Scientists unveil first spectral map of a black hole's event horizon

A team of Chinese and international researchers has produced the world's first spatially resolved spectral index map of a black hole at the scale of its event horizon, offering new insights into the extreme physical environment surrounding black holes.

The research, published on Monday in The Astrophysical Journal Letters, was led by researchers from the Shanghai Astronomical Observatory under the Chinese Academy of Sciences in collaboration with international partners.

The team analyzed observations of the supermassive black hole M87 using data collected in 2018 by the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array. By combining images captured at wavelengths of 1.3 millimeters and 3.5 millimeters, the researchers conducted the first dual-frequency spectral study at the event-horizon scale.

The study produced the first spatially resolved map of the black hole's spectral index, which describes how the brightness of radiation changes across different frequencies. Researchers likened the achievement to conducting a high-precision "spectral health check" of a black hole.

The results show that the radiation properties around the black hole change significantly with distance from its center. Near the event horizon, the spectral index is positive, indicating that radiation remains strongly affected by synchrotron self-absorption. Farther away, the spectral index gradually decreases and becomes negative, suggesting the radiation transitions to an optically thin state.

The transition occurs about 30 microarcseconds from the black hole's center, matching the size of the ring structure observed at the 3.5-millimeter wavelength. The finding suggests the ring is not only a visible feature of the black hole image but also reflects the radiation state of plasma near the event horizon.

Researchers said future observations at additional frequencies could enable more precise measurements of plasma conditions around black holes and help track the evolution of accretion flows and jets over time, advancing black hole studies from static imaging to dynamic physical diagnostics.