  In a previous paper we investigated the energy transfer of massive stars to
the interstellar medium as a function of time and the geometrical configuration
of three massive stars via 3D-mesh-refining hydrodynamics simulations,
following the complete evolution of the massive stars and their supernovae
except non-thermal processes . We analysed our ISM simulation results with the
help of spectra for plasma temperatures between 0.1 and 10 keV and computed the
spectral evolution and the spatio-temporal distribution of the hot gas.
Results. Despite significant input of high temperature gas from supernovae and
fast stellar winds, the resulting thermal X-ray spectra are generally very
soft, with most of the emission well below 1 keV. We show that this is due to
mixing triggered by resolved hydrodynamic instabilities. Supernovae enhance the
X-ray luminosity of a superbubble by 1-2 orders of magnitude for a time span of
about 0.1 Myr; longer if a supernova occurs in a larger superbubble and shorter
in higher energy bands. Peak superbubble luminosities of the order of 10^{36}
erg/s are reproduced well. The strong decay of the X-ray luminosity is due to
bubble expansion, hydrodynamic instabilities related to the acceleration of the
superbubble's shell thanks to the sudden energy input, and subsequent mixing.
We also find global oscillations of our simulated superbubbles, which produce
spatial variations of the X-ray spectrum, similar to what we see in the
Orion-Eridanus cavity. We calculated the fraction of energy emitted in X-rays
and find that with a value of a few times 10^{-4}, it is about a factor of ten
below the measurements for nearby galaxies.
