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.
Highlights
Karl Schwarzschild first derived the spacetime metric for the region exterior to a static, spherically symmetric source in 1916 [1]; only some 50 years later was it properly understood that this spacetime could be extrapolated inwards to describe a black hole
Continuing the analysis of [62], we calculate the location of the photon sphere and extremal stable circular orbit (ESCO) for the regular black hole with line element given by equation (2)
We investigate astrophysically observable quantities of a specific novel regular black hole model based on an asymptotically Minkowski core [62,63]: we investigate the photon sphere and extremal stable timelike circular orbit (ESCO)
Summary
Karl Schwarzschild first derived the spacetime metric for the region exterior to a static, spherically symmetric source in 1916 [1]; only some 50 years later was it properly understood that this spacetime could be extrapolated inwards to describe a black hole. The model spacetime investigated in this work is a specific regular black hole with an asymptotically Minkowski core, as discussed in [62,63] This is an example of a metric with an exponential mass suppression, and is described by the line element ds2 = −. If a > 2m/e, there are no horizons, and one is dealing with a regular horizonless extended but compact object (the energy density peaks at r = a/4) This object could either be extended all the way down to r = 0, or alternatively be truncated at some finite value of r, to be used as the exterior geometry for some static and spherically symmetric mass source that is not a black hole. We incorporate aspects of the analysis for a > 2m/e as and when required to generate astrophysical observables in the case when Equation (2) is modeling a compact object other than a black hole
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