Variability of the Faraday Rotation Measure in the Parsec-scale Jet of AGN 1633+382

Abstract

We study the Faraday rotation measure (RM) variability of flat spectrum radio quasar 1633+382 on 5 epochs spanning from 2004 to 2008. We used 4 to 43~GHz VLBI polarization data from VLBA. Core RM across 4 to 15 GHz scales with a power index $a\sim2$, for $a$ in RM$\propto \nu^a$. We detected sign changes across epochs, both in the core and in the jet region. RM time variability in the core and jet region are not correlated, hence limiting the size of a possible Faraday screen. We relate the core RM variability to a new component emerging from the core region. For the jet region, we consider the jet-medium interaction to be a less likely cause of the RM variability because of the uniform spectral index distribution. The observed RM value variation requires a huge fluctuation in electron density or magnetic field, hence a foreground Faraday screen is less favoured. We further discuss other possibilities of the RM variability based on jet kinematics.