Optical circular polarization in quasars

TL;DR

This study measures optical circular polarization in quasars, finding mostly null results but detecting significant polarization in two blazars, which informs models of magnetic fields and scattering processes in quasar environments.

Contribution

The paper provides new high-precision circular polarization measurements for quasars and explores their implications for magnetic fields and scattering mechanisms.

Findings

Most quasars show null circular polarization.

Significant circular polarization detected in two blazars.

Results suggest magnetic fields up to 1 kG or inverse Compton scattering influence.

Abstract

We present new optical circular polarization measurements with typical uncertainties < 0.1% for a sample of 21 quasars. All but two objects have null circular polarization. We use this result to constrain the polarization due to photon-pseudoscalar mixing along the line of sight. We detect significant (> 3 sigma) circular polarization in two blazars with high linear polarization and discuss the implications of this result for quasar physics. In particular, the recorded polarization degrees may be indicative of magnetic fields as strong as 1 kG or a significant contribution of inverse Compton scattering to the optical continuum.