Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A*

TL;DR

This study investigates how varying positron-to-electron ratios in plasma affect polarized emission from M87* and Sgr A*, revealing that positron content influences polarization signatures and can be constrained by EHT observations.

Contribution

It introduces models incorporating positron fractions into jet and accretion flow simulations, analyzing their impact on polarization and comparing with EHT data to constrain pair content.

Findings

Higher positron fractions decrease infrared circular polarization.

Increased positrons enhance polarization in sub-millimeter images due to Faraday effects.

A ~10% positron fraction best fits M87* broadband polarization data.

Abstract

We study the effects of including a nonzero positron-to-electron fraction in emitting plasma on the polarized SEDs and sub-millimeter images of jet and accretion flow models for near-horizon emission from M87* and Sgr A*. For M87*, we consider a semi-analytic fit to the force-free plasma regions of a general relativistic magnetohydrodynamic jet simulation which we populate with power-law leptons with a constant electron-to-magnetic pressure ratio. For Sgr A*, we consider a standard self-similar radiatively inefficient accretion flow where the emission is predominantly from thermal leptons with a small fraction in a power-law tail. In both models, we fix the positron-to-electron ratio throughout the emission region. We generate polarized images and spectra from our models using the general-relativistic ray tracing and radiative transfer from GRTRANS. We find that a substantial positron fraction reduces the circular polarization fraction at infrared and higher frequencies. However, in sub-millimeter images higher positron fractions increase polarization fractions due to strong effects of Faraday conversion. We find a M87* jet model that best matches the available broadband total intensity and 230 GHz polarization data is a sub-equipartition, with positron fraction of $\simeq$ 10%. We show that jet models with significant positron fractions do not satisfy the polarimetric constraints at 230 GHz from the Event Horizon Telescope (EHT). Sgr A* models show similar trends in their polarization fractions with increasing pair fraction. Both models suggest that resolved, polarized EHT images are useful to constrain the presence of pairs at 230 GHz emitting regions of M87* and Sgr A*.