Search Results (31)

By “nethotron”, from the ancient Greek verb for “to spin”, it is meant here a natural or artificial rotating object, like a pulsar or an artificial satellite, whose rotational axis is cumulatively displaced by the post-Newtonian static (gravitoelectric) and stationary (gravitomagnetic) components of the gravitational field of some massive body around which it freely moves. Until now, both relativistic effects have been measured only by the dedicated space-based mission Gravity Probe B in the terrestrial environment. It detected the gravitoelectric de Sitter and gravitomagnetic Pugh–Schiff spin precessions of four superconducting gyroscopes accumulated within a year after about 50 years from conception to completion of data analysis at a cost of 750 million US dollars to $0.3$ and 19 percent accuracy, respectively. The perspectives to measure them with Earth’s long-lived laser-ranged geodetic satellites, like those of the LAGEOS family or possibly one or more of them to be built specifically from scratch, and pulsars orbiting the supermassive black hole in the Galactic Centre, yet to be discovered, are preliminarily investigated. The double pulsar PSR J0737-3039A/B is examined as well. Full article

We investigate the evolution of a set of viscous rings, solving a diffusion-like evolution equation in the (Keplerian disk) Newtonian regime. The Lynden-Bell and Pringle approach for a single disk regime is applied to a disk with a ring profile mimicking a set of orbiting viscous rings. We discuss the time evolution of the disk, adopting different initial wavy (ringed) density profiles. Four different stages of the ring-cluster evolution are distinguished. In the second part of this analysis, we also explore the general relativistic framework by investigating the time evolution of composed systems of general relativistic co-rotating and counter-rotating equatorial disks orbiting a central Kerr black hole for faster spinning and slowly spinning black holes. In the sideline of this analysis, we consider a modified viscosity prescription mimicking an effective viscosity in the general relativistic ring interspace acting in the early phases of the rings’ evolutions, exploring the double system dynamics. Each ring of the separate sequence spreads inside the cluster modifying its inner structure following the rings merging. As the original ringed structure disappears, a single disk appears. The final configuration has a (well-defined) density peak, and its evolution turns in the final stages are dominated by its activity at the inner edge. Full article

The relativistic jets of gamma-ray bursts (GRBs) might be powered by a black-hole (BH) hyperaccretion system. The inherent asymmetry in these jets generates recoil forces, inducing oscillations and positional deviations of the BH from equilibrium. In this study, we explore the influence of different initial BH mass, spin, and mass accretion rate, as well as their evolutions on the dynamical properties of BH under the effect of asymmetric jets. Our results reveal that the initial mass and accretion rate significantly impact the BH’s acceleration, velocity, and displacement, while the different initial spin plays a negligible role in shaping the overall dynamical evolution. Additionally, we calculate the gravitational wave (GW) strains associated with the asymmetric jets, finding that the resulting GW signals are too weak to be detected, even for nearby GRBs. These findings provide critical insights into the dynamical response of BHs to asymmetric jets and the associated GW radiation, advancing our understanding of BH physics in GRBs. Full article

In this study, we investigate the properties of a thin accretion disk around a static spherically symmetric black hole in 4D Einstein–Gauss–Bonnet gravity, with an additional coupling constant, $\alpha$, appearing in the spacetime metric. Using the Novikov–Thorne accretion disk model, we examine the thermal properties of the disk, finding that increasing $\alpha$ reduces the energy, angular momentum, and effective potential of a test particle orbiting the black hole. We demonstrate that $\alpha$ can mimic the spin of a Kerr black hole in general relativity up to $a\simeq$ 0.23 M for the maximum value of $\alpha$. Our analysis of the thermal radiation flux shows that larger $\alpha$ values increase the flux and shift its maximum towards the central black hole, while far from the black hole, the solution recovers the Schwarzschild limit. The impact of $\alpha$ on the radiative efficiency of the disk is weak but can slightly alter it. Assuming black-body radiation, we observe that the disk’s temperature peaks near its inner edge and is higher for larger $\alpha$ values. Lastly, the electromagnetic spectra reveal that the disk’s luminosity is lower in Einstein–Gauss–Bonnet gravity compared to general relativity, with the peak luminosity shifting toward higher frequencies, corresponding to the soft X-ray band as $\alpha$ increases. Full article

