  We present a study of the X-ray emission from numerical simulations of hot
bubbles in planetary nebulae (PNe). High-resolution, two-dimensional,
radiation-hydrodynamical simulations of the formation and evolution of hot
bubbles in PNe, with and without thermal conduction, are used to calculate the
X-ray emission and study its time-dependence and relationship to the changing
stellar parameters. Instabilities in the wind-wind interaction zone produce
clumps and filaments in the swept-up shell of nebular material. Turbulent
mixing and thermal conduction at the corrugated interface can produce
quantities of intermediate temperature and density gas between the hot, shocked
wind bubble and the swept-up photoionized nebular material, which can emit in
soft, diffuse X-rays. We use the CHIANTI software to compute synthetic spectra
for the models and calculate their luminosities. We find that models both with
conduction and those without can produce the X-ray temperatures and
luminosities that are in the ranges reported in observations, although the
models including thermal conduction are an order of magnitude more luminous
than those without. Our results show that at early times the diffuse X-ray
emission should be dominated by the contribution from the hot, shocked stellar
wind, whereas at later times the nebular gas will dominate the spectrum. We
analyse the effect of sampling on the resultant spectra and conclude that a
minimum of 200 counts is required to reliably reproduce the spectral shape.
Likewise, heavily smoothed surface-brightness profiles obtained from low-count
detections of PNe do not provide a reliable description of the spatial
distribution of the X-ray emitting gas.
