2017MNRAS.468.2699L

Multifrequency Very Long Baseline Array (VLBA) polarimetry observation of the GHz-peaked spectrum (GPS) quasar OQ172 (J1445+0958) has been performed at 1.6, 2.2, 4.8, 8.3 and 15.3 GHz in 2005. Core-jet structures are detected in all bands with the jet strongly bent at about 3 mas from the core. The radio emission of the source is polarized at all five bands. We study the Faraday rotation in the core and jet components at all five bands, and find good linear fits of Faraday rotation in the core and jet components at 4.8 and 8.3 GHz. At these two frequencies, the rotation measure (RM) is ∼ 2000 rad m^- 2^ in the core and ∼ 700 rad m^- 2^ in the inner jet components and continues to decrease at the outer jet parts. We find that the depolarization at 4.8 and 8.3 GHz might be caused by the internal medium in the source. We investigate consistency of the turnover spectra of VLBI components with the synchrotron self-absorption and free-free absorption models. Although these two models cannot be easily distinguished due to the lack of low-frequency data, the physical parameters can be constrained for each model. We find that the large width of the [O  III]₅₀₀₇ line is likely caused by a jet interaction with a narrow line region (NLR) medium. The jet bending, significant RM variations, Faraday depolarization, spectral turnover and broad line width of [O  III]₅₀₀₇ could be closely related, likely caused by the same nucleus medium, presumably NLR.