Direct observation of the Broad Line Region in X-rays during the low
  state of NGC4051
  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 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
$\sim3\times10^{10}$ cm$^{-3}$ and distance of a few $10^{15}$ 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$^{+7}$ Ly$\alpha$ match those of the He$^{+1}\,\lambda4686$\,\AA\ 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.
