Probing plasma composition with the next generation Event Horizon Telescope (ngEHT)

TL;DR

This study investigates how the plasma composition, especially positron-electron ratios, affects polarimetric images of M87* at radio frequencies, aiding future anti-matter signatures detection with ngEHT.

Contribution

It models the impact of positron fractions on polarized emissions in M87* using various emission models and GRMHD simulations, highlighting sensitivities and degeneracies.

Findings

Polarization is enhanced at radio frequencies with increased positron content.

Higher frequencies show circular polarization suppression with more positrons.

Non-thermal models are more sensitive to pair fractions than thermal models.

Abstract

We explore the plasma matter content in the innermost accretion disk/jet in M87* as relevant for an enthusiastic search for the signatures of anti-matter in the next generation of the Event Horizon Telescope (ngEHT). We model the impact of non-zero positron-to-electron ratio using different emission models including a constant electron to magnetic pressure (constant $\beta_e$ model) with a population of non-thermal electrons as well as a R-beta model populated with thermal electrons. In the former case, we pick a semi-analytic fit to the force-free region of a general relativistic magnetohydrodynamic (GRMHD) simulation, while in the latter case, we analyze the GRMHD simulations directly. In both cases, positrons are being added at the post-processing level. We generate polarized images and spectra for some of these models and find out that at the radio frequencies, both of the linear and the circular polarizations get enhanced per adding pairs. On the contrary, we show that at higher frequencies a substantial positron fraction washes out the circular polarization. We report strong degeneracies between different emission models and the positron fraction, though our non-thermal models show more sensitivities to the pair fraction than the thermal models. We conclude that a large theoretical image library is indeed required to fully understand the trends probed in this study, and to place them in the context of large set of parameters which also affect polarimetric images, such as magnetic field strength, black hole spin, and detailed aspects of the electron temperature and the distribution function.