The first-ever image of a black hole—a glowing, ring-like structure in the heart of galaxy M87—was unveiled to the world in a landmark moment in 2019. Now, researchers have taken the next big step—by moving beyond capturing a picture of the black hole to understanding its physics through dual-frequency images.

The research was conducted by scientists at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS), together with international collaborators, who combined horizon-scale images obtained in 2018 from the Event Horizon Telescope and the Global Millimeter Very Long Baseline Interferometry (VLBI) Array at two frequencies—1.3 mm and 3.5 mm.

By carrying out the first dual-frequency spectral study of its kind, the researchers produced a spatially resolved spectral-index map on event-horizon scales and revealed how the spectral index changes with distance from the black hole. This allowed them to determine the physical conditions of the plasma around the black hole and the processes that generate the observed radiation.

The study was published in The Astrophysical Journal Letters.

The results show that the radiation properties surrounding the black hole vary systematically with distance, as revealed by the spatial distribution of the spectral index. In the innermost region, the spectral index is positive and increases slightly with radius, suggesting that the emission remains significantly affected by synchrotron self-absorption. Farther from the black hole, the spectral index decreases and changes from positive to negative values, indicating a transition toward a more optically thin emission regime.

Remarkably, this transition occurs at a distance of about 30 μas from the black hole, consistent with the radius of the ring-like structure observed at 3.5 mm. This result suggests that the ring-like structure seen in black hole images is not merely a feature of the emission morphology, but is closely connected to the physical state of the plasma near the event horizon.

"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 the 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."

According to the researchers, continued advances in millimeter VLBI will enable observations at more frequencies, with higher sensitivity and time-resolved imaging capabilities. These improvements will provide much richer information about black hole accretion, jet formation, and radiation processes in strong gravitational fields, further deepening our understanding of the extreme environments surrounding black holes.

Dr. LU Rusen, a researcher at SHAO and the corresponding author of the study, noted that multi-frequency horizon-scale imaging will enable more precise studies of black hole accretion, jet formation, and strong-field gravity by disentangling the effects of plasma physics from gravitational signatures in black hole images.

This study was funded by the National Natural Science Foundation of China, China's National Major Science and Technology Projects, CAS, and the Shanghai Municipal Government.

Spatially resolved spectral-index map of the M87 black hole on event-horizon scales. (Image by SHAO)