Photoionisation Modelling of the X-ray Emission Line Regions within the
  Seyfert 2 AGN NGC 1068
  We investigate the photoionised X-ray emission line regions (ELRs) within the
Seyfert 2 galaxy NGC 1068, to determine if there are any characteristic changes
between observations taken fourteen years apart. We compare XMM-Newton
observations collected in 2000 and 2014, simultaneously fitting the reflection
grating spectrometer (RGS) and EPIC-pn spectra of each epoch, for the first
time, with the photoionisation model, PION, in SPEX. We find that four PION
components are required to fit the majority of the emission lines in the
spectra of NGC 1068, with $\log \xi=1-4$, $\log N_H>26 m^{-2}$, and
$v_{out}=-100$ to $-600 kms^{-1}$ for both epochs. Comparing the ionisation
state of the components shows almost no difference between the two epochs,
while there is an increase in the total column density. To estimate the
locations of these plasma regions from the central black hole we compare
distance methods, excluding the variability arguments as there is no spectral
change between observations. Although the methods are unable to constrain the
distances, the locations are consistent with the narrow line region, with the
possibility of the higher ionised component being part of the broad line
region, but we cannot conclude this for certain. In addition, we find evidence
for emission from collisionally ionised plasma, while previous analysis had
suggested that collisional plasma emission was unlikely. However, although PION
is unable to account for the FeXVII emission lines at 15 and 17 \AA, we do not
rule out that photoexcitation is a valid processes to produce these lines too.
NGC 1068 has not changed, both in terms of the observed spectra or from our
modelling, within the 14 year time period between observations. This suggests
that the ELRs are fairly static relative to the 14 year time frame between
observations, or there is no dramatic change in the black hole variability.
