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

Abstract

Non-VLBI measurements of Faraday rotation at mm wavelengths have been used to constrain mass accretion rates (Ṁ ) 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 holeṀ increases by a factor of ≃ 100. 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 increasingṀ 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, withṀ much larger than the previously inferred limit of 9 × 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.

Modeling Polarized Emission from Black Hole Jets: Application to M87 Core Jet

“…We show the model in polarized light at three different wavelengths (λ= 7.0, 3.5 and 1.3 mm) at which the source is often observed [35][36][37]. The 1.3 mm synthetic images have been published in [16,18]. Here, we have extended our previous results to the other wavelengths.…”

“…In combination with radiative transfer model, this finally makes our numerical simulations of accretion flows resemble observations of underluminous accreting black holes [13,14,16,18,31].…”

“…The electron temperatures, T e , are computed assuming that the proton-to-electron coupling depends on plasma magnetization [14,16,18]:…”

“…Bottom panels: maps of the circular polarization degree. The 1.3 mm image in the topright panel has been published in [16,18].…”

“…Until recently, the least understood part of these models was a proper treatment of radiating electrons. Finally, however, this issue has been addressed [13][14][15][16][17][18][19]. With new and more detailed observations, we are beginning to directly compare the numerical simulations of accretion flows to real astronomical systems such as Galactic center Sgr A* and the core of M87 galaxy [20].…”