# Faraday rotation in GRMHD simulations of the jet launching zone of M87

## TL;DR

This study models Faraday rotation in M87 using GRMHD simulations, revealing how jet and accretion disk properties influence observed polarization and rotation measures, and constraining the jet's viewing angle.

## Contribution

It introduces comprehensive GRMHD models that incorporate polarized radiative transfer to explain M87's Faraday rotation and polarization, linking jet dynamics with observational data.

## Key findings

- Low RM at small viewing angles despite increased accretion rates.
- Polarization mainly produced in the forward jet, with the disk causing depolarization.
- Models suggest higher RMs at larger inclination angles, constraining jet orientation.

## Abstract

Non-VLBI measurements of Faraday rotation at mm wavelengths have been used to constrain mass accretion rates ($\mdot$) onto supermassive black holes in the centre of the Milky Way and in the centre of M87. We constructed general relativistic magnetohydrodynamics models for these sources that qualitatively well describe their spectra and radio/mm images invoking a coupled jet-disk system. Using general relativistic polarized radiative transfer, we now also model the observed mm rotation measure (RM) of M87. The models are tied to the observed radio flux, however, electron temperature and accretion rate are degenerate parameters and are allowed to vary. For the inferred low viewing angles of the M87 jet, the RM is low even as the black hole $\mdot$ increases by a factor of $\simeq100$. In jet-dominated models, the observed linear polarization is produced in the forward-jet, while the dense accretion disk depolarizes the bulk of the near-horizon scale emission which originates in the counter-jet. In the jet-dominated models, with increasing $\mdot$ and increasing Faraday optical depth one is progressively sensitive only to polarized emission in the forward-jet, keeping the measured RM relatively constant. The jet-dominated model reproduces a low net-polarization of $\simeq1$ per cent and RMs in agreement with observed values due to Faraday depolarization, however, with $\mdot$ much larger than the previously inferred limit of $9\times10^{-4}\,\mdotu$. All jet-dominated models produce much higher RMs for inclination angles $i\gtrsim30^\circ$, where the line-of-sight passes through the accretion flow, thereby providing independent constraints on the viewing geometry of the M87 jet.