Insights into the long-term behavior of the optical polarization from the blazar 1ES 1959+650

Abstract

A high degree of linear polarization measured in the optical emission is an important observational feature of blazars. It provides strong evidence of the presence of relativistic particles and magnetic field ordering in the non-thermal emission regions of blazars owing to the synchrotron nature of low energy radiation. Thus, the polarization studies of blazars are emerging as a promising approach to probe the particle acceleration and the physical processes involved in their broadband emission. In this work, we investigate the behavior of the optical polarization of the blazar 1ES 1959+650 measured over a decade using the spectropolarimetry (SPOL) at the Steward Observatory. We use measurements of the degree of linear polarization and angle of polarization in the wavelength range 500 - 700 nm available during the period October 1, 2008 and June 30, 2018 (MJD 54739 - 58299) from the SPOL observations. Near simultaneous photometry data in the R and V bands are also used to study the optical emission from the source. The maximum degree of linear polarization, measured as ∼ 8.5%, is significantly larger than the long term average value of ∼ 4.6%. Analysis of the light curves indicates that the optical emission from the blazar 1ES 1959+650 is highly variable and variability in the degree of linear polarization can be quantified by a fractional variability amplitude of ∼ 39% over the period of about ten years. Long term optical emission in the R and V bands is very weakly anti-correlated with the degree of linear polarization. Modeling of the polarization due to the synchrotron emission suggests that the observed degree of linear polarization can be broadly reproduced by a power law distribution of relativistic electrons gyrating in a spherical emission region permeated with chaotic and ordered magnetic fields. Variation in the measured degree of polarization may be attributed to the interplay between the two magnetic field components in the emission region. The effect of stellar emission from the host galaxy of the blazar 1ES 1959+650 on the degree of synchrotron polarization originating from the jet is also discussed.