Variability of the coronal line region in NGC 4151
  We present the first extensive study of the coronal line variability in an
active galaxy. Our data set for the nearby source NGC 4151 consists of six
epochs of quasi-simultaneous optical and near-infrared spectroscopy spanning a
period of about eight years and five epochs of X-ray spectroscopy overlapping
in time with it. None of the coronal lines showed the variability behaviour
observed for the broad emission lines and hot dust emission. In general, the
coronal lines varied only weakly, if at all. Using the optical [Fe VII] and
X-ray O VII emission lines we estimate that the coronal line gas has a
relatively low density of n~10^3 cm^-3 and a relatively high ionisation
parameter of log U~1. The resultant distance of the coronal line gas from the
ionising source is about two light years, which puts this region well beyond
the hot inner face of the obscuring dusty torus. The high ionisation parameter
implies that the coronal line region is an independent entity rather than part
of a continuous gas distribution connecting the broad and narrow emission line
regions. We present tentative evidence for the X-ray heated wind scenario of
Pier & Voit. We find that the increased ionising radiation that heats the dusty
torus also increases the cooling efficiency of the coronal line gas, most
likely due to a stronger adiabatic expansion.
