This is a solicited proposal following up GT spectral observations
of an handful of active galaxies, mostly type 2 Seyferts, which show
prominent lines from highly ionized species OIV, NeV, NeVI, MgVII,
MgVIII, SiIX. These coronal lines can be used both to determine the
physical conditions (e.g. ionization mechanism, abundances) of the
coronal line region, and to constrain the shape of the AGN ionizing
spectrum (20-200 eV) by means of photoionization models.
Preliminary results indicate that type 2 Seyferts have a prominent
UV bump peaking at about 70 eV (Moorwood et al. 1996), and its strength
relative to the underlying power-law continuum may vary between different
objects. One of the most puzzling results is that the only Seyfert 1 so
far observed (NGC4151) displays a much lower degree excitation, i.e.
progressively weaker lines from higher ionization species.
This may indicate an intrinsic difference between the ionizing spectra
of type 1 and 2 Seyferts, i.e. that the popular unified models for AGNs
could be incorrect.
Verifying this possibility is the main aim of this proposal where
we plan SWS observations of 4 bona fide Seyfert 1 galaxies which
should provide a large enough sample to check for systematic differences
between the two Seyfert types.