arXiv subjects

Daniele Malafarina

Publications and source records attributed to Daniele Malafarina.

At least 37 records · Page 2Linked to original sources

arXiv2022

Effects of gravitational lensing on neutrino oscillation in $ γ$-spacetime

We study the effects of gravitational lensing on neutrino oscillations in the $γ$-spacetime which describes a static, axially-symmetric and asymptotically flat solution of the Einstein's field equations in vacuum. Using the quantum-mechanical treatment for relativistic neutrinos, we calculate the phase of neutrino oscillations in this spacetime by considering both radial and non-radial propagation. We show the dependence of the oscillation probability on the absolute neutrino masses, which in the two-flavor case also depends upon the sign of mass squared difference, in sharp contrast with the well-known results of vacuum oscillation in flat spacetime. We also show the effects of the deformation parameter $γ$ on neutrino oscillations and reproduce previously known results for the Schwarzschild metric. We then extend these to a more realistic three flavors neutrino scenario and study the effects of the parameter $γ$ and the lightest neutrino mass while using best fit values of neutrino oscillation parameters.

gr-qc↗

arXiv2021

What a difference a quadrupole makes?

We consider some implications of the departure from spherical symmetry for static solutions of the vacuum Einstein's equations describing black hole mimickers. In particular, we investigate how the presence of mass quadrupole moment affects the collision energy of test particles. We show that collision processes in the vicinity of such exotic compact sources with non vanishing mass quadrupole moment may be significantly different from those in the vicinity of black holes.

gr-qc↗

arXiv2021

Accretion disk luminosity for black holes surrounded by dark matter with anisotropic pressure

We investigate the luminosity of the accretion disk for a static black hole surrounded by dark matter with anisotropic pressure. We calculate all basic orbital parameters of test particles in the accretion disk, such as angular velocity, angular momentum, energy and radius of the innermost circular stable orbit as functions of the dark matter density, radial pressure and anisotropic parameter, which establishes the relationship between the radial and tangential pressures. We show that the presence of dark matter with anisotropic pressure makes a noticeable difference in the geometry around a Schwarzschild black hole, affecting the radiative flux, differential luminosity and spectral luminosity of the accretion disk.

astro-ph.HE↗

arXiv2021

Effect of perfect fluid dark matter on particle motion around a static black hole immersed in an external magnetic field

We investigate particle and photon motion in the vicinity of a static and spherically symmetric black hole surrounded by perfect fluid dark matter in the presence of an external asymptotically uniform magnetic field. We determine the radius of the innermost stable circular orbit (ISCO) for charged test particles and the radius for unstable circular photon orbits and show that the effect of the presence of dark matter shrinks the values of ISCO and photon sphere radii. Based on the analysis of the ISCO radius we further show that the combined effects of dark matter and magnetic field can mimic the spin of a Kerr black hole up to $ a/M \approx 0.75-0.8$. Finally, we consider the effect of the presence of dark matter on the center of mass energy of colliding particles in the black hole vicinity. We show that the center of mass energy grows as the value of the dark matter parameter increases. This result, in conjunction with the fact that, in the presence of an external magnetic field, the ISCO radius can become arbitrarily close to the horizon leads to arbitrarily high energy that can be extracted by the collision process, similarly to what is observed in the super-spinning Kerr case.

gr-qc↗

arXiv2021

Particle motion around a static axially symmetric wormhole

We consider the properties of a static axially symmetric wormhole described by an exact solution of Einstein's field equations and investigate how we can distinguish such a hypothetical object from a black hole. To this aim, we explore the motion of test particles and photons in the wormhole's space-time and compare it with the particle dynamics in the well known space-times of Schwarzschild and Kerr black holes. We show that precise simultaneous measurement of test particle motion and photon motion may provide the means to distinguish the wormhole geometry from that of a black hole.

gr-qc↗

arXiv2021

Spinning test particle motion around a traversable wormhole

The motion of spinning test particles around a traversable wormhole is investigated using the Mathisson Papapetrous Dixon equations, which couple the Riemann tensor with the antisymmetric tensor $S^{ab}$, related to the spin of the particle. Hence, we study the effective potential, circular orbits, and innermost stable circular orbit ISCO of spinning particles. We found that the spin affects significantly the location of the ISCO, in contrast with the motion of nonspinning particles, where the ISCO is the same in both the upper and lower universes. On the other hand, since the dynamical fourmomentum and kinematical fourvelocity of the spinning particle are not always parallel, we also consider a superluminal bound on the particles motion. In the case of circular orbits at the ISCO, we found that the motion of particles with an adimensional spin parameter lower greater than $s=-1.5$ $(1.5)$ is forbidden. The spin interaction becomes important for Kerr black hole orbiting super massive wormholes SMWH.

