Thermal Absorption

The interaction of photons with electrons in the plasma gives rise to absorption of photons as the electron is accelerated in the Coulomb field of the ion. Although thermal emission is usually weak compared to synchrotron emission in most radio sources, many radio sources show evidences for thermal absorption as its radiation passes through the ionized region of the associated galaxy or our own galaxy. It may be theoretically shown that the thermal absorption coefficient is dependent on the frequency as,

$$\rm {k_\nu \propto -\nu^{-2}}.$$ (5..6)

The characteristic cutoff frequency in the spectrum of the radio source can be associated with the frequency at which the optical depth of the absorbing source equals 1. The effect can be very important for radio sources at low galactic latitudes, but will not be further discussed here. (Ref : Alan T. Moffet, "Strong non-thermal radio emission from galaxies").