arXiv subjects

Pankaj S. Joshi

Publications and source records attributed to Pankaj S. Joshi.

At least 19 recordsLinked to original sources

arXiv2026

Generalized JMN Naked Singularity Models

We construct a generalized class of Joshi-Malafarina-Narayan (JMN) naked singularity spacetimes that arise as equilibrium end states of gravitational collapse with non-vanishing tangential pressure. The generalization introduces density inhomogeneity through a radially dependent mass function $F(r)=(M_0+M_n r^n)r^3$, leading to a two-parameter family of solutions matched smoothly to an exterior Schwarzschild spacetime. The observational properties of the spacetime are then examined through shadow formation and thin accretion disk emission. We find that when the photon sphere lies in the exterior Schwarzschild region, the shadow is identical to that of a Schwarzschild black hole. Accretion disk spectra show enhanced high-frequency emission compared to Schwarzschild, while deviations from the original JMN model remain small due to strong constraints on the inhomogeneity parameter. These results indicate that the generalized model effectively serves as a small perturbation of the JMN spacetime, demonstrating the robustness of JMN-type naked-singularity geometries.

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arXiv2026

GRMHD accretion beyond the black hole paradigm: Light from within the shadow

We present the first three-dimensional general relativistic magnetohydrodynamic simulation of sustained accretion onto a horizonless singularity in which matter reaches the central object rather than being accumulated outside of it or expelled in outflows. We consider a Joshi-Malafarina-Narayan (JMN-1) spacetime, a well-motivated black hole mimicker that arises from gravitational collapse with anisotropic pressure in general relativity, and adopt a compactness parameter for which the central singularity is null. We find that the system evolves into a sustained magnetically arrested disk state. For parameters appropriate to the low-luminosity active galactic nucleus M87*, we obtain an accretion rate of $\sim(3.0 \pm 0.5)\times 10^{-6} \dot{M}_{\rm Edd}$, in full agreement with estimates based on black hole models and, in particular, comparable to that of our reference Schwarzschild black hole simulation. Synthetic ray-traced images at $230\,{\rm GHz}$, computed using polarized general relativistic radiative transfer, are broadly consistent with the Event Horizon Telescope observations of M87*. We identify a key observational discriminant between a black hole and JMN-1: the presence of detectable brightness inside of the ``observable" shadow of JMN-1. This emission originates very close to the central singularity, in a region that would be hidden behind the event horizon in a black hole spacetime. Although this signature is beyond the reach of current observations, it falls within the projected imaging dynamic range of next-generation radio interferometric instruments, offering a robust test of the black hole paradigm.

astro-ph.HE↗

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

Astrophysical Black holes: An Explanation for the Galaxy Quenching

In light of increasing observational evidence supporting the existence of ultra-compact objects, we adopt the term astrophysical black hole to refer to any object having a huge mass confined within a sufficiently small region of spacetime. This terminology encompasses both the classical black hole solutions predicted by general relativity, as well as alternative compact objects that may not possess an event horizon. We propose models of Astrophysical Black holes (ABHs) without event horizons (EHs), as a more viable explanation for the long-term quenching phenomenon in galaxies. At the same time, the short-term quenching is explained here in terms of an efficient feedback expected in the models of stellar-mass astrophysical black holes (StMABHs). We have calculated the radiative flux from the disk in a general spherically symmetric metric background and used it to contrast the distinctive features of the BHs and ABHs scenarios. We demonstrate the relative ease of wind generation from the accretion disk surrounding an ABH without an event horizon, compared to a BH, and highlight the significant strength of these winds. The nature of the feedbacks arising from accretion onto a BH and an ABH in the `quasar' and `radio' modes are compared and some possible observational signatures of the StMABHs are pointed out.

gr-qc↗

arXiv2026

Tidal disruption of a neutron star near naked singularity

We investigate the tidal disruption of a neutron star (NS) near a black hole (BH), and for the first time, to the best of our knowledge, near a naked singularity (NaS). For a BH with a mass greater than about $10 M_{\odot}$, the tidal disruption of NS should occur within the event horizon, and hence neither can the stellar material escape nor a distant observer observe the disruption. Since NaS does not have an event horizon, a significant portion of the NS's material can escape, and the tidal disruption can be observed by a distant observer. One could identify such an event from the observed emission from the disrupted NS's material and the decay of the light curve of the disruption event. The escape of a significant fraction of the NS's material may also have implications for the heavy elements in the universe. Moreover, observing such an event can be useful for confirming a NaS, probing its spacetime, and studying the motion of matter in such a geometry. This may help constrain the NS parameters and equation of state models. As a first step in this direction, we calculate here the tidal disruption radius and other parameters for a specific type (Joshi-Malafarina-Narayan type 1) of NaS and compare our results with observations.

