Shanghai Astronomical Observatory Maps Plasma Behavior Near M87 Black Hole

In a milestone for high-resolution space observation, a team led by the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS) has transitioned from simply capturing images of a black hole to actively diagnosing its physical environment. The M87 black hole, located 55 million light-years away at the center of the M87 galaxy in the Virgo constellation, weighs 6.5 billion times the mass of our Sun. By combining international observational data from global telescope networks, including the Event Horizon Telescope and the Global Millimeter Very Long Baseline Interferometry (VLBI) Array, researchers have mapped the spectral index of the black hole at two distinct frequencies: 1.3 mm and 3.5 mm.

Mapping Plasma Behavior Near the Event Horizon

The research, reported by Xinhua, reveals a clear gradient in how radiation behaves as it moves away from the black hole’s center. In the innermost region, the spectral index is positive and rises slightly with distance, a signature that the emission remains significantly affected by synchrotron self-absorption. As the distance increases, the index decreases and shifts from positive to negative values, signaling a transition toward a more optically thin emission regime.

This structural shift occurs at approximately 30 microarcseconds from the center of the black hole, a distance that matches the radius of the ring-like structure identified in previous 3.5-mm observations. According to researchers at the Chinese Academy of Sciences, this correlation confirms that the ring is not merely a feature of the emission morphology, but is fundamentally linked to the physical state of the plasma near the event horizon. This allows scientists to determine the physical conditions of the plasma around the black hole and the processes that generate the observed radiation.

Dr. ZHAO Shanshan, an assistant researcher at SHAO and the first author of the study, emphasized the significance of this achievement. “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. 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,” said Zhao.

For more on this story, see M87* Black Hole Study Maps Plasma Physics Using Dual-Frequency Images.

Advancing Beyond Static Imaging

The project represents a significant evolution in astrophysical techniques. While the 2019 capture of M87’s image made history as humanity’s first-ever image of a black hole, scientists noted that single-frequency images were limited in their ability to explain the underlying mechanics of the black hole environment. By integrating dual-frequency data, the team can now disentangle plasma physics from gravitational signatures.

Lu Rusen, a researcher at the SHAO, emphasized the broader utility of this approach. 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. He added that while previous single-frequency images could only show spatial structures, the combination of different frequencies allows scientists to decode the underlying physical properties and reveal the state of the plasma. This method pushes black hole research from static imaging to dynamic physical diagnosis.

Future Prospects for Millimeter VLBI Technology

The success of this dual-frequency study has provided a framework for future investigations into the extreme conditions near the event horizon. By analyzing the transition from optically thick to optically thin regimes, researchers have established that the ring-like structure is closely connected to the radiation state of nearby plasma. This discovery provides new insights into black hole accretion and jets, which remain central questions in modern astrophysics.

The study, which involved international collaborators and utilized data obtained in 2018, demonstrates the efficacy of using global telescope networks to conduct dual-frequency joint analysis. As researchers continue to refine these techniques, the ability to map spectral indices at varying scales will likely provide a deeper, more granular understanding of the processes that power these massive celestial objects. The findings confirm that the M87 black hole continues to serve as a vital laboratory for testing physics at the edge of the observable universe.