Optical Spectropolarimetric Variability Properties in Blazars PKS 0637-75 and PKS 1510-089

TL;DR

This study uses optical spectropolarimetry to analyze the polarization variability in two blazars, revealing how jet orientation and emission processes influence polarization in the broad lines and continuum.

Contribution

It provides new spectropolarimetric data on PKS 0637-75 and PKS 1510-089, linking polarization variability to disk-jet emission contributions and jet orientation effects.

Findings

Continuum polarization varies with average levels of 2.5% and 7.5%.

Broad emission lines show minimal polarization, around 0.2-0.6%.

PKS 0637-75's polarization is disk-dominated, while PKS 1510-089 shows combined disk and jet contributions.

Abstract

Spectropolarimetry is a powerful tool to investigate the central regions of active galactic nuclei (AGNs) as polarization signatures are key to probing magnetic field structure, evolution, and the physics of particle acceleration in jets. Optical linear polarization of blazars is typically greater than a few percent, indicating the emission is dominated by nonthermal synchrotron radiation, while polarization less than a few percent is common for other type 1 AGNs. We present a spectropolarimetric study of PKS 0637-75 and PKS 1510-089 to determine how the head-on orientation of a jet and dominant emission processes influence polarimetric variations in the broad lines and continuum. Observations were obtained biweekly from the Robert Stobie Spectrograph on the Southern African Large Telescope. Variability in the continuum polarization is detected for both PKS 0637-75 and PKS 1510-089, with a total average level of 2.5% +/- 0.1% and 7.5% +/- 0.1%, respectively. There is no clear polarization in the broad Balmer emission lines and weak polarization in Mg II as the average level across all observations is 0.2% +/- 0.1% for Hbeta, 0.2% +/- 0.3% for Hgamma, and 0.6% +/- 0.2% for Mg II. We find that polarization measurements confirm the conclusions drawn from spectral energy distribution modeling of the disk-jet contributions to the emission as optical polarization and time variability for PKS 0637-75 are shown to be dominated by accretion disk emission while those of PKS 1510-089 are due to both disk and jet emission, with greater jet contribution during flaring states.