Optical circular polarization induced by axionlike particles in blazars

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 $\sim 0.1\%$ level, a rough limit on ALP-photon coupling can be obtained, specifically $g_{a\gamma }\cdot B_{T0}\lesssim 7.9\times 10^{-12}{\mathrm{G}\cdot \mathrm{GeV}}^{-1}$ 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.

Comments: 15 pages, 9 figures, 4 tables