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We explore the plasma matter content in the innermost accretion disk/jet inM87* as relevant for an enthusiastic search for the signatures of anti-matterin the next generation of the Event Horizon Telescope (ngEHT). We model theimpact of non-zero positron-to-electron ratio using different emission modelsincluding a constant electron to magnetic pressure (constant $\beta_e$ model)with a population of non-thermal electrons as well as a R-beta model populatedwith thermal electrons. In the former case, we pick a semi-analytic fit to theforce-free region of a general relativistic magnetohydrodynamic (GRMHD)simulation, while in the latter case, we analyze the GRMHD simulationsdirectly. In both cases, positrons are being added at the post-processinglevel. We generate polarized images and spectra for some of these models andfind out that at the radio frequencies, both of the linear and the circularpolarizations get enhanced per adding pairs. On the contrary, we show that athigher frequencies a substantial positron fraction washes out the circularpolarization. We report strong degeneracies between different emission modelsand the positron fraction, though our non-thermal models show moresensitivities to the pair fraction than the thermal models. We conclude that alarge theoretical image library is indeed required to fully understand thetrends probed in this study, and to place them in the context of large set ofparameters which also affect polarimetric images, such as magnetic fieldstrength, black hole spin, and detailed aspects of the electron temperature andthe distribution function.

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