Can we constrain the maximum value for the spin parameter of the super-massive objects in galactic nuclei without knowing their actual nature?

TL;DR

This paper explores the maximum possible spin parameter for super-massive objects in galactic nuclei, suggesting it cannot exceed about 1.2, regardless of their true nature, based on observational efficiency constraints.

Contribution

It relaxes the Kerr black hole assumption and proposes a universal spin bound for super-massive objects based on radiative efficiency observations.

Findings

Maximum spin parameter estimated at about 1.2.

Higher spins would conflict with observed radiative efficiencies.

Bound appears independent of the objects' true nature.

Abstract

In 4-dimensional General Relativity, black holes are described by the Kerr solution and are subject to the bound $|a_*| \le 1$, where $a_*$ is the black hole spin parameter. If current black hole candidates are not the black holes predicted in General Relativity, this bound does not hold and $a_*$ might exceed 1. In this letter, I relax the Kerr black hole hypothesis and I find that the value of the spin parameter of the super-massive black hole candidates in galactic nuclei cannot be higher than about 1.2. A higher spin parameter would not be consistent with a radiative efficiency $\eta > 0.15$, as observed at least for the most luminous AGN. While a rigorous proof is lacking, I conjecture that the bound $|a_*| \lesssim 1.2$ is independent of the exact nature of these objects.