Black Hole Parameter Estimation from Its Shadow

Understand

The Event Horizon Telescope (EHT), a global submillimeter wavelength very long baseline interferometry array, unveiled event-horizon-scale images of the supermassive black hole M87* as an asymmetric bright emission ring with a diameter of $ 42 \pm 3\; \mu$as, and it is consistent with the shadow of a Kerr black hole of general relativity.

• A Kerr black hole is also a solution of some alternative theories of gravity, while several modified theories of gravity admit non-Kerr black holes.
• While earlier estimates for the M87* black hole mass, depending on the method used, fall in the range $ \approx 3\times 10^9 M_\odot- 7 \times 10^9 $$ M_\odot $, the EHT data indicated a mass for the M87* black hole of $(6.5 \pm 0.7) \times 10^9 M_\odot $.
• This offers another promising tool to estimate black hole parameters and to probe theories of gravity in its most extreme region near the event horizon.