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NGC 4051 is one of the X-ray brightest and most variable 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 ofthe He-like emission lines of N$^{+5}$ and O$^{+6}$. The exceptionally highintercombination line flux is a direct measurement of either a high density ora UV irradiated photoionized plasma. Either way, this provides an unambiguousdistance diagnostic. We find that the line emitting region has a density of$\sim3\times10^{10}$ cm$^{-3}$ and distance of a few $10^{15}$ cm from thecontinuum source, placing it within the UV-optical Broad Line Region of NGC4051. Both N$^{+5}$ and O$^{+6}$ independently result in similar measurementsfor density and distance. In addition, we find the kinematics of the broadO$^{+7}$ Ly$\alpha$ match those of the He$^{+1}\,\lambda4686$\,\AA\ line, whichis associated with the NGC 4051 Broad Line Region. This is the first directX-ray measurement of a Seyfert Broad Line Region.

Direct observation of the Broad Line Region in X-rays during the low state of NGC4051