optical broad line region of NGC 4051. Both N⁺⁵ and O⁺⁶ independently result in similar measurements for density and distance. In addition, we find the kinematics of the broad O⁺⁷ Lyα match those of the He⁺¹ λ4686 Å line, which is associated with the NGC 4051 broad line region. This is the first direct X-ray measurement of a Seyfert broad line region.