Spectrum

On top of the thermal bremsstrahlung emission spectrum, observed X-ray spectra also exhibit spectral lines. Given the high temperature of the gas, it won't come as a surprise that we observe lines of highly ionised species - Fe-lines are especially important in the wave-bands observed by modern X-ray telescopes. And just as we did for HII regions, we can try to constrain the physical conditions in the hot gas, by modelling the relative strength of different transitions. This is how one determines the temperature $T$ of the hot gas.