UBVRI photopolarimetry of BL Lac objects - The connection between frequency-dependent polarization and growing radio shocks

We present 61 truly simultaneous polarimetric and photometric observations in UBVRI bands of 10 BL Lac objects. The aim of the observations has been to search for frequency-dependent polarization properties in these sources and to investigate the characteristics of their variations. We report first detections of frequency dependent polarization in OF 038 (0422+004) and 3C 371, and the first detection of frequency dependent position angle in Mkn 421. We also find that the strength of the frequency dependence in polarization, P(U)/P(I), increases with increasing P(U) but decreases with increasing P(I). Comparison of optical data with radio outbursts indicates that the observed variations in the wavelength dependence of the optical polarization could be caused by growing shocks responsible for the radio outbursts.

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