New Spectral Map Reveals Extreme Physics of the M87 Black Hole

• Researchers have produced the first spatially resolved dual-frequency spectral map revealing how radiation properties of the M87 black hole change with distance.
• Observations confirm that the inner region is dominated by synchrotron self-absorption, while a transition to freely escaping photons occurs at approximately 30 microarcseconds (μas).
• The findings demonstrate that the ring-like structure observed by the EHT in 2019 is intrinsically linked to the physical state of plasma near the event horizon.

Research Methodology

• An international team, led by the Shanghai Astronomical Observatory (SAO), combined data from the 2018 Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA).
• Observations were conducted at two frequencies—1.3 mm and 3.5 mm—leveraging advances in millimeter very long baseline interferometry (VLBI).

Key Findings

• Spectral index measurements show distinct zones: the innermost region exhibits a positive index, indicating significant synchrotron self-absorption where electrons reabsorb previously emitted photons.
• A transition to a negative spectral index occurs at 30 μas, signifying a regime where photons escape without re-absorption or scattering.
• This boundary coincides with the ring-like structure identified in previous imaging, confirming it represents a physical plasma state rather than just morphology.

Implications and Future Outlook

• This technique provides a novel method for characterizing extreme physics beyond simple spatial imaging, offering insights into accretion flows and jet formation.
• Ongoing improvements in millimeter VLBI will allow for multi-frequency observations with higher sensitivity and time resolution.
• Future studies aim to disentangle plasma physics from gravitational signatures, leading to more precise models of strong-field gravity and radiation processes.