On Testing the Kerr Metric of the Massive Black Hole in the Galactic Center via Stellar Orbital Motion: Full General Relativistic Treatment

TL;DR

This paper presents a comprehensive general relativistic method to constrain the spin, mass, and orientation of the Galactic center's black hole using stellar orbits, promising high-precision measurements with upcoming observations.

Contribution

It introduces a full general relativistic approach to simultaneously determine black hole parameters from stellar motion and photon propagation, improving constraints over previous methods.

Findings

Spin effects significantly influence stellar trajectories and redshifts.

High-precision astrometric and spectroscopic data can constrain black hole spin within ~10 years.

Mass and distance measurements achieve unprecedented accuracy.

Abstract

The S-stars in the Galactic center (GC) are anticipated to provide unique dynamical constraint on the spin of the GC massive black hole (MBH). In this paper, we develop a fast full general relativistic method to simultaneously constrain the MBH mass, spin, and spin direction by considering both the motion of a star and the propagation of photons from the star to a distant observer. Assuming some example stars, we demonstrate that the spin-induced effects on the projected trajectory and redshift curve of a star depend on both the value and the direction of the spin. The maximum effects over a full orbit can differ by a factor upto more than one order of magnitude for cases with significantly different spin directions. Adopting the Markov Chain Monte Carlo fitting technique, we illustrate that the spin of the GC MBH is likely to be well constrained by using the motion of S0-2/S2 over a period of ~45yr if it is close to one and the astrometric and spectroscopic precisions (sigma_p,sigma_Z) can be as high as (10muas, 1km/s). In the mean time, the distance from the sun to the GC and the MBH mass can also be constrained to an unprecedented accuracy (0.01%-0.1%). If there exists a star with semimajor axis significantly smaller than that of S0-2/S2 and eccentricity larger than that of S0-2/S2, the MBH spin can be constrained with high accuracy over a period of <~10yr for (sigma_p,sigma_Z) ~ (10muas,1km/s), even if the spin is only moderately large (>~0.2).