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