Spinning super-massive objects in galactic nuclei up to $a_* > 1$

TL;DR

This paper explores the possibility that supermassive objects in galactic nuclei could spin faster than the Kerr bound, challenging the traditional understanding of black holes and suggesting new observational tests.

Contribution

It provides theoretical evidence that supermassive objects might exceed the Kerr spin limit, implying they could be non-black-hole compact objects.

Findings

Supermassive objects may have spin parameters greater than 1.

Accretion processes could spin non-black-hole objects beyond the Kerr bound.

Potential observational tests include gravitational wave detection and very long baseline interferometry.

Abstract

Nowadays we believe that a typical galaxy contains about $10^7$ stellar-mass black holes and a single super-massive black hole at its center. According to general relativity, these objects are characterized solely by their mass $M$ and by their spin parameter $a_*$. A fundamental limit for a black hole in general relativity is the Kerr bound $|a_*| \le 1$, but the accretion process can spin it up to $a_* \approx 0.998$. If a compact object is not a black hole, the Kerr bound does not hold and in this letter I provide some evidences suggesting that the accretion process could spin the body up to $a_* > 1$. While this fact should be negligible for stellar-mass objects, some of the super-massive objects at the center of galaxies may actually be super-spinning bodies exceeding the Kerr bound. Such a possibility can be tested by gravitational wave detectors like LISA or by sub-millimeter very long baseline interferometry facilities.