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 spectral energy distributions and submillimeter 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 IR and higher frequencies. However, in submillimeter images, higher positron fractions increase polarization fractions due to strong effects of Faraday conversion. We find an M87* jet model that best matches the available broadband total intensity, and 230 GHz polarization data is a sub-equipartition, with positron fraction of similar or equal to 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*.

Automated summary

The study shows that including a higher positron fraction in emitting plasma near the horizon of M87* and Sgr A* can affect polarized images and spectra, with higher positron fractions increasing polarization fractions in submillimeter images but reducing circular polarization fractions at infrared and higher frequencies. Jet models with significant positron fractions do not meet polarimetric constraints at 230 GHz from the Event Horizon Telescope (EHT), and resolved, polarized EHT images are useful for constraining the presence of pairs in the emitting regions of M87* and Sgr A*.