An international team led by researchers at the Shanghai Astronomical Observatory, part of the Chinese Academy of Sciences, has produced the first dual-frequency spectral study of the M87* black hole on event-horizon scales, combining 1.3-millimeter observations from the Event Horizon Telescope with 3.5-millimeter data from the Global Millimeter VLBI Array, both gathered in 2018.

The resulting spatially resolved spectral-index map shows the spectral index is positive close to the black hole, consistent with synchrotron self-absorption dominating there, before transitioning to negative, an optically thin regime, at roughly 30 microarcseconds out, matching the radius of the ring seen at 3.5 millimeters. That match suggests the ring structure researchers observe reflects the actual physical state of the surrounding plasma, not merely the geometry of its emission.

“By obtaining the first spatially resolved spectral-index distribution of the M87 black hole, we can quantitatively characterize how the radiation properties change across the region surrounding the black hole… This allows us to directly explore how the plasma properties vary on horizon scales and provides new clues for understanding accretion flows and jet formation,” said first author Dr. Zhao Shanshan of SHAO.

Corresponding author Dr. Lu Rusen, also of SHAO, said multi-frequency horizon-scale imaging “will enable more precise studies of black hole accretion, jet formation, and strong-field gravity.”

Journal: The Astrophysical Journal Letters
DOI: 10.3847/2041-8213/ae84ca
Article Title: Spatially resolved spectral properties of M87* on event horizon scales
Publication Date: 20-Jul-2026
Funding: National Natural Science Foundation of China, China’s National Major Science and Technology Projects, Chinese Academy of Sciences, Shanghai Municipal Government

Source: EurekAlert