Chinese scientists have taken a crucial step from "seeing" to "reading" a black hole, unveiling the first spatially resolved spectral index map of the M87 black hole on the event horizon scale, revealing the gradient of the spectral index with distance from the center.

The study, led by a team from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, was recently published in the Astrophysical Journal Letters on July 20.

Located 55 million light-years away at the center of the M87 galaxy in the Virgo constellation, the M87 black hole, weighing 6.5 billion times the mass of our Sun, made history in 2019 when humanity captured its first-ever image.

By combining international observational data from global telescope networks, the Chinese-led team conducted a dual-frequency joint analysis.

Research reveals that radiation around a black hole changes with distance. Near the center, a positive spectral index indicates that the emission remains significantly affected by synchrotron self-absorption, but becomes optically thin further out. Notably, this transition occurs at about 30 microarcseconds, matching the ring seen in 3.5-mm observations. This proves the ring is closely connected to the radiation state of nearby plasma, offering new insights into black hole accretion and jets.

"The spectral index characterizes the frequency-dependent emission from the black hole environment, providing an important probe of the radiation processes in the accretion flow and jet," said Lu Rusen, a researcher at the SHAO.

Lu added that previous single-frequency images could only show spatial structures. Only by combining observations of different frequencies can scientists decode the underlying physical properties and reveal the state of the plasma, pushing black hole research from static imaging to dynamic physical diagnosis.