Extreme-mass-ratio inspirals (EMRIs) are promising gravitational-wave (GW) sources for space-based GW detectors. EMRI signals typically have long durations, ranging from several months to several years, necessitating highly accurate GW signal templates for detection. In most waveform models, compact objects in EMRIs are treated as test particles without accounting for their spin, mass quadrupole, or tidal deformation. In this study, we simulate GW signals from EMRIs by incorporating the spin and mass quadrupole moments of the compact objects. We evaluate the accuracy of parameter estimation for these simulated waveforms using the Fisher Information Matrix (FIM) and find that the spin, tidal-induced quadruple, and spin-induced quadruple can all be measured with precision ranging from ${10}^{-2}$ to ${10}^{-1}$, particularly for a mass ratio of ∼${10}^{-4}$. Assuming the “true” GW signals originate from an extended body inspiraling into a supermassive black hole, we compute the signal-to-noise ratio (SNR) and Bayes factors between a test-particle waveform template and our model, which includes the spin and quadrupole of the compact object. Our results show that the spin of compact objects can produce detectable deviations in the waveforms across all object types, while tidal-induced quadrupoles are only significant for white dwarfs, especially in cases approaching an intermediate-mass ratio. Spin-induced quadrupoles, however, have negligible effects on the waveforms. Therefore, our findings suggest that it is possible to distinguish primordial black holes from white dwarfs, and, under certain conditions, neutron stars can also be differentiated from primordial black holes. Full article

Black hole X-ray binary systems consist of a black hole accreting mass from its binary companion, forming an accretion disk. As a result, twin relativistic plasma ejections (jets) are launched towards opposite and perpendicular directions. Moreover, multiple broadband emission observations from X-ray binary systems range from radio to high-energy gamma rays. The emission mechanisms exhibit thermal origins from the disk, stellar companion, and non-thermal jet-related components (i.e., synchrotron emission, inverse comptonization of less energetic photons, etc.). In many attempts at fitting the emitted spectra, a static black hole is often assumed regarding the accretion disk modeling, ignoring the Kerr metric properties that significantly impact the geometry around the usually rotating black hole. In this work, we study the possible implications of the spin inclusion in predictions of the X-ray binary spectrum. We mainly focus on the most significant aspect inserted by the Kerr geometry, the innermost stable circular orbit radius dictating the disk’s inner boundary. The outcome suggests a higher-peaked and hardened X-ray spectrum from the accretion disk and a substantial increase in the inverse Compton component of disk-originated photons. Jet-photon absorption is also heavily affected at higher energy regimes dominated by hadron-induced emission mechanisms. Nevertheless, a complete investigation requires the full examination of the spin contribution and the resulting relativistic effects beyond the disk truncation. Full article

To the first post–Newtonian order, the gravitational action of mass–energy currents is encoded by the off–diagonal gravitomagnetic components of the spacetime metric tensor. If they are time–dependent, a further acceleration enters the equations of motion of a moving test particle. Let the source of the gravitational field be an isolated, massive body rigidly rotating whose spin angular momentum experiences a slow precessional motion. The impact of the aforementioned acceleration on the orbital motion of a test particle is analytically worked out in full generality. The resulting averaged rates of change are valid for any orbital configuration of the satellite; furthermore, they hold for an arbitrary orientation of the precessional velocity vector of the spin of the central object. In general, all the orbital elements, with the exception of the mean anomaly at epoch, undergo nonvanishing long–term variations which, in the case of the Juno spacecraft currently orbiting Jupiter and the double pulsar PSR J0737–3039 A/B turn out to be quite small. Such effects might become much more relevant in a star–supermassive black hole scenario; as an example, the relative change of the semimajor axis of a putative test particle orbiting a Kerr black hole as massive as the one at the Galactic Centre at, say, 100 Schwarzschild radii may amount up to about $7\%$ per year if the hole’s spin precessional frequency is $10\%$ of the particle’s orbital one. Full article

