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

Abstract

OJ 287 is a BL Lac object at redshift z= 0.306 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 to 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 find six such events, all on the time‐scales of 10–20 d. 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 OJ 287. We compose a list of requirements a model has to fulfil to explain the mysterious behaviour observed in OJ 287. The list includes not only characteristics of the light curve but also other properties of OJ 287, 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 OJ 287. 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.

Polarimetry of optically selected BL Lacertae candidates from the SDSS

“…Since we used filters that are slightly different from the R-band and since the polarization position angle in high polarization standards is typically wavelength-dependent, though not strongly, a small systematic error in our PA calibration was expected. We compared the derived r-band zero points to the ones derived in Villforth et al (2010) for the R-band and found the two to differ by 1.4 ± 1.3 and by 1.8 ± 1.1 degrees for the CA and NOT data, respectively. As a result, any PA offsets due to filter mismatch are likely to be smaller than 2 degrees.…”

“…Valtonen et al 2008;Villforth et al 2010;Fermi-Lat Collaboration et al 2010), polarization measurements have rarely been used to verify them. So far, only Impey & Brand (1982), Borra & Corriveau (1984), Impey & Tapia (1988), Fugmann & Meisenheimer (1988), Kuehr & Schmidt (1990), Jannuzi et al (1993a), Kock et al (1996), Marcha et al (1996), and S07 used polarization measurements as a diagnostic tool for detecting or confirming BL Lac candidates.…”

“…Since even very prominent BL Lacs like OJ 287 are unpolarized from time to time (e.g. Villforth et al 2010), the number of bona fide BL Lac objects in the sample is presumably even higher. We have further options for testing this result.…”

Optical polarization of high-energy BL Lacertae objects

“…Our analysis also relies on the assumption that the mean EVPA represents a stable EVPA of the jet, which may not be the case considering the fairly short time span of our observations. As discussed in e.g., Villforth et al (2010) and Sakimoto et al (2013), it is also possible that the same sources occasionally show a preferred polarization angle while at another time they may not, so clearly long-term polarization observations are required to better understand this in the HSP objects.…”

The “Far Site” Scenario for Gamma-ray Emission in Blazars

“…In particular, for OJ287 we also found a coherent relation in the multi-spectral range behavior of the source that underwent a series to two γ-ray flares at the end of [39] (Fig. 6).…”