Semi-analytic studies of the accretion disk and magnetic field geometry in M 87*

¹ Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 D-53121 Bonn, Germany
² Instituto de Astrofísica de Andalucía-CSIC Glorieta de la Astronomía s/n E-18008 Granada, Spain
³ Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava CZ-74601 Opava, Czech Republic
⁴ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA

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Received: 15 August 2025
Accepted: 3 November 2025

Abstract

Context. Magnetic fields play a pivotal role in the dynamics of black hole accretion flows and in the 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 that links polarized emission to the underlying system properties and magnetic field geometries.

Aims. We investigated how the system properties, in particular, the magnetic field geometry in the region of the event horizon scale, affect the structure of the observable synchrotron emission in M 87*. Specifically, we characterized the sensitivity of observables used by the EHT to black hole spin, plasma dynamics, accretion disk thickness, and magnetic field geometry.

Methods. We adopted a semi-analytic radiatively inefficient accretion flow model in Kerr spacetime. We varied the magnetic field geometry, black hole spin, accretion disk dynamics, and geometric thickness of the disk. We performed general relativistic ray-tracing with a full polarized radiative transfer to obtain synthetic images of M 87*. We extracted EHT observables, such as disk diameter, asymmetry, and polarimetric metrics from synthetic models. We also considered a number of general relativistic magnetohydrodynamics simulations and compared them with the semi-analytical models.

Results. The effect of the disk thickness on the observables is limited. On the other hand, magnetic configurations dominated by the toroidal and poloidal fields can be distinguished reliably. The flow dynamics, in particular, radial inflow, also significantly affects the EHT observables.

Conclusions. The M 87* system is most consistent with a flow dominated by the poloidal magnetic field with partially radial inflow. While the spin remains elusive, moderate or high positive values are preferred.