arXiv subjects

Sudipta Hensh

Publications and source records attributed to Sudipta Hensh.

8 recordsLinked to original sources

arXiv2026

Timing of echoes in electromagnetic radiation from hot spots orbiting a Kerr black hole impostor

We investigate the light curve of an isolated bright spot in a Keplerian orbit around a Kerr black hole candidate to probe possible deviations from the standard event-horizon picture. In particular, we explore the presence of a reflective surface, namely a "mirror" near the horizon, and its observable consequences. We present estimates on the time delays of the echoes of electromagnetic radiation from the mirror or some other reflecting area related to black hole mimickers. Our results show that photon reflections from such a surface imprint distinct late-time features in the light curve, producing an electromagnetic analogue of gravitational wave echoes. For lower values of the spin, the reflected signal appears within the same orbital period and leads to a noticeable enhancement of the flux. In contrast, for higher spins, the effect emerges only after several orbits, giving rise to a characteristic echo-wave train. These features provide a potential observational signature of horizon-scale modifications in compact objects.

gr-qc↗

arXiv2025

Neutron-quark stars: Discerning viable alternatives for the higher-density part of the equation of state of compact stars

We investigate binary neutron star (BNS) mergers using general-relativistic numerical simulations with hadronic and hybrid equations of state (EOSs), incorporating the latest observations and theoretical constraints. We address two viable scenarios for the transition to quark matter: a quark-hadron crossover (QHC) or a strong first-order phase transition (1PT). To distinguish between different models, we define neutron-quark stars (NQS) as configurations where quark effects emerge at masses below the lowest observed neutron-star mass. While traditional "hybrid stars" may be distinguished by purely hadronic configurations through mass-radius measurements, the mass-radius relations of NQSs resemble those of purely hadronic models, with no sharp boundary between hadrons and quarks. The name NQS effectively captures the absence of a phase boundary between hadrons and quarks in QHC scenarios. Our results indicate that QHC models can be distinguished from hadronic ones if both the inspiral and postmerger gravitational waves (GWs) are observed. In particular, the dominant postmerger frequency ($f_2$) tends to be lower than in hadronic models with the same tidal deformability ($Λ$). We also present the first general-relativistic simulations of BNS mergers where the stars already contain quark matter before merging. These involve a strong first-order phase transition (1PT) at 1.8 times nuclear saturation density, followed by a stiff quark EOS. Finally, we identify a robust linear correlation between the total GW energy emitted after the merger and the $f_2$ frequency. Remarkably, this relation holds regardless of the quark presence.

astro-ph.HE↗

arXiv2023

Tidal heating in a Riemann-Cartan spacetime

We consider possible perturbations of the black hole event horizon induced by matter with spin, extending the derivation of the Hawking-Hartle formula (tidal heating) in the presence of torsion. When specialised to theories with a non-vanishing (pseudo-)traceless component of the (con)torsion tensor, we remarkably find that the tidal heating phenomenon gets modified by additional torsion-dependent terms, in agreement with previous investigations based on Jacobson's spacetime thermodynamics approach. These results lead to relevant phenomenological and theoretical consequences: modifications in the Hawking-Hartle term change the Bondi mass associated with the gravitational radiation observed at infinity, and modify the Hawking radiation spectrum of evaporating black holes.

gr-qc↗

arXiv2022

Evolution of Braneworld Kerr-Newman Naked Singularities

We study evolution of the braneworld Kerr--Newman (K-N) naked singularities, namely their mass $M$ , spin $a$, and tidal charge $b$ characterizing the role of the bulk space, due to matter in-falling from Keplerian accretion disk. We construct the evolution in two limiting cases applied to the tidal charge. In the first case we assume $b$ = const during the evolution, in the second one we assume that the dimensionless tidal charge $β\equiv b/M^2$ = const. For positive values of the tidal charge the evolution is equivalent to the case of the standard K-N naked singularity under accretion of electrically neutral matter. We demonstrate that counter-rotating accretion always converts a K-N naked singularity into an extreme K-N black hole and that the corotating accretion leads to variety of outcomes. The conversion to an extreme K-N black hole is possible for naked singularity with dimensionless tidal charge $β< 0.25$, and $β\in (0.25, 1)$ with sufficiently low spin. In other cases the accretion ends in a transcendental state. For $0.25 < β< 1$ this is a mining unstable K-N naked singularity enabling formally unlimited energy extraction from the naked singularity. In the case of $β> 1$, the corotating accretion creates unlimited torodial structure of mater orbiting the naked singularity. Both non-standard outcomes of the corotating accretion imply a transcendence of such naked singularity due to nonlinear gravitational effects.

gr-qc↗

arXiv2021

Strong gravitational lensing around Kehagias-Sfetsos compact objects surrounded by plasma

We present the analysis how Hořava gravity and plasma influence the strong lensing phenomena around Kehagias-Sfetsos (KS) black holes. Using the semi-analytical Bozza method of strong lensing limit, we determine the multiple images, namely their separation S, and magnification R. We apply our calculations to the case of supermassive black hole having mass $M=6.5\times 10^{9}\textrm{M}_\odot$ and being at distance $d_0=16.8\textrm{Mpc}$ from observer corresponding to those observed in M87. We show that the sensitivity of image magnification, image separation, and shadow angular size on KS parameter $ω$ and plasma parameter $k$ are of order from $1\%$ to $10\%$ for $R$ and $~\,16\%$ for $S$.

gr-qc↗

arXiv2021

Raychaudhuri equations and gravitational collapse in Einstein-Cartan theory

The Raychaudhuri equations for the expansion, shear and vorticity are generalized in a spacetime with torsion for timelike as well as null congruences. These equations are purely geometrical like the original Raychaudhuri equations and could be reduced to them when there is no torsion. Using the Einstein-Cartan-Sciama-Kibble field equations the effective stress-energy tensor is derived. We also consider an Oppenheimer-Snyder model for the gravitational collapse of dust. It is shown that the null energy condition (NEC) is violated before the density of the collapsing dust reaches the Planck density, hinting that the spacetime singularity may be avoided if there is a non-zero torsion,i.e. if the collapsing dust particles possess intrinsic spin.

gr-qc↗

arXiv2019

Anisotropic Tolman VII solution by gravitational decoupling

Using the gravitational decoupling by the minimal geometric deformation approach, we build an anisotropic version of the well-known Tolman VII solution, determining an exact and physically acceptable interior two-fluid solution that can represent behavior of compact objects. Comparison of the effective density and density of the perfect fluid is demonstrated explicitly. We show that the radial and tangential pressure are different in magnitude giving thus the anisotropy of the modified Tolman VII solution. The dependence of the anisotropy on the coupling constant is also shown.

gr-qc↗

arXiv2019

Gravitational lensing around Kehagias-Sfetsos compact objects surrounded by plasma

We study the optical properties of the Kehagias-Sfetsos (KS) compact objects, characterized by the "Hořava" parameter $ω_{_{KS}}$, in the presence of plasma, considering its homogeneous or power-law density distribution. The strong effects of both "Hořava" parameter $ω_{_{KS}}$ and plasma on the shadow cast by the KS compact objects are demonstrated. Using the weak field approximation, we investigate the gravitational lensing effect. Strong dependence of the deflection angle of the light on both the "Hořava" and plasma parameter is explicitly shown. The magnification of image source due to the weak gravitational lensing is given for both the homogeneous and inhomogeneous plasma.

gr-qc↗