A black hole’s spin effects on the jet emissions of high-energy neutrinos and $\gamma$-rays from black hole X-ray binary systems (BHXRBs) are investigated. The BHXRBs consist of a stellar black hole, a companion (donor) star, a BH accretion disk, a BH corona, and two jets emitted from the black hole perpendicular to the accretion disk. For their description, properties of the accretion disk, specifically the accretion disk’s inner radius $R_{in}$ and the accretion disk’s temperature profile $T\left(R\right)$, play key roles since they depend on the black hole’s dimensionless spin parameter ${\alpha }_{\ast }$. In this work, we focus on the main reaction mechanisms taking place inside jets from which high-energy $\gamma$-rays and neutrinos are created. The intensities and integral fluxes of neutrinos and $\gamma$-rays are obtained by integrating the respective source functions. Lastly, the $\gamma$-ray absorption due to $e^{-}$-$e^{+}$ pair production is considered, particularly absorption from the accretion disk. For concrete applications, we have chosen the BHXRB systems MAXI J1820+070, XTE J1550-564, and XTE J1859+226. Full article

In the last few decades, galactic stellar black hole X-ray binary systems (BHXRBs) have aroused intense observational and theoretical research efforts specifically focusing on their multi-messenger emissions (radio waves, X-rays, $\gamma$-rays, neutrinos, etc.). In this work, we investigate jet emissions of high-energy neutrinos and gamma-rays created through several hadronic and leptonic processes taking place within the jets. We pay special attention to the effect of the black hole’s spin (Kerr black holes) on the differential fluxes of photons originating from synchrotron emission and inverse Compton scattering and specifically on their absorption due to the accretion disk’s black-body radiation. The black hole’s spin (dimensionless spin parameter $a_{*}$) enters into the calculations through the radius of the innermost circular orbit around the black hole, the $R_{ISCO}$ parameter, assumed to be the inner radius of the accretion disk, which determines its optical depth ${\tau }_{disk}$. In our results, the differential photon fluxes after the absorption effect are depicted as a function of the photon energy in the range $1$GeV $\le E\le {10}^3$GeV. It is worth noting that when the black holes’ spin (${\alpha }_{*}$) increases, the differential photon flux becomes significantly lower. Full article

The origin of the spins of stellar-mass black holes is still controversial, and angular momentum transport inside massive stars is one of the main sources of uncertainty. Here, we apply hierarchical Bayesian inference to derive constraints on spin models from the 59 most confident binary black hole merger events in the third gravitational-wave transient catalogue (GWTC-3). We consider up to five parameters: chirp mass, mass ratio, redshift, effective spin, and precessing spin. For the model selection, we use a set of binary population synthesis simulations spanning drastically different assumptions for black hole spins and natal kicks. In particular, our spin models range from the maximal to minimal efficiency of angular momentum transport in stars. We find that if we include the precessing spin parameter into our analysis, models predicting only vanishingly small spins are in tension with GWTC-3 data. On the other hand, models in which most spins are vanishingly small but that also include a subpopulation of tidally spun-up black holes are a good match to the data. Our results show that the precessing spin parameter has a crucial impact on model selection. Full article

One of the open problems in black hole physics is testing spacetime around black holes through astrophysical observations in the strong field regime. In fact, black holes cannot produce radiation themselves in the electromagnetic spectrum. However, a black hole’s gravity plays an important role in the production of the radiation of the accretion disc around it. One may obtain valuable information from the electromagnetic radiation of accretion discs about the gravitational properties of the spacetime around black holes. In this work, we study particle dynamics in the spacetime of quasi- and non-Schwarzschild black holes. We compare the gravitational effects of the spacetime deformation parameters of both black hole solutions on the innermost stable circular orbit (ISCO) radius, position, energy, and angular momentum of test particles at the ISCO, together with the energy efficiency of the accretion disc in the thin Novikov–Thorn model. Furthermore, we study the frequencies of particle oscillations in the radial and angular directions along circular stable orbits around both deformed black holes. Furthermore, we investigate quasiperiodic oscillations around the black holes in the relativistic precession model. We show the dependence of the deviation parameters on the orbits of twin peak QPOs with the frequency ratio 3:2. In the obtained results, we compare the gravitational effects of deviation parameters with the spin of a rotating Kerr black hole. Finally, we obtain constraints on the values of the deviation parameter of the spacetime around the black hole at the center of the microquasars GRO J1655-40 and GRS 1915-105 and their mass, using the ${\chi }^2$ method. Full article