gr-qc↗

arXiv2021

Effects of non-vanishing dark matter pressure in the Milky Way galaxy

We consider the possibility that the Milky Way's dark matter halo possesses a non vanishing equation of state. Consequently, we evaluate the contribution due to the speed of sound, assuming that the dark matter content of the galaxy behaves like a fluid with pressure. In particular, we model the dark matter distribution via an exponential sphere profile in the galactic core, and inner parts of the galaxy whereas we compare the exponential sphere with three widely-used profiles for the halo, i.e. the Einasto, Burkert and Isothermal profile. For the galactic core we also compare the effects due to a dark matter distribution without black hole with the case of a supermassive black hole in vacuum and show that present observations are unable to distinguish them. Finally we investigate the expected experimental signature provided by gravitational lensing due to the presence of dark matter in the core.

astro-ph.GA↗

arXiv2021

Luminosity of accretion disks in compact objects with quadrupole

We consider the circular motion of test particles in the gravitational field of a static and axially-symmetric compact object described by the $q$-metric. To this end, we calculate orbital parameters of test particles on accretion disks such as angular velocity ($Ω$), total energy ($E$), angular momentum ($L$), and radius of the innermost stable circular orbit ($r_{ISCO}$) as functions of the mass ($m$) and quadrupole ($q$) parameters of the source. The radiative flux, differential, and spectral luminosity of the accretion disk, which are quantities that can be experimentally measured, are then explored in detail. The obtained results are compared with the corresponding ones for the Schwarzschild and Kerr black holes in order to establish whether black holes may be distinguished from the $q$-metric via observations of the accretion disk's spectrum.

gr-qc↗

arXiv2021

Can a light ray distinguish charge of a black hole in nonlinear electrodynamics?

It is a well-known fact that light rays do not follow the null geodesics of the space-time in nonlinear electrodynamics; instead, they follow the null geodesics of the so-called effective space-time. Taking this into account, in this paper, we aim to discuss the possibility of distinguishing the type of charge with which the black hole is endowed, via the motion of light rays. The results show that, for any black hole being a charged solution of the field equations of general relativity coupled to the nonlinear electrodynamics, one cannot distinguish the two types of charge (magnetic or electric) through the motion of light rays around it.

gr-qc↗

arXiv2020

On the properties of a deformed extension of the NUT space-time

We consider a class of space-times given by a stationary extension of the Zipoy-Voorhees metric that was found by Halilsoy. We show that the solutions do not describe rotating sources but must be interpreted, similarly to the NUT case, as deformed sources endowed with a gravitomagnetic charge. We show that the Halilsoy family is directly linked to the NUT space-time, which can be obtained in the limit of vanishing deformations. We investigate the motion of test particles and photons in this class of space-times, in particular the innermost stable circular orbits and photon capture radius. Finally we show that this class of solutions possesses a sub-manifold where closed time-like curves are allowed.

gr-qc↗

arXiv2020

Motion of spinning particles in non asymptotically flat spacetimes

The assumption of asymptotic flatness for isolated astrophysical bodies may be considered an approximation when one considers a cosmological context where a cosmological constant or vacuum energy is present. In this framework we study the motion of spinning particles in static, spherically symmetric and asymptotically non-flat spacetimes with repulsive cosmological vacuum energy and quintessential field. Due to the combined effects of gravitational attraction and cosmological repulsion, the region where stable circular orbits are allowed is restricted by an innermost and an outermost stable circular orbits. We show that taking into account the spin of test particles may enlarge or shrink the region of allowed stable circular orbits depending on whether the spin is co-rotating or counter-rotating with the angular momentum of the particles.

gr-qc↗

arXiv2020

Charged particle and epicyclic motions around $4D$ Einstein-Gauss-Bonnet black hole immersed in an external magnetic field

We investigate particle motion in the vicinity of a $4D$ Einstein-Gauss-Bonnet (EGB) black hole immersed in external asymptotically uniform magnetic field. It is well known that magnetic fields can strongly affect charged particle motion in the black hole vicinity due to the Lorenz force. We find that the presence of the Gauss-Bonnet (GB) coupling gives rise to a similar effect, reducing the radius of the innermost stable circular orbit (ISCO) with respect to the purely relativistic Schwarzschild black hole. Further, we consider particle collisions in the black hole vicinity to determine the center of mass energy and show that this energy increases with respect to the Schwarzschild case due to the effect of the GB term. Finally, we consider epicyclic motion and its frequencies and resonance as a mean to test the predictions of the model against astrophysical observations. In particular we test which values of the parameters of the theory best fit the 3:2 resonance of high-frequency quasi-periodic oscillations in three low-mass X-ray binaries.

