Photon spheres, ISCOs, and OSCOs: Astrophysical observables for regular black holes with asymptotically Minkowski cores

TL;DR

This paper investigates observable features like photon spheres and stable orbits in a regular black hole model with no singularity, revealing complex behaviors that could help distinguish such objects from classical black holes.

Contribution

It introduces calculations of photon spheres and stable orbits for a regular black hole with an asymptotically Minkowski core, highlighting complex phenomenology relevant for astrophysical observations.

Findings

Photon spheres can approach near extremal horizons.

Some photon spheres become stable under certain conditions.

Locations of photon spheres and stable orbits can be multi-valued.

Abstract

Classical black holes contain a singularity at their core. This has prompted various researchers to propose a multitude of modified spacetimes that mimic the physically observable characteristics of classical black holes as best as possible, but that crucially do not contain singularities at their cores. Due to recent advances in near-horizon astronomy, the ability to observationally distinguish between a classical black hole and a potential black hole mimicker is becoming increasingly feasible. Herein, we calculate some physically observable quantities for a recently proposed regular black hole with an asymptotically Minkowski core -- the radius of the photon sphere and the extremal stable timelike circular orbit (ESCO). The manner in which the photon sphere and ESCO relate to the presence (or absence) of horizons is much more complex than for the Schwarzschild black hole. We find situations in which photon spheres can approach arbitrarily close to (near extremal) horizons, situations in which some photon spheres become stable, and situations in which the locations of both photon spheres and ESCOs become multi-valued, with both ISCOs (innermost stable circular orbits) and OSCOs (outermost stable circular orbits). This provides an extremely rich phenomenology of potential astrophysical interest.