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

We consider the effects of including a nonzero positron-to-electron fraction in the emitting plasma in semi-analytic models for M87 and Sgr A* on the polarized near-horizon submillimeter images observed by the Event Horizon Telescope (EHT). Our model for M87 is a semi-analytic fit to the force-free regions of a GRMHD jet simulation, and our model for Sgr A* is a standard radiatively inefficient accretion flow (RIAF). For the RIAF disk, we include emission from electrons and positrons in a dominant thermal population with a small non-thermal tail. For the semi-analytic jet model, we include emission from a nonthermal electron and positron distribution and apply two classes of parametric emission models: a model with constant electron-to-magnetic pressure ratio, and a multi-zone model where synchrotron emission is exponentially suppressed above a critical plasma beta. In all of our models, the positron-to-electron ratio is fixed throughout the emission region. We generate polarized Stokes images and spectra from our models using general-relativistic ray tracing and radiative transfer. In the parameter space considered, we find that the polarized spectra are sensitive indicators of the plasma positron fraction. A high positron fraction enhances the circular polarization in submillimeter images due to Faraday conversion. The M87 jet model that best matches the spectral data is a sub-equipartition, evenly mixed pair/ionic plasma. Positron disk models for Sgr A* tend to have crescent-shaped intensity and electric vector polarization angle distributions, though they vary greatly in their circular polarization degree patterns and polarized spectra.