Measuring spin of a supermassive black hole at the Galactic centre -- Implications for a unique spin

TL;DR

This study estimates the spin of the supermassive black hole at the Galactic centre using discseismology and QPOs, revealing a lower-than-expected spin value indicative of angular momentum loss during growth.

Contribution

It introduces a novel method of measuring black hole spin via QPOs and compares supermassive and stellar-mass black holes to understand their spin evolution.

Findings

Black hole spin in Sgr A* is approximately 0.44.

Black hole spins tend to approach an equilibrium value.

Spin measurements suggest angular momentum extraction during growth.

Abstract

We determine the spin of a supermassive black hole in the context of discseismology by comparing newly detected quasi-periodic oscillations (QPOs) of radio emission in the Galactic centre, Sagittarius A* (Sgr A*), as well as infrared and X-ray emissions with those of the Galactic black holes. We find that the spin parameters of black holes in Sgr A* and in Galactic X-ray sources have a unique value of $\approx 0.44$ which is smaller than the generally accepted value for supermassive black holes, suggesting evidence for the angular momentum extraction of black holes during the growth of supermassive black holes. Our results demonstrate that the spin parameter approaches the equilibrium value where spin-up via accretion is balanced by spin-down via the Blandford-Znajek mechanism regardless of its initial spin. We anticipate that measuring the spin of black holes by using QPOs will open a new window for exploring the evolution of black holes in the Universe.