Circular geodesics of Bardeen and Ayon-Beato-Garcia regular black-hole and no-horizon spacetimes

TL;DR

This paper analyzes the circular geodesic motion of particles and photons in Bardeen and ABG regular black-hole and no-horizon spacetimes, revealing unique optical signatures and accretion disk structures that distinguish these models from classical black holes.

Contribution

It provides a detailed study of geodesic structures and optical phenomena in Bardeen and ABG spacetimes, highlighting observable differences from Schwarzschild black holes and identifying signatures of no-horizon solutions.

Findings

Existence of antigravity static spheres in no-horizon solutions

Additional inner Keplerian disks in ABG spacetimes with high charge parameter

Distinct spectral line profiles for no-horizon spacetimes compared to Schwarzschild black holes

Abstract

We study circular geodesic motion of test particles and photons in the Bardeen and Ayon-Beato-Garcia (ABG) geometry describing spherically symmetric regular black-hole or no-horizon spacetimes. While the Bardeen geometry is not exact solution of Einstein's equations, the ABG spacetime is related to self-gravitating charged sources governed by Einstein's gravity and non-linear electrodynamics. They both are characterized by the mass parameter $m$ and the charge parameter $g$. We demonstrate that in similarity to the Reissner-Nordstrom (RN) naked singularity spacetimes an antigravity static sphere should exist in all the no-horizon Bardeen and ABG solutions that can be sorrounded by a Keplerian accretion disc. However, contrary to the RN naked singularity spacetimes, the ABG no-horizon spacetimes with parameter $g/m > 2$ can contain also an additional inner Keplerian disc hidden under the static antigravity sphere. Properties of the geodesic structure are reflected by simple observationally relevant optical phenomena. We give silhouette of the regular black hole and no-horizon spacetimes, and profiled spectral lines generated by Keplerian rings radiating at a fixed frequency and located in strong gravity region at or nearby the marginally stable circular geodesics. We demonstrate that the profiled spectral lines related to the regular black holes are qualitatively similar to those of the Schwarzschild black holes, giving only small quantitative differences. On the other hand, the regular no-horizon spacetimes give clear qualitative signatures of their presence while compared to the Schwarschild spacetimes. Moreover, it is possible to distinguish the Bardeen and ABG no-horizon spacetimes, if the inclination angle to the observer is known.