9.5. 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: nei.spo. Again the response is the same as corona.res. Adopt a source distance of 3 kpc, and fix the Galactic foreground absorption to 5 \times 10^{24}  \mathrm{m}^{-2}. Define your spectral model.

  1. Fit the spectrum with a CIE model. Is the fit acceptable?

  2. 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?

  3. In SPEX there is also a component which can fit a non-equilibrium spectrum called neij. The most important parameter is U. It is defined as follows: U = \int_{t_0}^{t_n} n_e dt. When U is big, it means the ionization is in equilibrium. Fit the spectrum with neij. What is the temperature after the shock?

  4. Now vary the pre-shock temperature. Does that make any difference?

  5. 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.