Variability and stability in blazar jets on time scales of years:
  Optical polarization monitoring of OJ287 in 2005-2009
  (Abridged) OJ287 is a BL Lac object that has shown double-peaked bursts at
regular intervals of ~12 yr during the last ~40 yr. We analyse optical
photopolarimetric monitoring data from 2005-2009, during which the latest
double-peaked outburst occurred. The aim of this study is twofold: firstly, we
aim to analyse variability patterns and statistical properties of the optical
polarization light-curve. We find a strong preferred position angle in optical
polarization. The preferred position angle can be explained by separating the
jet emission into two components: an optical polarization core and chaotic jet
emission. The optical polarization core is stable on time scales of years and
can be explained as emission from an underlying quiescent jet component. The
chaotic jet emission sometimes exhibits a circular movement in the Stokes
plane. We interpret these events as a shock front moving forwards and backwards
in the jet, swiping through a helical magnetic field. Secondly, we use our data
to assess different binary black hole models proposed to explain the regularly
appearing double-peaked bursts in OJ287. We compose a list of requirements a
model has to fulfil. The list includes not only characteristics of the
light-curve but also other properties of OJ287, such as the black hole mass and
restrictions on accretion flow properties. We rate all existing models using
this list and conclude that none of the models is able to explain all
observations. We discuss possible new explanations and propose a new approach
to understanding OJ287. We suggest that both the double-peaked bursts and the
evolution of the optical polarization position angle could be explained as a
sign of resonant accretion of magnetic field lines, a 'magnetic breathing' of
the disc.
