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
nei.spo. Again the
response is the same as
Adopt a source distance of 3 kpc, and fix the Galactic foreground
absorption to . 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 . It is defined as follows: . When 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
neijmodel 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.
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.