Peering Inside the Ring: New Spectral Maps Reveal M87’s Black Hole in Unprecedented Detail
The iconic ring of light around the supermassive black hole at the center of galaxy M87 is not a uniform glow. A new study combining data from two global networks of radio telescopes has mapped how the ring’s radiation changes with distance from the event horizon, offering the sharpest physical picture yet of the plasma environment just outside a black hole.
An international team led by the Shanghai Astronomical Observatory (SAO) of the Chinese Academy of Sciences analyzed observations of M87 taken in 2018 at two different wavelengths: 1.3 millimeters with the Event Horizon Telescope (EHT) and 3.5 millimeters with the Global Millimeter VLBI Array (GMVA). By comparing images at both frequencies, the researchers produced the first spatially resolved map of the spectral index of the M87 black hole, a measure of how the brightness of radiation changes between the two wavelengths at every point in the image.
The result reveals a clear transition. In the innermost region, the spectral index is positive, meaning the plasma is optically thick: electrons spiraling in magnetic fields reabsorb the photons they emit, a process known as synchrotron self-absorption. Moving outward, around 30 microarcseconds from the black hole, the index flips to negative, marking the boundary where photons can escape freely. This transition point aligns closely with the ring structure visible in the iconic 2019 EHT image.
The work demonstrates that the ring seen in 2019 is more than a striking visual feature. It is a direct signature of a change in the physical state of the infalling plasma. The new technique gives astronomers a way to distinguish between gravitational effects and plasma physics in black hole images, a distinction that has been difficult to make with imaging alone.
The paper was published in the Astrophysical Journal Letters.
Images:
- EHT image of M87 black hole: https://eventhorizontelescope.org/sites/default/files/styles/large/public/images/20190410-m87-black-hole.jpg
- Shanghai Astronomical Observatory announcement: https://english.shao.ac.cn/news/202607/t20260714_1178114.html
Source: 1ban.news