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arXiv2026

Obstructions to global visibility of singularities in asymptotically flat spacetimes

Consider an $(N+1)$-dimensional asymptotically flat spacetime and a future-directed, affinely parametrized outgoing null generator $γ$ of an achronal boundary $\partial J^+(S_\varepsilon)$, where $\{S_\varepsilon\}$ is a nested family of smooth compact codimension $2$ surfaces approaching a singular boundary set $S$ in the past. In the twist-free case and under the null energy condition, the Raychaudhuri equation on the $m:=N-1$ dimensional screen bundle reads, $$ θ'=-\frac1mθ^2-\|σ\|^2-\mathrm{Ric}(k,k), $$ where $k$ is the tangent to $γ$. This equation linearizes, via the rescaling $u:=A^{1/m}$ with $A := |\det D|$ the Jacobi-map $m$-volume, to the Sturm-type ODE $$ u''+\frac1m f\,u=0,\qquad f:=\|σ\|^2+\mathrm{Ric}(k,k)\ge 0. $$ We develop two purely generator-wise criteria forcing a first zero of $u$: (i) an exact Volterra identity combined with concavity leads to a barrier-weighted integral inequality, and (ii) Sturm comparison and a Prüfer-angle estimate yields failure of disconjugacy whenever $\int_c^d \sqrt{f/m}\,dλ>π$ on a subinterval. We prove that $u(λ_\ast)=0$ is equivalent to the existence of a focal (conjugate) point and implies $θ= m u'/u\to-\infty$ at $λ_\ast$. Using the standard structure of achronal boundaries, this yields a geodesic-wise obstruction: if every generator that could reach $\mathscr I^+$ satisfies one of the above conditions in the regular spacetime region, then $J^+(S_\varepsilon)\cap \mathscr I^+=\emptyset$, and hence $S$ is not globally visible. As an application, we illustrate one of these criteria in the Einstein-massless scalar field collapse model of Christodoulou.

gr-qc↗

arXiv2025

Universality and Criticality in Mass-less Scalar Field Collapse

In this paper, we observe the collapse of a mass-less scalar field covariantly. We show that the strengths of the collapsing and dispersing modes of this scalar field will decide whether the collapse will end up in a black-hole or disperse. We find a locally naked null singularity as a critical case between these two and confirm that there is a single dimensionless parameter that determine the end state. This work is ansatz-independent, hence, true for all mass-less scalar field families. We also show that the geometrical mass of these singularities go to zero.

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arXiv2025

Quasinormal Modes and Stability Analysis of the JMN-1 Naked Singularity

In this paper, we perform a comprehensive analysis of the quasinormal modes in an external geometry of the Joshi-Malafarina-Narayan (JMN-1) naked singularity by investigating its response to linear perturbations, including gravitational and electromagnetic perturbations. To analyze the stability of the JMN-1 naked singularity under axial perturbations, we compute the quasinormal mode frequencies using the Wentzel-Kramers-Brillouin method. The quasinormal mode frequencies provides information about the stability of spacetime, with the real part of the frequency determining the oscillation rate and the imaginary part governing the decay or growth of perturbations. Our results indicate that by imposing appropriate boundary conditions, we find that the background spacetime of JMN-1 naked singularity remains dynamically stable under axial perturbations.

