Semi-analytic studies of accretion disk and magnetic field geometry in M87*

Abstract

Context: Magnetic fields play a pivotal role in dynamics of black hole accretion flows and formation of relativistic jets. Observations by the Event Horizon Telescope (EHT) provided unprecedented insights into accretion structures near black holes. Interpreting these observations requires a theoretical framework linking polarized emission to underlying system properties and magnetic field geometries. Aims: We investigate how system properties, particularly magnetic field geometry in the event horizon scale region, influence the structure of the observable synchrotron emission in M87*. Specifically, we aim to quantify the sensitivity of observables used by the EHT to black hole spin, plasma dynamics, accretion disk thickness, and magnetic field geometry. Methods: We adopt a semi-analytic radiatively inefficient accretion flow model in Kerr spacetime. We vary magnetic field geometry, black hole spin, accretion disk dynamics, and geometric thickness of the disk. We perform general relativistic ray tracing with a full polarized radiative transfer to obtain synthetic images of M87*. We extract EHT observables, such as disk diameter, asymmetry, and polarimetric metrics from synthetic models. We also consider a number of general relativistic magnetohydrodynamics simulations and compare them with the semi-analytical models. Results: We see limited impact of the disk thickness on observables. On the other hand, toroidal field dominated and poloidal field dominated magnetic configurations can be distinguished reliably. The flow dynamics, in particular presence of radial inflow, also significantly impacts the EHT observables. Conclusions: The M87* system is most consistent with a poloidal magnetic field dominated flow with partially radial inflow. While the spin remain elusive, moderate or large positive values are preferred.

12 pages and 8 figures

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