Testing the No-Hair Theorem with Observations in the Electromagnetic Spectrum: II. Black-Hole Images

Understand

According to the no-hair theorem, all astrophysical black holes are fully described by their masses and spins.

• This theorem can be tested observationally by measuring (at least) three different multipole moments of the spacetimes of black holes.
• In this paper, we analyze images of black holes within a framework that allows us to calculate observables in the electromagnetic spectrum as a function of the mass, spin, and, independently, the quadrupole moment of a black hole.
• We show that a deviation of the quadrupole moment from the expected Kerr value leads to images of black holes that are either prolate or oblate depending on the sign and magnitude of the deviation.