The heart of a black hole beats unevenly. One thump is brighter, another dimmer, and this rhythm encodes the temperature of the electrons swirling right at the horizon. Astronomers call this rhythm the spectral index: a number that shows how much brightness changes as you move from one radio frequency to another. For M87*, the black hole at the center of the Virgo galaxy, this number does not sit still.
To hear this pulse, scientists built a virtual telescope. They took computer models of a disk where the magnetic field holds the plasma tightly, and ran them five times: with different spins of the black hole itself. For each variant, they made 500 snapshots, then calculated how each snapshot would look at six frequencies: from 86 to 345 gigahertz. In total, that yielded 240,000 images. On each one, they measured the spectral index at every point, and then watched how it jittered from frame to frame.
The jitter turned out to differ between models. If the electrons in the disk are cold and the protons hot, the spectral index jumps wildly: the roof now stands on end, now lies almost flat. If the electrons are hot, the jitter is weaker. And if the electrons have a "tail" of very fast particles, as in a power-law distribution, the jitter almost freezes: the tail holds the roof's slope, like a guy wire holds a mast.
Particularly interesting is the photon ring, a thin bright loop that light draws around the black hole's shadow after bending around it. There, the spectral index is often near zero or even positive: the higher the frequency, the brighter. Outside the ring, the index goes negative. The transition is sharp, like the boundary between fire and smoke.
Checking against real 2018 observations has already ruled out several models. For example, a rapidly spinning black hole with cold electrons and strong proton heating did not fit: its image came out too small. And ahead lies the 2026 Event Horizon Telescope movie campaign. Then, for the first time, they will film how the black hole changes over time at several frequencies at once. From the amplitude of the spectral index jitter, it will be possible to distinguish thermal from non-thermal models, and thus peer into the microphysics of the plasma right at the horizon.
The black hole's heart beats in every frame. All that remains is to learn how to read the cardiogram.
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.