Shadows and photon rings of regular black holes and geonic horizonless compact objects

Abstract

The optical appearance of a body compact enough to feature an unstable bound orbit, when surrounded by an accretion disk, is expected to be dominated by a luminous ring of radiation enclosing a central brightness depression typically known as the shadow. Despite observational limitations, the rough details of this picture have been now confirmed by the results of the EHT Collaboration on the imaging of the M87 and Milky Way supermassive central objects. However, the precise characterization of both features - ring and shadow - depends on the interaction between the background geometry and the accretion disk, thus being a fertile playground to test our theories on the nature of compact objects and the gravitational field itself in the strong-field regime. In this work we use both features in order to test a continuous family of solutions interpolating between regular black holes and horizonless compact objects, which arise within the Eddington-inspired Born-Infeld theory of gravity, a viable extension of Einstein's General Relativity (GR). To this end we consider seven distinctive classes of such configurations (five black holes and two traversable wormholes) and study their optical appearances under illumination by a geometrically and optically thin accretion disk, emitting monochromatically with three analytic intensity profiles previously suggested in the literature. We build such images and consider the sub-ring structure created by light rays crossing the disk more than once and existing on top of the main ring of radiation. We discuss in detail the modifications as compared to their GR counterparts, the Lyapunov exponents of unstable nearly-bound orbits, as well as the differences between black hole and traversable wormholes for the three intensity profiles. In addition we use the claim by the EHT Collaboration on the radius of the bright ring acting (under proper calibrations) as a proxy for the radius of the shadow itself to explore the parameter space of our solutions compatible with such a result.

Black hole shadows in Einstein-Bel-Robinson gravity*

“…In this section, we aim to investigate how the plasma background affects the black hole shadow in EBR gravity. The main motivation for this is the idea that in most cases, black holes are surrounded by a medium that changes the geodesic of photons [24,36]. We begin by considering a plasma with a refractive index , where ω denotes photon frequency.…”

Shadow images of compact objects in beyond Horndeski theory

“…On the other hand, theoretical investigations of shadow images of compact objects are quite fascinating. This applies not only to black holes in General Relativity [36][37][38][39][40][41][42][43][44][45] and in alternative gravities [46][47][48][49][50][51][52][53][54][55][56], but also to exotic compact objects like wormholes [57][58][59][60][61][62][63][64][65][66][67], naked singlarities [68][69][70][71][72][73][74][75][76][77][78] or boson stars [79,80]. Previous studies have shown that some of these exotic compact objects can mimic black holes with respect to their shadow images if their light ring properties are similar.…”

Observational appearance and extra photon rings of an asymmetric thin-shell wormhole with a Bardeen profile*

“…In fact, certain UCOs have the potential to create a shadow resembling that of a BH [53][54][55][56][57][58][59]. Therefore, it is imperative to develop a methodology capable of discerning BHs from other UCOs, such as worm-holes [60][61][62][63][64][65][66][67][68][69][70][71][72][73] and boson stars [74][75][76][77]. As an innovative attempt, Wang et al studied the optical appearance of an asymmetric thin-shell wormhole (ATSW), demonstrating that the shadow of the ATSW exhibits a smaller size compared to that generated by a BH [60].…”