Physical Origin of the Variability Discrepancy Between GRMHD Simulations and the Observed 230 GHz Emission of Sagittarius A*

The first Event Horizon Telescope (EHT) observations of Sagittarius A* (April 2017, 230 GHz) revealed a 51.8 +/- 2.3 microarcsecond emission ring consistent with a Kerr black hole, yet the general relativistic magnetohydrodynamic (GRMHD) simulation suites used to interpret the images are systematically more variable than the observations. Follow-up work through 2026 has partially mapped this discrepancy: realistic wind-fed outer boundary conditions and two-temperature electron thermodynamics with radiative cooling each partially reconcile the variability statistics in isolated studies, but no self-consistent model class yet reproduces the observed variability together with the full image, spectral, and polarimetric constraint set, and attribution remains contested across simulation setups. The precise open core is to identify which physical ingredients -- collisionless kinetic effects, electron heating prescriptions, radiative cooling, non-thermal particle populations, wind feeding, alone or in combination -- damp the modeled variability to the observed level, thereby placing horizon-scale model-based tests of the Kerr spacetime for the Galactic-center black hole on fully consistent footing.

Background

Sagittarius A* (Sgr A*) is the $4\times10^6\,M_\odot$ supermassive black hole at the center of the Milky Way and the nearest object of its kind. In April 2017 the Event Horizon Telescope (EHT), a global very long baseline interferometry (VLBI) array of eight radio telescopes operating at a wavelength of 1.3 mm (frequency 230 GHz), resolved its emission on event-horizon scales, finding a thick, bright ring of diameter $51.8\pm2.3$ microarcseconds whose size matches the shadow (the dark central depression in the image produced by strongly lensed photon orbits near the horizon) predicted by general relativity for a Kerr black hole -- the unique stationary rotating black-hole solution of Einstein's equations -- of the mass inferred from stellar orbits (EHT Collaboration 2022, Paper I).

The observed emission is synchrotron radiation from hot plasma in a radiatively inefficient accretion flow (RIAF), a magnetized, quasi-spherical inflow in which the turbulent heat released by accretion is not efficiently radiated away. Because the EHT observes the electrons while the dynamics are governed mainly by the ions, theoretical interpretation relies on suites of general relativistic magnetohydrodynamic (GRMHD) simulations -- fluid simulations of magnetized plasma in curved spacetime, treating the plasma as a single conducting gas with a heuristic electron-temperature prescription parametrized by $R_{\rm high}$, the ion-to-electron temperature ratio in weakly magnetized regions -- followed by relativistic ray tracing to produce synthetic images and light curves. The suites span two magnetic states: magnetically arrested disks (MADs), in which accumulated vertical magnetic field throttles the accretion, and standard-and-normal-evolution (SANE) disks, in which the field remains dynamically sub-dominant.

Paper I and the companion model-testing paper (Paper V) compared these suites against the 2017 data and found that, although the models reproduce the ring morphology and broadband spectrum remarkably well, the simulations 'tend to be more variable than the observations': the fractional variability of the 230 GHz flux is approximately 9% across the campaign, with variations of 4-13% within individual nights, and although the structural variability reaches order unity on some interferometric baselines, it is lower than most simulations produce. Only a couple of MAD configurations satisfy every observational constraint except variability. The papers propose candidate missing physics: collisionless (kinetic) effects, since the true plasma mean free path exceeds macroscopic scales so a single-fluid treatment may fail; improved electron heating prescriptions; radiative cooling; non-thermal particle populations; and more realistic outer boundary feeding by winds from the surrounding Wolf-Rayet stars (the two wind-fed models in the suite were too sparsely sampled to judge). The same variability hampers time-resolved ('movie') reconstruction of horizon-scale structures, since the April 7 dynamic reconstructions depend strongly on prior assumptions (Paper III).

Resolving the discrepancy matters because the EHT's spacetime inferences -- the shadow diameter $d_{\rm sh}$, the angular gravitational radius $\theta_{\rm g}$, and limits on metric deviations $\delta$ -- are calibrated against these same model suites; the variability excess is their dominant failure mode.

Problem Statement

Identify the physical origin of the discrepancy between the 230 GHz variability produced by state-of-the-art suites of general relativistic magnetohydrodynamic (GRMHD) simulations -- fluid simulations of magnetized plasma in the curved spacetime of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way -- and the comparatively lower variability observed at 230 GHz by the Event Horizon Telescope (EHT), the global very long baseline interferometry array that observed Sgr A* in April 2017. Specifically, determine which physical ingredients -- collisionless (kinetic) plasma effects, electron heating prescriptions, radiative cooling, non-thermal particle populations, and realistic wind-fed outer boundary conditions, alone or in combination -- reconcile the simulated light-curve variability (fractional RMS variability on intranight timescales of order hours, and the variability structure function, i.e., the RMS fluctuation amplitude as a function of time lag) and horizon-scale structural variability with the observed statistics, within model classes that simultaneously satisfy the full set of EHT and multiwavelength constraints (ring diameter and morphology, broadband spectrum, polarization). The physical origin sought is causal rather than parametric tuning: the modeled variability must be brought to the observed level -- a campaign-level fractional RMS of about 9% with intranight variations inside the measured 4-13% band, and a variability structure function matching the observed one -- and the reduction must be traced to an identified physical mechanism, not to re-tuning the phenomenological electron-temperature prescription (e.g. the ion-to-electron ratio $R_{\rm high}$) alone. The reconciled model class must satisfy the ring-diameter, morphological, broadband-spectrum, and polarization constraints jointly -- not merely inherit them from a model that remains over-variable -- and it must establish which ingredient damps or modifies the modeled variability and why, so that model-based shadow and spacetime constraints on Sgr A* and physically grounded time-resolved imaging of its horizon-scale plasma can be placed on consistent footing.

References

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