Sudipta Hensh

By taking into account the latest observations and theoretical constraints, we investigate the merger and post-merger of binary neutron stars (NSs) with general-relativistic numerical simulations employing hadronic and hybrid equations of state (EOSs). We name our hybrid stars neutron-quark stars (NQS), because the transition from hadrons to quarks starts at a density lower than the central density of ∼1M⊙ stars. We address two viable scenarios for the transition to quark matter: a crossover or a strong first-order phase transition (1PT). We find that a crossover transition is in principle observable when both the inspiral and post-merger signals are detected because the post-merger gravitational-wave (GW) main frequency f2 is generally lower than that of hadronic models with the same tidal deformability (Λ). Since it is viable according to current multi-messenger constraints, we also highlight the possibility of an EOS with a strong 1PT that takes place at 1.8 times the nuclear saturation density, with a stiff quark EOS after the transition. It is the first time that mergers of binary NQSs with a deconfined quark-matter core are studied numerically in full general relativity. In this case, although (Λ, compactness) lies significantly outside the hadronic relation, (Λ, f2) is close to the relation valid for hadronic EOSs. We also point out a linear correlation, valid within the observational constraints and not sensitive to the presence of a hadron-quark transition, between the emitted energy in GWs and their frequency.

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 specialized to theories with a nonvanishing (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.

The light curve of an isolated bright spot in a Keplarian orbit is studied to investigate the signature of the firewall around the event horizon of the black hole. An increase in total observed flux is found. In addition to that, for firewall case comparatively a longer time radiation is observed.

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=6.5×109M⊙ and being at distance d0=16.8Mpc 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.

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=constduring the evolution, in the second one we assume that the dimensionless tidal charge 𝛽≡𝑏/𝑀2=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 counterrotating accretion always converts a K-N naked singularity into an extreme K-N black hole and that the corotating accretion leads to a 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∈(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 nonstandard outcomes of the corotating accretion imply a transcendence of such naked singularity due to nonlinear gravitational effects.

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 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 nonzero torsion, i.e., if the collapsing dust particles possess intrinsic spin.

We consider electrostatic effect on the fluid distribution of compact star. We modify the energy-momentum tensor including the electric field and current density terms and get a set of hydrostatic equilibrium equations which are an extended version of Tolman-Openheimer-Volkoff (TOV) equations. We expect that solutions of set of hydrostatic equations will lead to a mass-radius relation of the compact star configuration.

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.

We study the optical properties of the Kehagias–Sfetsos (KS) compact objects, characterized by the “Hořava” parameter , in the presence of plasma, considering its homogeneous or power-law density distribution. The strong effects of both “Hořava” parameter 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.