.. SPDX-FileCopyrightText: 1992-2026 NWO-I/SRON Space Research Organisation Netherlands .. .. SPDX-License-Identifier: CC-BY-4.0 Supernova remnants ================== Up to now we have only fitted an object which was in collisional ionization equilibrium (CIE), but when there are plasma shocks in a (low density) medium, equilibrium might not be reached yet. This is often the case in supernova remnants. We will illustrate this with the following spectrum: :download:`nei.spo `. Again the response is the same as :download:`corona.res `. Adopt a source distance of 3 kpc, and fix the Galactic foreground absorption to :math:`5 \times 10^{24}  \mathrm{m}^{-2}`. Define your spectral model. #. Fit the spectrum with a CIE model. Is the fit acceptable? #. With the parameter ``rt``, which is the ratio between the temperature in ionization balance and spectral temperature, we can obtain a better fit. Set the parameter to thawn, but be aware that this ratio is not allowed to get too close to 0! Is the fit acceptable? #. In SPEX there is also a component which can fit a non-equilibrium spectrum called ``neij``. The most important parameter is :math:`U`. It is defined as follows: :math:`U = \int_{t_0}^{t_n} n_e dt`. When :math:`U` is big, it means the ionization is in equilibrium. Fit the spectrum with ``neij``. What is the temperature after the shock? #. Now vary the pre-shock temperature. Does that make any difference? #. In order to see the effect of Non-Equilibrium Ionisation (NEI), make the parameter "U" of the ``neij`` model 10 times smaller and 10 times larger than your best-fit value (leave all other parameters the same!), calculate the spectrum using the "calc" command (no fitting here!) and plot the spectrum. You will see large differences. **Learning goals:** After having done this spectrum, you should know: - How to check your data for Non-Equilibrium Ionisation (NEI) effects by using the parameter RT. - How to use proper NEI models and get a basic understanding of these spectra.