Turn Over in the Spectra due to Self Absorption

Radio sources which are compact ($<$ 1 kilo-parsec) can have high density of synchrotron electron and become optically thick at lower frequencies. The typically self absorbed spectrum has a steep spectrum behavior at high frequencies and $S_\nu \propto \nu^{5/2}$ behavior at low radio frequencies. The turn over frequency depends on the size of the source and the magnetic field (see eq. 5.7) and can be used to estimate the magnetic field in the source if the angular size is known. The figure 5.5 shows the spectrum with a turn over due to synchrotron self absorption.

Figure 5.5: Spectrum with a turn over due to synchrotron self absorption.

Depending on where the turn over frequency occurs such spectra have been variously classified as synchrotron self absorbed spectra, GHz peaked sources, compact steep spectrum objects etc.