The Event Horizon Telescope has already measured the size and shape of the emission around M87*, but the black hole's spin and the properties of the plasma near the horizon remain uncertain. The main difficulty is that the electron temperature depends on microphysics that global simulations do not capture. Work on the shadow, the ring, and polarization has crowded around this problem, yet the spectral index as a diagnostic tool has rarely been used. This work stands apart because it systematically studies the variability of spectral maps, not just their average values.
The authors took a set of three-dimensional simulations of magnetically arrested accretion disks from the EBHLIGHT code for five spin values: -0.9375, -0.5, 0, 0.5, 0.9375. Each simulation contained 500 snapshots with a step of 10M, which yielded 240,000 images after post-processing. The emission was computed with the IPOLE code using ray tracing along geodesics, taking into account synchrotron emission, absorption, and Faraday rotation. Electrons were described either by a thermal Maxwell-Jüttner distribution or by a non-thermal kappa distribution with a fixed index of 4.25. The electron temperature was set through the ratio to the proton temperature with the parameters $R_{low}$ and $R_{high}$. For each pair of frequencies, spectral index maps were constructed and their mean and standard deviation were calculated.
Thermal models with $R_{low}$=10 show significantly stronger variability of the spectral index than models with $R_{low}$=1, regardless of the frequency range. Non-thermal models with a kappa distribution demonstrate even weaker variability because of the contribution of the power-law tail with a fixed slope. The photon ring stands out with a higher spectral index, often near zero or positive, while the outer regions go into negative values down to -3 in the thermal models. Comparison with the measured integrated spectral index of -0.30 (+0.56/-0.11) between 86 and 230 GHz rules out some models, for example thermal ones with (a*=0.9375, $R_{low}$=10, $R_{high}$=10 or 40) because of their too compact size.
The work differs from neighboring studies in that it is the first to systematically link the variability of the spectral index to the parameters of the electron distribution. While others focused on the shape of the shadow or on polarization, here it is shown that future EHT movies will be able to distinguish thermal from non-thermal models by the amplitude of fluctuations in the spectral slope. This opens a new channel for testing the microphysics of accretion flows that has not been used before.
The topic could develop toward including anisotropic electron distributions, a variable kappa index, and more realistic heating models. The next step will be to compare the predictions with real data from the EHT 2026 movie campaign, when the first time series of images at several frequencies become available. This will make it possible to place direct constraints on the spin and the plasma parameters.
The results will affect the interpretation of Event Horizon Telescope observations and future multi-frequency campaigns, as well as the modeling of accretion disks in other active galactic nuclei.
It is necessary to carry out a comparison with real data from the 2026 campaign and to expand the model grid by including anisotropic distributions and a variable kappa index.
The work is directly connected to the unsolved problem of determining the black hole's spin and the mechanism of electron heating in accretion flows. It shows that the variability of the spectral index can become a new tool for distinguishing models, which until now has been a weak spot in black hole astrophysics.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The work models spectral maps and their temporal variability for M87* within the framework of magnetically arrested disks, covering five spin values, thermal and non-thermal electron distributions, and frequencies from 86 to 345 GHz. The authors use general relativity, magnetohydrodynamics, and radiative transfer, showing that thermal models with cold electrons produce stronger variability in the spectral index, while non-thermal models are more stable. Comparison with 2018 observations rules out some parameters and provides predictions for the future EHT cinema campaign.
Where its strength is
A systematic study of spectral index variability as a diagnostic of the electron distribution, opening a new channel for testing the microphysics of accretion flows.
Where it could go next
Worth trying
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.55, against a usual 0.64. It looks like few have drilled nearby.
The photon ring of M87* has a spectral index near zero, while the outer regions can reach -3, which means a sharp change in the emission properties on a scale of just a few gravitational radii.