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Gamma-ray emission from the Galactic center shows a significant break, pointing to an energy limit for cosmic-ray protons.
At the center of the Milky Way, where the supermassive black hole Sagittarius A* churns gas clouds like a millstone, astronomers have caught an irregular pulse. Gamma-ray emission, the most energetic light in the universe, arrives not as a smooth wave but with a break, as if the Galaxy's heart beats with arrhythmia. For sixteen years, four telescopes in the Namibian desert have been capturing these flashes, accumulating 365 hours of clean signal. The result is a map that shows not only bright points but also a faint ridge: a band of glow along the galactic plane.
The ridge is not a separate object but the combined glow of gas bombarded by cosmic rays, charged particles accelerated to enormous speeds.
To understand where this light comes from, scientists built a three-dimensional model: where the gas lies in space, where the particles travel, and how they collide. It turned out that the density of cosmic rays falls from the center according to a simple law: inversely proportional to distance. But in the spectrum, that is, in the distribution of gamma-ray energies, a break appeared: at energies around 10–20 TeV, the emission sharply weakens. It is as if the Galaxy's pulse suddenly began to fade.
Such a break points to an energy limit for the parent protons, roughly 15–50 TeV. Above this mark, the accelerator at the Galactic center likely cannot boost particles any further. The injection source has been localized to within about 20 parsecs of the black hole, which is less than the distance from the Sun to the nearest star. The Arches and Quintuplet star clusters, previously considered prime candidates, are ruled out: they are too far away.
The pulsar wind nebula G0.9+0.1 has for the first time shown a spectral cutoff at an energy of about 5 TeV, another sign of the limits nature imposes on particle accelerators.
Previously, astronomers could not separate the ridge emission from background sources, leaving the nature of the accelerator an open question. Now, thanks to the three-dimensional approach, the true spectrum of the ridge has been isolated for the first time. This allows a rigorous test of the hypothesis of continuous particle injection from the center. The Galaxy's heart beats irregularly, but now we know exactly where its rhythm falters.
The central molecular zone contains about 10% of the Galaxy's total molecular gas while occupying only 1% of its volume, making it the densest gas reservoir in the Milky Way.