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arXiv2025

Apparent horizon and causal structure of spacetime singularities

A major issue in general relativity and blackhole physics today is to determine the conditions when the spacetime singularities forming as end-states of gravitational collapse are visible to external observers, and when these are hidden within the event horizon of a black-hole. We show here that such a causal structure of singularity, in terms of its visibility or otherwise, is determined by the dynamics of the apparent horizon and trapped surfaces forming during collapse of massive matter clouds. It turns out that the relative timing of formation of trapped surfaces and the singularity plays a crucial role here. The dynamics of apparent horizon governs the visibility of singularity, and we characterize precise conditions here for spherically symmetric collapse with a general type-I matter field. This is done in terms of the existence of outgoing null geodesic families from the central singularity. These results hold under generic initial data satisfying $\mathcal{C}^2$ regularity and the weak energy condition.

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arXiv2025

On Formation of Primordial Naked Singularities

The density fluctuations in the nearly homogeneous background in the very early universe are argued to be the origin of the cosmic structures we observe in our present universe. Along with many other structures, these fluctuations would have also given rise to primordial black holes at the end of unhindered gravitational collapse of high-density matter blobs that developed due to these fluctuations. We study here such a collapse, which are seeded by a scalar field $ϕ$ associated to a non-trivial potential function $V(ϕ)$, minimally coupled to gravity. Such a continual collapse is presumed to form a black hole always and is named a primordial black hole (PBH). Examining the dynamics of such a collapse, we find the parameter range where the apparent horizon does not form, thus resulting in the visibility of the final singularity of collapse for faraway external observers. This treatment is within the classical limits dictated by Planck's constraints. The slow-roll parameters are analysed here to keep the relic abundance of the scalar field high enough so that the abundance of produced primordial naked singularities (PNaSs) falls within the range of resolution of possible observational probes.

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arXiv2025

Regular black hole from regular initial data

Recently, there has been an interest in exploring black holes that are regular in the sense that the central curvature singularity is avoided. Here, we depict a method to obtain a regular black hole (RBH) spacetime from the unhindered gravitational collapse, beginning with regular initial data of a spherically symmetric perfect fluid. In other words, we obtain the equilibrium (static) spacetime $(\mathcal{M}, \Tilde{g})$ as a limiting case of the time-evolving (non-stationary) spacetime $(\mathcal{M}, g)$. In the spirit of Joshi, Malafarina and Narayan (\textit{Class. Quantum Grav. 31, 015002, 2014}), our description of gravitational collapse is implicit in nature in the sense that we do not describe the data at each time-slice. Rather, we impose a condition in terms of geometric and matter variables for the collapse to have an end-state that is devoid of incomplete geodesics but admits a marginally trapped surface (MTS). The admission of MTS causally disconnects two mutually exclusive regions $\Hat{\mathcal{M}}_1$ and $\Hat{\mathcal{M}}_2\subset \mathcal{M}$ in the sense that $\forall~p\in\Hat{\mathcal{M}_2}$, the causal past of $p$ does not intersect $\Hat{\mathcal{M}}_1$. While the classic Oppenheimer-Snyder collapse model necessarily produces a black hole with a Schwarzschild singularity at the centre, we show here that there are classes of regular initial conditions for which the collapse gives rise to a RBH.

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arXiv2025

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.

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arXiv2025

Tipler Naked Singularities in $N$ Dimensions

A spacetime singularity, identified by the existence of incomplete causal geodesics in the spacetime, is called a (Tipler) strong curvature singularity if the volume form acting on independent Jacobi fields along causal geodesics vanishes in the approach of the singularity. It is called naked if at least one of these causal geodesics is past incomplete. Here, we study the formation of strong curvature naked singularities arising from spherically symmetric gravitational collapse of general type-I matter fields in an arbitrarily finite number of dimensions. In the spirit of Joshi and Dwivedi [26], and Goswami and Joshi [31], we first construct regular initial data in terms of matter variables and geometric quantities, subject to the dominant and null energy conditions. Using this initial data, we derive two distinct (but not mutually exclusive) conditions, which we call the positive root condition (PRC) and the simple positive root condition (SPRC), that serve as necessary and sufficient conditions, respectively, for the existence of naked singularities. In doing so, we generalize the results of [26] and [31]. We further restrict the PRC and the SPRC by imposing the curvature growth condition (CGC) of Clarke and Krolak [24] on all causal curves that satisfy the causal convergence condition. The CGC gives a sufficient condition for the naked singularities implying the PRC and implied by the SPRC, to be of strong curvature type; thereby also implying the $C^2$ inextendibility of the spacetime. Using the CGC, we extend the results of [28] (that hold for dimension $N=4$) to the case $N=5$, showing that strong curvature naked singularities can occur in this case. However, for the case $N\geq6$, we show that past-incomplete causal curves that identify naked singularities do not satisfy the CGC. These results shed light on the validity of the cosmic censorship conjectures in arbitrary dimensions.

