Survey of Non-thermal Electron around Supermassive Black Holes through Polarization Flips

TL;DR

This paper reviews polarization flip phenomena in non-thermal synchrotron sources near supermassive black holes, linking polarization behavior to electron energy distribution and magnetic field structure, with implications for jet physics.

Contribution

It provides a detailed analysis of polarization flips in simulated black hole jet models, connecting these features to non-thermal electron populations and magnetic field configurations.

Findings

Polarization vectors flip by 90 degrees near the jet base and photon ring due to optical depth effects.

Circular polarization sign also flips on the counter jet and photon ring.

Polarization flips correlate with features in spectral index and Faraday RM maps.

Abstract

Optically thick non-thermal synchrotron sources notably produce linear polarization vectors being parallel to projected magnetic field lines on the observer's screen, although they are perpendicular in well-known optically thin cases. To elucidate the complex relationship between the vectors and fields and to investigate the energy and spatial distribution of non-thermal electrons through the images, we perform polarization radiative transfer calculations at submillimeter wavelengths. Here the calculations are based on semi-analytic force-free jet models with non-thermal electrons with a power-law energy distribution. In calculated images, we find a $90^\circ$-flip of linear polarization (LP) vectors at the base of counter-side (receding) jet near a black hole, which occurs because of large optical depths for synchrotron self-absorption effect. The $90^\circ$-flip of LP vectors is also seen on the photon ring at a high frequency, since the optical depth along the rays is large there due to the light bending effect. In addition, we see the flip of the sign of circular polarization (CP) components on the counter jet and photon ring. Furthermore, we show that these polarization flips are synthesized with large values in the spectral index map, and also give rise to outstanding features in the Faraday Rotation Measure (RM) map. Since the conditions of flipping depend on the magnetic field strength and configuration and the energy distribution of electrons, we can expect that the polarization flips will provide us with an observational evidence for the presence of non-thermal electrons around the black hole, and a clue to the magnetically driving mechanism of plasma jets.