Taking the generalized uncertainty principle (GUP) into account, we apply the corrected state density to investigate the entropy density, energy density, pressure and equation of state for the perfect relativistic gases of massless particles with an arbitrary spin of s ≤ 2 surrounding a new four-dimensional neutral Gauss–Bonnet black hole. The modifications of these thermodynamic quantities by the gravity correction factor and particle spin are shown, and the expressions have completely different forms from those in flat space-times. For example, the energy density is not proportional to the fourth power of the temperature. In other words, the energy density differs from that of blackbody radiation. The quantum gravity effects reduce these quantities and are proportional to the gravity correction factor. The result that the equation of state is not zero is compatible with the non-vanishing trace of the stress tensor. Full article

The ability of bumblebee gravity models to explain dark energy, which is the phenomenon responsible for the universe’s observed accelerated expansion, is one of their most significant applications. An effect that causes faster expansion can be linked to how much the Lorentz symmetry of our universe is violated. Moreover, since we do not know what generates dark energy, the bumblebee gravity theory seems highly plausible. By utilizing the physical changes happening around a rotating bumblebee black hole (RBBH), we aim to obtain more specific details about the bumblebee black hole’s spacetime and our universe. However, as researched in the literature, slow-spinning RBBH (SRBBH) spacetime, which has a higher accuracy, will be considered instead of general RBBH. To this end, we first employ the Rindler–Ishak method (RIM), which enables us to study how light is bent in the vicinity of a gravitational lens. We evaluate the deflection angle of null geodesics in the equatorial plane of the SRBBH spacetime. Then, we use astrophysical data to see the effect of the Lorentz symmetry breaking (LSB) parameter on the bending angle of light for numerous astrophysical stars and black holes. We also acquire the analytical greybody factors (GFs) and quasinormal modes (QNMs) of the SRBBH. Finally, we visualize and discuss the results obtained in the conclusion section. Full article

We estimated the spin values of the supermassive black holes (SMBHs) of the active galactic nuclei (AGN) for a large set of Narrow Line Seyfert 1 (NLS1) galaxies assuming the inclination angle between the line of sight and the axis of the accretion disk to be approximately 45 degrees. We found that for these objects the spin values are on average less than for the Seyfert 1 galaxies that we studied previously. In addition, we found that the dependencies of the spin on the bolometric luminosity and the SMBH mass are two to three times stronger that for Seyfert 1 galaxies, which could mean that at early stages of evolution NLS1 galaxies either have a low accretion rate or chaotic accretion, while at later stages they have standard disk accretion, which very effectively increases the spin value. Full article

We investigate the dynamics of test particles endowed with both electric charge and a magnetic dipole moment around a Schwarzschild black hole (BH) immersed in an externally asymptotically uniform magnetic field. We further analyse the effective potential and specific angular momentum and energy of the particles. Furthermore, we show that the upper limit for magnetic interaction parameter $\beta$ increases with increasing cyclotron frequency ${\omega }_{\mathrm{B}}$, while the radius of the innermost stable circular orbit (ISCO) for charged test particles decreases for the upper value of $\beta ={\beta }_{\mathrm{upper}}$. Furthermore, we show that the energy efficiency released from the BH increases up to about 90% due to the presence of the magnetic dipole moment of the test particle. We explore a degeneracy between the spin parameter of rotating Kerr BH and the magnetic parameter for the values of the ISCO radius and energy efficiency. We study in detail the centre of mass energy for collisions of charged and magnetized particles in the environment surrounding the Schwarzchild BH. Finally, as an astrophysical application, we explore the magnetized parameter and cyclotron frequency numerically for a rotating magnetized neutron star. Interestingly, we show that the corresponding values of the above-mentioned parameters for the magnetar PSR J1745-2900 that orbits around the supermassive black hole (SMBH) that exists at the centre of the Milky Way galaxy are ${\omega }_{\mathrm{B}}\simeq 5$ and $\beta \simeq 0.67$, respectively, for the magnetic field is about 10 G. Full article