Kerr black hole parameters and its distance from the Earth in terms of directly measurable quantities of accretion disk

Abstract

We extract elegant and concise analytic formulae for the mass and rotation parameters of the Kerr black hole as well as its distance from the Earth only in terms of directly measurable quantities of the accretion disk revolving in the black hole spacetime background. To this end, we consider massive geodesic particles circularly orbiting the Kerr black hole in the equatorial plane and emitting frequency-shifted photons toward a distant observer. We calculate the frequency shift and redshift rapidity at the detector location, and by solving an inverse problem, we express the Kerr black hole parameters and its distance from a distant observer in terms of a handful of observable elements, such as frequency shift, aperture angle of the telescope, and redshift rapidity, a newly introduced concept in [1]. The aperture angle of the telescope (angular distance) characterizes the emitter position on the sky, and the redshift rapidity is an observable relativistic invariant representing the proper time evolution of the frequency shift. The relations presented in this article allow us to disentangle mass, spin, and distance to the black holes in the Kerr spacetime background and obtain these parameters separately. Our analytic formulae are valid on the midline and close to the line of sight, and they can be directly applied to supermassive black holes hosted at the core of active galactic nuclei orbited by water vapor clouds within their accretion disks. The generic exact relations are valid for an arbitrary point of the emitter's orbit, and they can be employed in black hole parameter estimation studies.