arXiv subjects

Vishva Patel

Publications and source records attributed to Vishva Patel.

8 recordsLinked to original sources

arXiv2026

Degeneracy in Accretion Disk Spectra from Naked Singularities and Kerr Black Holes: Application to the AGN MCG-06-30-15

Theoretical studies suggest that gravitational collapse can form either a black hole or a visible (naked) singularity. Identifying observational signatures that distinguish these two types of collapsed objects is a holy grail of physics. Here, we examine whether relativistic accretion disk spectra can provide such a test. We construct an additive table model for a thin accretion disk in the Joshi-Malafarina-Narayan (JMN-1) naked singularity geometry matched to a Schwarzschild exterior and fit it to NuSTAR X-ray data from the AGN MCG-06-30-15. Our results are compared with standard Kerr and Schwarzschild black hole models. We also include the relativistic reflection spectral component relxill. Despite their different underlying geometries, the spinning (Kerr) black hole and the non-spinning JMN-1 naked singularity provide similar spectral fits, which are significantly better fits than the Schwarzschild black hole. This degeneracy between the naked singularity and the Kerr black hole could lead to incorrect spin measurements of collapsed objects using disk spectra. The degeneracy could be broken with an independent spin measurement, which could also help identify a naked singularity. Our results could also have a role in different spin distributions of collapsed objects measured from gravitational-wave sources and X-ray binaries.

astro-ph.HE↗

arXiv2026

Energy extraction-driven instability and horizon formation in Kerr-Newman naked singularities and their limiting cases

Energy extraction from compact objects has been a central topic in general relativity since the introduction of the Penrose process. In this work we present a unified analysis of rotational and electromagnetic energy extraction in Kerr, Reissner-Nordstrom, and Kerr-Newman spacetimes. Using particle energetics and the irreducible mass formalism, we compare the efficiencies of these mechanisms and examine their consequences for horizonless objects. While purely rotational extraction in Kerr spacetime is fundamentally limited by geometric constraints, electromagnetic interactions enlarge the region of negative energy orbits through an effective ergoregion, allowing significantly higher efficiencies. In Kerr-Newman geometry, the combined effect of rotation and charge further enhances the extractable energy. We then study the long-term evolution of over-extremal cases under continuous extraction. By deriving coupled evolution equations for the mass, spin, and charge parameters, we show that continuous extraction can gradually drive a naked singularity toward the extremal bound. For astrophysically realistic luminosities, the characteristic evolution timescale is of order about 10^9 years. These results suggest that energy extraction provides an energetic indication of instability in Reissner-Nordstrom, Kerr, and Kerr-Newman naked singularities and may lead to horizon formation as a long-term stabilizing outcome.

gr-qc↗

arXiv2025

Optical appearance of regularized compact objects without an exterior photon sphere

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 unstable circular null geodesics outside the compact object (hereafter referred to as an exterior exterior photon sphere) observationally remains challenging. Motivated by this, we investigate shadow formation in spacetimes that lack an exterior 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. Apart from the shadow-like dark region without an exterior photon sphere in the charged SV spacetime, our investigation shows that, for a specific range of parameters, the shadow boundary (dark region) is controlled by the regular core rather than by an exterior photon sphere. Our results reveal that shadows arising from these regularized null singularity spacetimes closely mimic those of Schwarzschild and charged black-bounce spacetimes, even though no exterior exterior photon sphere exists. We also perform a phenomenological comparison of the predicted shadow sizes with the EHT observations of Sgr A* and M87, identifying parameter ranges compatible with the observed angular diameters.

gr-qc↗

arXiv2024

Magnetic Penrose Process and Kerr Black Hole Mimickers

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.

gr-qc↗

arXiv2023

High Energy Particle Collisions in the vicinity of Naked Singularity

In this paper, we investigate particle acceleration and high-energy collisions in the Joshi-Malafarina-Narayan (JMN-1) naked singularity, which, in the absence of an event horizon, allows infalling particles to turn back under specific angular momentum conditions. These outgoing particles can then collide with infalling ones, enabling the JMN-1 singularity to act as a natural high-energy particle accelerator. We derive the necessary expressions to compute the center-of-mass energy of two colliding particles and find that this energy can reach extremely high values, potentially even approaching Planck energy scales. We also explore the implications of these results, including the possible formation of microscopic black holes that could decay via Hawking radiation, releasing energy on the order of $10^{26} eV$ due to the extreme gravitational fields near the naked singularity. This scenario offers significant advantages. If horizonless compact objects exist in nature, these high-energy collisions could substantially influence the surrounding physical processes and might give rise to distinct observational signatures.

gr-qc↗

arXiv2023

Energy extraction from Janis-Newman-Winicour naked singularity

In general, energy extraction methods such as the Penrose process and the magnetic Penrose process are thought to be reliant on the existence of an ergoregion. Inside an ergoregion, there are negative energy states that allow a particle to extract energy and escape to an observer at infinity. In this paper, we considered the electromagnetic field in the rotating Janis-Newman-Winicour (JNW) spacetime. This concept is feasible because an accretion disc forms an electromagnetic field around compact objects. After that, we briefly examine negative energy orbits and their significance in energy extraction. The ergoregion is absent in a rotating JNW geometry, but we show that the effective ergoregion is there. The change in a negative energy orbit concerning the magnetic field (B), spin parameter (a), and electric charge (Q) is analyzed. We find that the total energy extraction efficiency within this process can be around $60\%$ for the rotating JNW naked singularity.

gr-qc↗

arXiv2022

Light trajectory and shadow shape in the rotating naked singularity

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.

gr-qc↗

arXiv2022

Rotational energy extraction from the Kerr black hole's mimickers

In this paper, the Penrose process is being 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 the Kerr black hole's mimickers. The Penrose process is then studied through conformally transformed rotating singular and regular black hole solutions. These both 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 $l$ affects 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 the Kerr black hole case. While, in the conformally transformed singular and regular black holes, we obtain the efficiency rate of extracted energies are extremely high compared to the Kerr black hole scenario. This distinct signature of the conformally transformed singular and regular black holes would be useful to distinguish them from the Kerr black hole in observation.

gr-qc↗