2017MNRAS.468.2214M

Non-very long baseline interferometry (VLBI) measurements of Faraday rotation at millimetre wavelengths have been used to constrain mass accretion rates ({dot}M) on to 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-disc 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, the 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 {dot}M increases by a factor of ≃100. In jet-dominated models, the observed linear polarization is produced in the forward jet, while the dense accretion disc depolarizes the bulk of the near-horizon scale emission that originates in the counter jet. In the jet-dominated models, with increasing {dot}M 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 ≃1 per cent and RMs in agreement with observed values due to Faraday depolarization, however, with {dot}M much larger than the previously inferred limit of 9 x 10^–4 M_☉ yr^–1. All jet-dominated models produce much higher RMs for inclination angles i >= 30°, where the line of sight passes through the accretion flow, thereby providing independent constraints on the viewing geometry of the M87 jet.