M87* in two colours — probing black-hole plasma on event-horizon scales

Seven years after the Event Horizon Telescope (EHT) unveiled the first-ever image of a black hole, an international team led by Shao Zhan-Zhan, together with Lu Ru-Sen and collaborators, has taken the next step: mapping how the physical properties of the plasma surrounding the supermassive black hole in M87 change across event-horizon scales.

By combining 2018 observations from the EHT and the Global mm-VLBI Array (GMVA) at two wavelengths — 1.3 mm and 3.5 mm — the team produced the first spatially resolved spectral-index map of M87*. The results reveal a clear transition from optically thick, self-absorbed emission near the black hole to optically thin emission farther out, occurring right at the radius of the iconic ring seen in the images. This shows that the ring is not just a morphological feature, but a direct signature of the plasma's physical state near the event horizon.

The MPIfR played a central role in making this result possible: all GMVA data were correlated and post-processed at the institute, and MPIfR contributed to half of the EHT correlation effort. The work was driven by Lu Ru-Sen, leader of an MPG Partner Group with the Chinese Academy of Sciences based at MPIfR, and by Rocco Lico, a collaborator who was postdoctoral researcher at MPIfR when the project began.

This study, published in The Astrophysical Journal Letters, opens a new path toward disentangling plasma physics from gravity in black hole imaging — a direction that mm-VLBI research at MPIfR will continue to pursue.