The Event Horizon Telescope made history in 2019 by producing the first-ever image of a black hole’s silhouette. Now, a China-led analysis of that same M87 dataset has gone a step further — mapping, for the first time, exactly how the black hole’s radio glow changes character as you move outward from the edge of the event horizon itself. The study, led by researchers at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, was published in The Astrophysical Journal Letters on 21 July 2026.
Reading the black hole’s glow like a spectrum
The famous 2019 image showed M87’s black hole as a bright ring surrounding a dark shadow, captured at a single radio frequency. This new study instead combines Event Horizon Telescope observations taken in 2018 at two different wavelengths — 1.3 millimeters and 3.5 millimeters — together with data from the Global Millimeter Very Long Baseline Interferometry (VLBI) Array. Comparing how brightness changes between the two frequencies at each point in the image produces what astronomers call a spectral index — a number that encodes information about the physical state of the hot, magnetized plasma emitting the radiation. The result is the first spatially resolved spectral-index map of a black hole’s surroundings at event-horizon scales: not just an image of the glow, but a map of what kind of plasma is producing it, point by point.
What the map found
The team reports that the spectral index transitions systematically with distance from the black hole — positive close to the event horizon, flipping to negative roughly 30 microarcseconds farther out — with a ring-like structure at the 3.5-millimeter wavelength marking the transition point between two different plasma states. “This allows us to directly explore how the plasma properties vary on horizon scales,” said Dr. Zhao Shanshan, the study’s first author and an assistant researcher at SHAO. Dr. Lu Rusen, a SHAO researcher, is the paper’s corresponding author. In practical terms, the map lets astronomers distinguish, region by region, between plasma dominated by synchrotron self-absorption near the black hole and optically thin plasma farther out — a distinction that matters directly for testing models of how supermassive black holes feed and launch jets.
A shared global instrument, a China-led result
The Event Horizon Telescope is not a single telescope but a globe-spanning array of radio dishes, from Antarctica to Hawaii to Chile, linked together through very-long-baseline interferometry to function as one Earth-sized virtual telescope — infrastructure built and operated collectively by dozens of institutions across the EHT Collaboration and the separate Global mm-VLBI Array. That shared infrastructure is precisely what let a China-led team, working from Shanghai, extract an entirely new scientific result from a dataset the international collaboration originally captured together in 2018. It is a useful illustration of how big, expensive, internationally shared astronomical infrastructure keeps paying dividends years after the original headline observation, as different groups within and beyond the founding collaboration bring new analysis techniques to the same archived data.
Why a spectral-index map is a methodological first, not just a prettier picture
Single-frequency black hole images, however striking, conflate two things: how bright the emission is, and what physical process produced it. Adding a second frequency and mapping the spectral index across the image separates those two questions — turning a picture of a black hole into a diagnostic tool for the plasma physics happening at its boundary. That is the specific methodological advance this paper claims, distinct from simply re-imaging M87 at higher resolution or with a prettier color palette.
What to watch
- Whether the same dual-frequency spectral-index method is applied to the EHT’s other primary target, the Milky Way’s own supermassive black hole, Sagittarius A*.
- Follow-up theoretical work testing which black hole accretion and jet-launching models best match the observed plasma-state transition.
- Further results mined from the EHT’s growing multi-year, multi-frequency archive as the collaboration’s data-sharing infrastructure matures.
Source: Phys.org, reporting on a paper published in The Astrophysical Journal Letters, 21 July 2026 (DOI: 10.3847/2041-8213/ae84ca). Featured image: the Event Horizon Telescope Collaboration’s 2019 image of the M87 black hole, shown here as the event-horizon-scale EHT observation this new spectral analysis builds on.
