Astronomy & Astrophysics

Free access article

A&A 385, 55-61 (2002)
DOI: 10.1051/0004-6361:20020145

Analysis of the long-term polarization behaviour of BL Lac

V. A. Hagen-Thorn^{1, 2}, E. G. Larionova¹, S. G. Jorstad^{1, 3}, C.-I. Björnsson⁴ and V. M. Larionov^{1, 2}

¹ Astronomical Institute of St. Petersburg State University, Universitetsky pr. 28, 198504 St. Petersburg, Russia
² Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
⁴ Stockholm Observatory, 13336 Saltsjöbaden, Sweden

(Received 29 August 2001 / Accepted 22 January 2002)

Abstract
An analysis is performed of polarimetric and photometric observations of BL Lac carried out in 1969-1991 at the Astronomical Institute of St. Petersburg State University. The distribution of polarization directions certainly points to the existence of the preferred direction of polarization ( $\theta= 22\degr$ ) close to the direction of the jet observed by VLBI. High polarization degree and colorimetric data are evidence of the synchrotron nature of the variable source. The relative Stokes parameters are distributed normally over some mean value. The correlations between different parameters characterizing the flux density and polarization of the central point source allow us to consider that in BL Lac there is a continually acting source of polarized radiation on which the sources with randomly distributed polarization directions are superimposed.

Key words: BL Lac objects: individual: BL Lac -- techniques: polarimetric

Offprint request: V. A. Hagen-Thorn, hth@vg3823.spb.edu

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