Spin and mass of the nearest supermassive black hole

TL;DR

This paper uses QPO observations of hot plasma spots orbiting the supermassive black hole at the Galactic center to determine its mass and spin, based on frequencies independent of accretion models.

Contribution

It introduces a method to measure black hole parameters using QPO frequencies linked to gravitational field properties, applicable to low accretion rate black holes.

Findings

Black hole spin parameter a=0.65±0.05

Mass of the black hole M=(4.2±0.2)×10^6 solar masses

Identification of QPO periods with horizon rotation and hot spot oscillations

Abstract

Quasi-Periodic Oscillations (QPOs) of the hot plasma spots or clumps orbiting an accreting black hole contain information on the black hole mass and spin. The promising observational signatures for the measurement of black hole mass and spin are the latitudinal oscillation frequency of the bright spots in the accretion flow and the frequency of black hole event horizon rotation. Both of these frequencies are independent of the accretion model and defined completely by the properties of the black hole gravitational field. Interpretation of the known QPO data by dint of a signal modulation from the hot spots in the accreting plasma reveals the Kerr metric rotation parameter, $a=0.65\pm0.05$, and mass, $M=(4.2\pm0.2)10^6M_\odot$, of the supermassive black hole in the Galactic center. At the same time, the observed 11.5 min QPO period is identified with a period of the black hole event horizon rotation, and, respectively, the 19 min period is identified with a latitudinal oscillation period of hot spots in the accretion flow. The described approach is applicable to black holes with a low accretion rate, when accreting plasma is transparent up to the event horizon region.