General Relativistic Magnetohydrodynamic Simulations of Sag-A

https://phys.org/news/2020-06-insight-behavior-black-hole-center.html

Innovative model provides insight into the behavior of the black hole at the center of our galaxy
by Harrison Tasoff, University of California - Santa Barbara

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Sagittarius A*. This image was taken with NASA's Chandra X-Ray Observatory. Credit: Public domain

Like most galaxies, the Milky Way hosts a supermassive black hole at its center. Called Sagittarius A*, the object has captured astronomers' curiosity for decades. And now there is an effort to image it directly.

Catching a good photo of the celestial beast will require a better understanding of what's going on around it, which has proved challenging due to the vastly different scales involved. "That's the biggest thing we had to overcome," said Sean Ressler, a postdoctoral researcher at UC Santa Barbara's Kavli Institute for Theoretical Physics (KITP), who just published a paper in the Astrophysical Journal Letters, investigating the magnetic properties of the accretion disk surrounding Sagittarius A*.

In the study, Ressler, fellow KITP postdoc Chris White and their colleagues, Eliot Quataert of UC Berkeley and James Stone at the Institute for Advanced Study, sought to determine whether the black hole's magnetic field, which is generated by in-falling matter, can build up to the point where it briefly chokes off this flow, a condition scientists call magnetically arrested. Answering this would require simulating the system all the way out to the closest orbiting stars.
more at link....................

the paper:
https://iopscience.iop.org/article/10.3847/2041-8213/ab9532

Ab Initio Horizon-scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds:

Abstract
We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the ~30 Wolf–Rayet (WR) stellar winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a ~10−8 M ⊙ yr−1 accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR stellar winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.