Space-VLBI polarimetry of the BL Lac object S5 0716+714: Rapid polarization variability in the VLBI core

To determine the location of the intra-day variable (IDV) emission region within the jet of the BL Lac object S5 0716+714, a multi-epoch VSOP polarization experiment was performed in Autumn 2000. To detect, image, and monitor the short term variability of the source, three space-VLBI experiments were performed with VSOP at 5 GHz, separated in time by six days and by one day. Quasi-contemporaneous flux density measurements with the Effelsberg 100 m radio telescope during the VSOP observations revealed variability of about 5 % in total intensity and up to 40 % in linear polarization in less than one day. Analysis of the VLBI data shows that the variations are located inside the VLBI core component of 0716+714. In good agreement with the single-dish measurements, the VLBI ground array images and the VSOP images, both show a decrease in the total flux density of ∼ 20 mJy and a drop of ∼ 5 mJy in the linear polarization of the VLBI core. During the observing interval, the polarization angle rotated by about 15 degrees. No variability was found in the jet. The high angular-resolution VSOP images are not able to resolve the variable component and set an upper limit of < 0.1 mas to the size of the core component. From the variability timescales we estimate a source size of a few micro-arcseconds and brightness temperatures exceeding 10 K. We discuss the results in the framework of source-extrinsic (interstellar scintillation induced) and source-intrinsic IDV models.