The famous ring around the supermassive black hole M87* isn't just a pretty picture. Its uneven brightness has helped scientists estimate how fast the black hole itself is spinning. The results are published in The Astrophysical Journal.
M87* is a 6.5-billion-solar-mass black hole at the center of galaxy M87, 55 million light-years away. In 2019, the Event Horizon Telescope (EHT) captured its first image: a bright, asymmetric ring — the black hole's shadow against the glowing plasma of its accretion disk.
The team analyzed three observing epochs — 2017, 2018, and 2021 — and compared the ring's brightness distribution against a library of general relativistic magnetohydrodynamic (GRMHD) simulations spanning different black hole spins.
The key result: for strongly magnetized (MAD) models, the three epochs of data disfavor very slow rotation — |a*| ≲ 0.2 (where 0 = no spin, 1 = maximum). In other words, M87* is spinning, and noticeably so. This is consistent with the Blandford–Znajek mechanism, which explains M87's powerful relativistic jet as being powered by the black hole's rotational energy.
The ring's asymmetry arises from a complex interplay of Doppler boosting, gravitational lensing, and frame-dragging effects — all spin-dependent. Plasma turbulence causes the bright spot to wander around the ring, but the statistical distribution of these fluctuations acts like a fingerprint, unique to each spin value.
The researchers also demonstrated how future observations could significantly tighten spin constraints and potentially distinguish between different jet-launching mechanisms and black hole growth scenarios.