Photopolarimetry of 3C 66A in 2003

Abstract

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 ~16% in February to ~3% at the end of our observations. In December 2003 a rapid change in the position angle from 15° to 40° was noticed. The spectral energy distribution F n is well described by a power law with a spectral index a (). 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.

Extreme photopolarimetric behaviour of the blazar AO 0235+164

“…This relation is in qualitatively good agreement with those found for 3C 66A (Efimov & Primak 2006) and OJ 287 (Efimov et al 2002), suggesting that, despite the different time-scales involved in those studies, a similar mechanism could be operating in all three objects. Efimov & Primak (2006) propose that the polarization degree decreases when the ratio between the strength of the regular and the chaotic components of the magnetic field decreases due to blobs moving along the jet, thus breaking up the ordered structure of the field. The higher degree of disorder in the magnetic field also leads to a hardening of the spectrum.…”