gr-qc↗

arXiv2025

Retrograde Precession of Relativistic Orbits and the Quest for Charged Black Holes

The S-stars around the center of the milky way galaxy provide us with detailed information about the nature of the supermassive compact object Sagittarius A* (Sgr A*). In this work, we derive the fully relativistic orbit equations for the case of the Reissner-Nordström (RN) and Kerr-Newman spacetimes. We solve these orbit equations numerically to analyze the periastron shift of relativistic orbits. We show that retrograde precession (or negative precession) of timelike bound orbits is possible in the case of naked singularity arising from these spacetimes. We have then compared our results with the non-charged Schwarzschild and Kerr spacetimes. This theoretical analysis of relativistic orbits would be helpful in either confirming or ruling out such charged black holes and naked singularities through the future trajectories of S-stars and will also help us constrain the geometry of Sgr A*.

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arXiv2024

Primordial naked singularities

Primordial black hole formation has been discussed widely, when density perturbations in the early universe cause matter to collapse gravitationally, giving rise to these ultra-compact objects. We propose and point out that such a gravitational collapse would also give rise to primordial naked singularities, that would play an important role in the observable features of present universe. We consider two types of collapse scenarios that give rise to event-like and object-like visible singularities within a cosmological background. We briefly discuss implications of primordial naked singularities, including those for dark matter, vis-a-vis primordial black holes.

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arXiv2024

Relativistic time delay analysis of pulsar signals near ultra-compact objects

The upcoming discoveries of pulsars orbiting the center of the Milky Way will present unparalleled opportunities to examine the causal structure of the spacetime geometry of Sagittarius A*. In this paper, we investigate the fully relativistic propagation time delay of pulsar signals in the Joshi-Malafarina-Narayan (JMN-1) and Janis-Newman-Winicour (JNW) spacetimes. This delay arises basically from the spacetime curvatures in the vicinity of these ultra-compact objects, induced by the intense gravitational field near the Galactic Center (GC). Using the principles of gravitational lensing, we compute the arrival time of photons originating from a pulsar in orbit around the GC. To validate our approach, we compare our time delay analysis of the Schwarzschild black hole with the corresponding delay in the post-Newtonian framework. Subsequently, we find that the propagation time of pulsar signal is greater and lesser for the given horizon-less ultra-compact objects for direct and indirect propagation respectively. Therefore, our results suggest quite significant propagation time delay differences in JMN-1 and JNW spacetimes, when compared to the Schwarzschild black hole case. This can be inferred as a possible distinguishing feature for these ultra-compact objects' geometries.

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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↗

arXiv2024

Tidal forces in collapsing compact objects

In this work, we investigate tidal forces in the Lemaitre-Tolman-Bondi (LTB) metric, focusing on both hidden and locally visible singularities. We discuss the strength of these singularities in terms of deformationally strong singularities. Specifically, we analyze tidal forces in LTB spacetime, calculating radial and angular tidal forces and Jacobi fields for the radially co-moving shell. To provide a comparative study, we consider both homogeneous and inhomogeneous cases. The matter field distribution at one-time slice can differ significantly from another, highlighting the potential for time-dependent tidal deformation as a distinct observational signature. We focus on a specific feature: the time-varying maximum of stretching in the radial tidal force, which we term the "critical tidal boundary." In the inhomogeneous case, close to singularity time ($t<t_{s}$), the magnitudes of tidal forces vary substantially, with significant differences in compressive and stretching forces within a small physical radius $R(t,r)$. The resulting singularity in the LTB metrics at the end state of gravitational collapse appears to be an Ori-strong singularity, characterized by infinite tidal deformation.

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