Scientists produce first spectral map of a black hole’s event horizon

Researchers have moved beyond photographing a black hole to diagnosing its physical properties, producing the first spatially resolved spectral-index map of the supermassive black hole M87* at the scale of its event horizon. The study, published Monday in The Astrophysical Journal Letters, offers a new window into how plasma behaves in one of the most extreme environments in the universe.eurekalert+2

A "Spectral Health Check" for a Black Hole

The team, led by scientists at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, combined images taken in 2018 by the Event Horizon Telescope and the Global Millimeter Very Long Baseline Interferometry Array at two radio wavelengths — 1.3 millimeters and 3.5 millimeters. By mapping how the brightness of radiation changes across frequencies at each point around the black hole, the researchers created what they described as a high-precision "spectral health check" of M87*.english.cas+2

The results reveal that radiation properties change systematically with distance from the black hole. Close to the event horizon, the spectral index is positive, indicating that synchrotron self-absorption still dominates the emission. Farther out, the index turns negative, marking a transition to optically thin radiation. That transition occurs at roughly 30 microarcseconds from the center — a distance that matches the ring-like structure seen in the 3.5-millimeter image.eurekalert+1

Ring Structure Tied to Plasma Physics

The finding suggests the iconic ring in black hole images is not simply a geometric feature but reflects a real change in the physical state of the surrounding plasma. "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," said Dr. Zhao Shanshan, an assistant researcher at SHAO and first author of the study. "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."english.cas+1

Looking Ahead

Dr. Lu Rusen, the study's corresponding author, said that future multi-frequency horizon-scale imaging will help disentangle the effects of plasma physics from gravitational signatures in black hole images, enabling more precise studies of accretion, jet formation, and strong-field gravity. The researchers noted that continued advances in millimeter VLBI could soon allow time-resolved observations, shifting black hole studies from static imaging toward dynamic physical diagnostics.news.cgtn+2