gr-qc↗

arXiv2020

Quasi-harmonic oscillations of charged particles in static axially symmetric space-times immersed in a uniform magnetic field

In continuation of our previous study [Phys. Rev. D 99 (2019) 4, 044012], we investigate the motion of charged particles in the $γ$-metric. We provide some examples of curled trajectories in the equatorial plane and escape trajectories outside the equatorial plane. Finally, we consider harmonic oscillations due to small displacements from stable circular orbits on the equatorial plane and compute the epicyclic frequencies for different values of deformation parameter and magnetic field.

gr-qc↗

arXiv2020

Accretion disk luminosity for black holes surrounded by dark matter

We consider the observational properties of a static black hole space-time immersed in a dark matter envelope. We thus investigate how the modifications to geometry, induced by the presence of dark matter affect the luminosity of the black hole's accretion disk. We show that the same disk's luminosity produced by a black hole in vacuum may be produced by a smaller black hole if surrounded by dark matter under certain conditions. In particular, we demonstrate that the luminosity of the disk is markedly altered by dark matter's presence, suggesting that mass estimation of distant super-massive black holes may be changed if they are immersed in dark matter. We argue that a similar effect holds in more realistic scenarios and we discuss about the refractive index related to dark matter lensing. Hence we show how this may help explain the observed luminosity of super-massive black holes in the early universe.

astro-ph.HE↗

arXiv2020

Dust collapse in 4D Einstein-Gauss-Bonnet gravity

We consider gravitational collapse in the recently proposed 4D limit of Einstein-Gauss-Bonnet gravity. We show that for collapse of a sphere made of homogeneous dust the process is qualitatively similar to the case of pure Einstein's gravity. The singularity forms as the endstate of collapse and it is trapped behind the horizon at all times. However, and differently from Einstein's theory, as a consequence of the Gauss-Bonnet term, the collapsing cloud reaches the singularity with zero velocity, and the time of formation of the singularity is delayed with respect to the pure Einstein case.

gr-qc↗

arXiv2020

A toy model for a baby universe inside a black hole

We present a dynamical toy model for an expanding universe inside a black hole. The model is built by matching a spherically symmetric collapsing matter cloud to an expanding Friedmann-Robertson-Walker universe through a phase transition that occurs in the quantum-gravity dominated region, here modeled with semi-classical corrections at high density. The matching is performed on a space-like hyper-surface identified by the co-moving time at which quantum-gravity induced effects halt collapse. The purpose of the model is to suggest a possible reconciliation between the observation that black holes are well described by the classical solutions and the fact that the theoretical resolution of space-time singularities leads to a bounce for the collapsing matter.

gr-qc↗

arXiv2020

Hamiltonian formulation of dust cloud collapse

We consider the gravitational collapse of self-gravitating spherical dust cloud in the Hamiltonian formalism. We address both homogeneous and inhomogeneous cases. Our novel derivation of the Hamiltonian of the system is based on the improved variational principle that was proposed in \cite{KMM}. The present derivation differs from usual treatments due to the presence of an extra boundary term added to the Hilbert action. As expected, the standard equations of motion are retrieved. However, differently from other treatments, the total Hamiltonian obtained with our procedure in the Schwarzschild time-gauge is identical to the total mass of the system as measured from infinity, as it would be expected. Implications for the quantization of the system are suggested.

gr-qc↗

arXiv2019

Spinning test particles in the $γ$ spacetime

We consider the motion of spinning particles in the field of a well known vacuum static axially-symmetric spacetime, known as $γ$ metric, that can be interpreted as a generalization of the Schwarzschild manifold to include prolate or oblate deformations. We derive the equations of motion for spinning test particles by using the Mathisson-Papapetrou-Dixon equations together with the Tulczyjew spin-supplementary condition, and restricting the motion to the equatorial plane. We determine the limit imposed by super-luminal velocity for the spin of the particle located at the innermost stable circular orbits (ISCO). We show that the particles on ISCO of the prolate $γ$ spacetime are allowed to have nigher spin than the corresponding ones in the the oblate case. We determine the value of the ISCO radius depending on the signature of the spin-angular momentum, ${\rm s-L}$ relation, and show that the value of the ISCO with respect to the non spinning case is bigger for ${\rm sL}<0$ and smaller for ${\rm sL}>0$. The results may be relevant for determining the properties of accretion disks and constraining the allowed values of quadrupole moments of astrophysical black hole candidates.

gr-qc↗