Shanghai Astronomers Map Plasma Radiation Around M87 Black Hole

The 2019 image of the M87 black hole served as a static testament to the power of global telescope networks. Now, a team led by the Shanghai Astronomical Observatory (SHAO) has moved beyond simple imaging, successfully “reading” the physical state of the plasma surrounding the 6.5-billion-solar-mass giant. By analyzing data collected in 2018 from the Event Horizon Telescope and the Global Millimeter Very Long Baseline Interferometry (VLBI) Array, the researchers have decoded how radiation changes with distance from the black hole’s center.

Mapping the Spectral Index of M87

The core of this achievement lies in the dual-frequency joint analysis at 1.3 mm and 3.5 mm. By comparing these two wavelengths, the team created a map of the spectral index—a measurement that reveals the frequency-dependent nature of the light emitted by the surrounding accretion flow.

“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.”

Lu Rusen, a researcher at the SHAO

The results show a clear transition in the plasma’s behavior. In the innermost region, the spectral index remains positive, which researchers note indicates that the emission is still heavily influenced by synchrotron self-absorption. As the distance from the black hole increases, the index shifts from positive to negative, signaling a move into an optically thin regime. This transition occurs at approximately 30 microarcseconds, a distance that aligns with the radius of the ring structure observed in 3.5-mm data.

Decoding Plasma Physics and Jet Formation

This study challenges the idea that the ring-like structure is merely a morphological artifact. Instead, the data suggests the ring is intrinsically tied 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. 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.”

Dr. Zhao Shanshan, assistant researcher at SHAO

By disentangling plasma physics from gravitational signatures, the team is setting the stage for more precise studies of strong-field gravity.

Future Directions for Millimeter VLBI Imaging

The transition from static, single-frequency imaging to dynamic physical diagnosis marks an evolution for the field. The researchers indicate that further advancements in millimeter VLBI will eventually allow for observations at a greater range of frequencies. With higher sensitivity and improved time-resolved imaging, future iterations of this work aim to provide a more comprehensive view of how energy is channeled into jets and how matter is consumed by the black hole.

As the team continues to refine these techniques, the focus remains on pushing beyond the “picture-taking” phase of black hole research. By isolating the effects of plasma, scientists are moving closer to a clearer view of the extreme gravitational environments that define the centers of galaxies.