Vishva Patel

Recent observations by the Event Horizon Telescope (EHT) indicate that the shadow of the compact object at our Galaxy's center (Sgr A*) closely resembles that of a Schwarzschild black hole. However, identifying the presence and exact location of a photon sphere observationally remains challenging. Motivated by this, we investigate shadow formation in spacetimes that lack a photon sphere by applying the Simpson-Visser (SV) regularization technique (originally designed to smooth black hole singularities) to null singularity and charged null singularity metrics. Remarkably, these regularized null and charged null singularity spacetimes can produce a shadow without a photon sphere. We analyze how the SV regularization parameter influences their geometry and shadow size, and show that the regularized null and charged null singularity spacetimes can correspond either to two-way traversable wormholes or retain singularities. Our results reveal that shadows arising from these regularized 'null singularity spacetimes' closely mimic those of Schwarzschild and charged black-bounce spacetimes, despite the absence of a photon sphere. We also constrain parameters in these geometries using observational data of Sgr A* and M87.

The present study investigates the negative energy orbits and energy extraction efficiency using the magnetic Penrose process in various regular black hole geometries surrounded by electromagnetic fields. Utilizing numerical simulations, we analyze the efficiency of this process in Kerr and Simpson-Visser geometries, focusing on extremal black holes. Interestingly, our study demonstrates that the energy extraction efficiency remains indistinguishable between Kerr and Simpson-Visser geometries, regardless of the regularization parameter ($l$); this trend is consistent with previous studies of the Penrose process and superradiance. Additionally, we present results for the rotating Hayward black hole, showing that efficiency is influenced by spin and deviation parameters ($g$), as well as the induced magnetic field and charge of the compact object. Notably, we observe that energy extraction efficiency is highest in the rotating Hayward black hole compared to Kerr and Simpson-Visser geometries, particularly in scenarios where the magnetic field and charge are minimal. Our study highlights the significant role of spin, charge and magnetic field characteristics in maximizing energy extraction efficiency, particularly in the rotating Hayward black hole context.

In this paper, the Penrose process is used to extract rotational energy from regular black holes. Initially, we consider the rotating Simpson–Visser regular spacetime, which describes the class of geometries of Kerr black hole mimickers. The Penrose process is then studied through conformally transformed rotating singular and regular black hole solutions. Both the Simpson–Visser and conformally transformed geometries depend on mass, spin, and an additional regularisation parameter l. In both cases, we investigate how the spin and regularisation parameter l affect the configuration of an ergoregion and event horizons. Surprisingly, we find that the energy extraction efficiency from the event horizon surface is not dependent on the regularisation parameter l in the Simpson–Visser regular spacetimes, and hence, it does not vary from that of the Kerr black hole. Meanwhile, in conformally transformed singular and regular black holes, we obtain that the efficiency rate of extracted energies is extremely high compared to that of the Kerr black hole. This distinct signature of conformally transformed singular and regular black holes is useful to distinguish them from Kerr black holes in observation.

In this paper, we investigate the light trajectories and shadow properties in the rotating version of null naked singularity (NNS) spacetime which is derived using the Newman–Janis algorithm without complexification method. We discuss some of the geometrical properties and causal structure of Rotating Naked Singularity (RNS) spacetime. The gravitational lensing in a rotating naked singularity is analyzed, and the results are compared to those of a Kerr black hole. In the case of a Kerr black hole, the photon sphere exists for both prograde and retrograde photon orbits, whereas for RNS, the photon sphere exists only for retrograde photon orbits. As a result, the naked singularity projects an arc-shaped shadow that differs from the contour-shaped shadow cast by a Kerr black hole.

Astrometric observations of S-stars provide a unique opportunity to probe the nature of Sagittarius-A* (Sgr-A*). In view of this, it has become important to understand the nature and behavior of timelike bound trajectories of particles around a massive central object. It is known now that whereas the Schwarzschild black hole does not allow the negative precession for the S-stars, the naked singularity spacetimes can admit the positive as well as negative precession for the bound timelike orbits. In this context, we study the perihelion precession of a test particle in the Kerr spacetime geometry. Considering some approximations, we investigate whether the timelike bound orbits of a test particle in Kerr spacetime can have negative precession. In this paper, we only consider low eccentric timelike equatorial orbits. With these considerations, we find that in Kerr spacetimes, negative precession of timelike bound orbits is not allowed.