Optical circular polarization induced by axionlike particles in blazars

TL;DR

This paper explores how axionlike particles interacting with photons could generate optical circular polarization in blazars, providing constraints on ALP properties and explaining tentative observations.

Contribution

It introduces a novel mechanism linking ALP-photon interactions to optical CP in blazars and derives new constraints on ALP parameters based on polarization measurements.

Findings

Optical CP in blazars can be explained by ALP-photon mixing.

Constraints on ALP-photon coupling are derived from polarization limits.

Tentative CP observations in specific blazars can be explained by ALPs with certain parameters.

Abstract

We propose that the interaction between the axionlike particles (ALPs) and photons can be a possible origin of optical circular polarization (CP) in blazars. Given that there is no definite detection of optical CP at $\sim0.1\%$ level, a rough limit on ALP-photon coupling can be obtained, specifically $g_{a\gamma}\cdot B_\mathrm{T0}\lesssim7.9\times10^{-12}~\mathrm{G\cdot GeV}^{-1}$ for $m_{a}\lesssim 10^{-13}~\mathrm{eV}$, depending on the magnetic field configuration of the blazar jet. Obviously, for the blazar models with a larger magnetic field strength, such as hadronic radiation models, this constraint could be more stringent. We also perform a dedicated analysis of the tentative observations of optical CP in two blazars, namely 3C 66A and OJ 287, and we find that these observations could be explained by the ALP-photon mixing with $g_{a\gamma} \sim 10^{-11}~\mathrm{GeV}^{-1}$. As an outlook, our analysis can be improved by further research on the radiation models of blazars and high-precision joint measurements of optical CP and linear polarization.