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

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.