Thanks to advances in millimeter very long baseline interferometry (VLBI), astronomers have obtained horizon-scale images of the supermassive black holes at the centers of M87 and the Milky Way. These groundbreaking observations have opened a new window into the extreme environment surrounding black holes, allowing scientists to probe accretion flows, relativistic jets, and high-energy radiation processes on scales close to the event horizon — the boundary beyond which nothing, not even light, can escape.

However, images obtained at a single observing frequency (a single "color") mainly show what a black hole looks like. Combining observations at different frequencies allows astronomers to determine the physical conditions of the plasma around the black hole and the processes that generate the observed radiation, moving the research from merely seeing a black hole to understanding how it works.

Recently, researchers from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, together with international collaborators, carried out the first spatially resolved dual-frequency spectral study of the M87 black hole using observations obtained in 2018 with the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA). By combining horizon-scale images obtained at 1.3 mm and 3.5 mm, the team produced the first spatially resolved spectral-index map on event-horizon scales and revealed how the spectral index changes with distance from the black hole (Figure 1). This work provides a new way to characterize the physical conditions of the plasma surrounding a black hole, going beyond imaging the structure alone. The results have been published in The Astrophysical Journal Letters.

The study shows that the radiation properties surrounding the black hole vary systematically with distance from the black hole, 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. The observations provide new constraints on models of black hole accretion flows, relativistic jets, and horizon-scale radiation processes.

“By obtaining the first spatially resolved spectral-index distribution of M87 black hole, we can quantitatively characterize how the radiation properties change across the region surrounding the black hole,” said Dr. Shan-Shan Zhao, an assistant researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and the first author of the paper. “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.”

Looking ahead, 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 for studying black hole accretion, jet formation, and radiation processes in strong gravitational fields, further deepening our understanding of the extreme physical environments surrounding black holes.

“Multi-frequency horizon-scale imaging will become a powerful tool for diagnosing the physical conditions of plasma near black holes,” said Dr. Ru-Sen Lu, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and the corresponding author of the paper, the corresponding author of the study. “With improved observational capabilities, multi-frequency observations will help disentangle the effects of plasma physics from gravitational signatures in black hole images, enabling more precise studies of black hole accretion, jet formation, and strong-field gravity.”

This study received funding support from the National Natural Science Foundation of China, China’s National Science and Technology Major Project, the Chinese Academy of Sciences, and the Shanghai Municipal Government.

Figure 1. Spatially resolved spectral-index map of the M87 black hole on event-horizon scales (above) and the radial spectral-index profile as a function of distance from the black hole (below).

DOI: https://doi.org/10.3847/2041-8213/ae84ca

Contact:

Dr. Shan-Shan Zhao; email: zhaoss@shao.ac.cn
Dr. Ru-Sen Lu; email: rslu@shao.ac.cn