A New Test for the Absorption Mechanism of GPS Radio Sources Using Polarization Properties

We consider the use of polarization properties as a means to discriminatebetween Synchrotron Self-Absorption (SSA) and Free--Free Absorption (FFA) inGHz-Peaked Spectrum (GPS) sources. The polarization position angle (PA) ofsynchrotron radiation at high frequencies for the optically thin regime isperpendicular to the magnetic field, whereas it is parallel to the magneticfield at low frequencies for the optically thick regime. Therefore, SSAproduces a change in PA of $90^{\circ}$ across the spectral peak, while FFAdoes not result in such a change. We analyzed polarization data from VLAobservations for six GPS sources to see if such a change in PA was present. Ourresults indicate that there is no significant evidence for $90^{\circ}$ changein PA across the spectral peak, suggesting that FFA is more likely than SSA forlow-frequency cutoffs in these sources