The intraday variations of polarization vector direction in radio source S5 0716+714
S5 0716+714 is a well-known bright radio source, commonly referred to the class of BL Lac objects, but characterized by unusually high variability in all spectral ranges: the changes in the brightness in optical band reach 5 mag on a long time scale and 0.5 mag within the night.
In 2018 February on the 6-m BTA telescope of the SAO RAS we carried out a 9-hour photometric and polarimetric monitoring of S5 0716+714 using the SCORPIO spectrograph. The technique of observations with a double Wollaston analyzer allows us to measure three Stokes parameters I, Q and U, which determine the intensity and the linear polarization, at the same time. Simultaneous observations of the source and the star in the field – the photometric and the zero polarization standard - minimize the effects introduced by the atmosphere: absorption and depolarization. This technique provided the polarization measurement accuracy of about 0.1%.
Due to the high temporal resolution of our monitoring – about 70 seconds – we were able to resolve the change in the direction of the polarization vector on the QU-plane during the night with a typical switching time of about 1.5 hours (Fig. 1). The same period was found by the wavelet analysis for the variability of the total flux. Fig. 2 demonstrates the «period – exposure moment» diagram, where the areas up to 95% (2σ) deviation are contoured, and the average periodogram, revealing the 77±10 min period of the variations. The equality of the periods of the variability in total and polarized light confirmed the assumption that the flux and polarization variability is generated by the motion of the radiating relativistic plasma in the helical magnetic field of the jet.
In the frame of the conventional model of jet radiation (Fig. 3) the observed optical light is generated at a distance of less than 0.01 pc from the central black hole. Then the calculated period of the variability in polarized and total light allows us to estimate the linear size of the spiral field associated with the radiating area - 1.5 light hours, or 10 astronomical units. Numerical simulation of polarization generation in the jet revealed that the observed pattern of oscillations of the polarization vector can be explained by only geometric effects – the motion of relativistic charged particles in a precessing helical magnetic field. The approximation of our observational data by such a model showed the presence of a magnetic field precession with a period of 15 days.
Contact person: Elena Shablovinskaya, graduate student of Laboratory of spectroscopy and photometry of extragalactic objects SAO RAS