Variability and stability in blazar jets on time-scales of years: optical polarization monitoring of OJ 287 in 2005-2009

Abstract

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