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We conducted multi-epoch, multi-frequency parsec-scale studies on thegigahertz-peaked spectrum quasar PKS 0858-279 with the Very Long Baseline Array(VLBA). Our observations on 2005-11-26 elucidated a weak core, characterized byan inverted spectrum, and a distinctly bent jet that exhibited a notable brightfeature in its Stokes I emission. Through comprehensive analysis ofpolarization and spectral data, we inferred the formation of a shock wavewithin this feature, stemming from interactions with a dense cloud in theambient medium. In this paper, VLBI-Gaia astrometry further reinforces the coreidentification. With a deep analysis of six additional VLBA epochs spanningfrom 2007 to 2018, we observed that while the quasar's parsec-scale structureremained largely consistent, there were discernible flux density changes. Thesevariations strongly imply the recurrent ejection of plasma into the jet.Complementing our VLBA data, RATAN-600 observations of the integrated spectrasuggested an interaction between standing and travelling shock waves in 2005.Moreover, our multi-epoch polarization analysis revealed a drastic drop inrotation measure values from 6000 rad/m^2 to 1000 rad/m^2 within a single year,attributable to diminishing magnetic fields and particle density in an externalcloud. This change is likely instigated by a shock in the cloud, triggered bythe cloud's interaction with the jet, subsequently prompting its expansion.Notably, we also observed a significant change in the magnetic field directionof the jet, from being perpendicular post its observed bend to beingperpendicular prior to the bend - an alteration possibly induced by thedynamics of shock waves.

Parsec-scale evolution of the gigahertz-peaked spectrum quasar PKS 0858-279