The intraday variations of the polarization vector direction in radio source S5 0716+714

The bright radio source S5 0716+714, which is usually classified as a BL Lac object, is one of the most intensively studied blazar. S5 0716+714 demonstrates extremely peculiar properties, such as the shortest time-scale of optical and polarimetric variations observed in blazars. In this paper, we present the results of a 9-h polarimetric monitoring of S5 0716+714 with a ∼70-s resolution carried out using the 6-m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS). An anaysis of the observations reveals variability in both the total and the polarized light on a time-scale of 1-1.5 hr. Because polarization is generated by plasma motion in the magnetic field, the variations of the polarization vector are bound to the magnetic field configuration and the average time-scales specify the size of the emitting region, which cannot be resolved directly. We present a numerically processed model of the polarization in the jet with a precessing helical magnetic field. Fitting the model reveals that the observed short-term variations, with a complicated trajectory of polarization vector, could be induced by a magnetic field precession with a period T ≈ 15 d.