Photopolarimetry of 3C 66A in 2003

摘要

The BL Lac-type object 3C 66A was observed at the Crimean Astrophysical Observatory during the international project OJ-94. Observations were made over 10 nights from February through December 2003 at the Cassegrain focus of the 125-cm AZT-11 telescope with a photopolarimeter capable of simultaneous measurements in the UBVRI bands. In the course of our measurements the brightness of the object increased by more than 1 magnitude in all these bands. Its color indices varied and the degree of polarization decreased from similar to 16% in February to similar to 3% at the end of our observations. In December 2003 a rapid change in the position angle from 15 degrees to 40 degrees was noticed. The spectral energy distribution F-n is well described by a power law with a spectral index a (F-nu proportional to nu (-)alpha). The increase in brightness was accompanied by a reduction in the spectral index. The most probable mechanism for the observed changes in the brightness, degree of polarization, and spectral index may be a decrease in the magnetic field strength or a change in its configuration owing to a increase in the chaotic component of the field.