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We analyse, using new analytical models and numerical general relativistic magnetohydrodynamic simulations, the three-dimensional properties of accretion flows inside the plunging region of black hole spacetimes (i.e. at radii smaller than the innermost stable circular orbit). These simulations are of thick discs, with aspect ratios of order unity |$h/r \sim 1$|⁠ , and with a magnetic field geometry given by the standard low-magnetization 'SANE' configuration. This work represents the first step in a wider analysis of this highly relativistic region. We show that analytical expressions derived in the 'thin disc' limit describe the numerical results remarkably well, despite the large aspect ratio of the flow. We further demonstrate that accretion within this region is typically mediated via spiral arms, and that the geometric properties of these spiral structures can be understood with a simple analytical model. These results highlight how accretion within the plunging region is fundamentally two-dimensional in character, which may have a number of observational implications. We derive a modified theoretical description of the pressure within the plunging region which accounts for turbulent heating and may be of use to black hole image modelling. [ABSTRACT FROM AUTHOR]

This paper presents new relativistic composite polytropic models for compact stars by simultaneously solving Einstein field equations with the polytropic state equation to simulate the spherically symmetric, static matter distribution. Using a non-uniform polytropic index, we get the Tolman–Oppenheimer–Volkoff equation for the relativistic composite polytrope (CTOV). To analyze the star's structure, we numerically solve the CTOV equation and compute the Emden and mass functions for various relativistic parameters and polytropic indices appropriate for neutron stars. The calculation results show that, as the relativistic parameter approaches zero, we recover the well-known Lane-Emden equation from the Newtonian theory of polytropic stars; thus, testing the computational code by comparing composite Newtonian models to those in the literature yields good agreement. We compute composite relativistic models for the neutron star candidates Cen X-3, SAXJ1808.4-3658, and PSR J1614-22304. We compare the findings with various existing models in the literature. Based on the accepted models for PSR J1614-22304 and Cen X-3, the star's core radius is predicted to be between 50 and 60% percent of its total radius, while we found that the radius of the core of star SAXJ1808.4-3658 is around 30% of the total radius. Our findings show that the neutron star structure may be approximated by a composite relativistic polytrope, resulting in masses and radii that are quite consistent with observation. [ABSTRACT FROM AUTHOR]

Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to β = v / c > 0.6. In a preceding paper, we derived an analytic description of the time-dependent radio to X-ray synchrotron flux produced by collision-less shocks driven by such fast ejecta into the interstellar medium, for spherical ejecta with broken power-law mass (or energy) distributions, M (> γβ)∝(γβ)− s with s = s KN at γβ < γ0β0 and s = s ft at γβ > γ0β0 (where γ is the Lorentz factor). Here, we extend our analysis and provide analytic expressions for the self-absorption frequency, the cooling frequency, and the observed angular size of the emitting region (which appears as a ring in the sky). For parameter values characteristic of merger calculation results – a 'shallow' mass distribution, 1 < s KN < 3, for the bulk of the ejecta (at γβ ≈ 0.2), and a steep, s ft > 5, 'fast tail' mass distribution – the analytic results reproduce well (to tens of per cent accuracy) the results of detailed numeric calculations, a significant improvement over earlier order-of-magnitude estimates (based on extrapolations of results valid for γβ ≪ 1). [ABSTRACT FROM AUTHOR]

Polarized general-relativistic radiative transfer in the vicinity of black holes and other compact objects has become a crucial tool for probing the properties of relativistic astrophysics plasmas. Instruments like GRAVITY, the Event Horizon telescope, Atacama Large Millimeter/submillimeter Array, or Imaging x-ray Polarimetry Explorer make it very timely to develop such numerical frameworks. In this article, we present the polarized extension of the public ray-tracing code Gyoto, and offer a python notebook allowing to easily perform a first realistic computation. The code is very modular and allows to conveniently add extensions for the specific needs of the user. It is agnostic about the spacetime and can be used for arbitrary compact objects. We demonstrate the validity of the code by providing tests, and show in particular a perfect agreement with the ipole code. Our article also aims at pedagogically introducing all the relevant formalism in a self-contained manner. [ABSTRACT FROM AUTHOR]

Every numerical general relativistic investigation starts from the solution of the initial value equations at a given time. Astrophysically relevant initial values for different systems lead to distinct sets of equations that obey specific assumptions tied to the particular problem. Therefore, a robust and efficient solver for a variety of strongly gravitating sources is needed. In this work, we present the OpenMP version of the Compact Object CALculator (COCAL) on shared memory processors. We performed extensive profiling of the core COCAL modules in order to identify bottlenecks in efficiency, which we addressed. Using modest resources, the new parallel code achieves speedups of approximately one order of magnitude relative to the original serial COCAL code, which is crucial for parameter studies of computationally expensive systems such as magnetized neutron stars, as well as its further development towards more realistic scenarios. As a novel example of our new code, we compute a binary quark system where each companion has a dimensionless spin of 0.43 aligned with the orbital angular momentum. [ABSTRACT FROM AUTHOR]

In the 1960s, the remnants of supernova explosions (SNRs) were indicated as a possible source of galactic cosmic rays through the Diffusive Shock Acceleration (DSA) mechanism. Since then, the observation of gamma-ray emission from relativistic ions in these objects has been one of the main goals of high-energy astrophysics. A few dozen SNRs have been detected at GeV and TeV photon energies in the last two decades. However, these observations have shown a complex phenomenology that is not easy to reduce to the standard paradigm based on DSA acceleration. Although the understanding of these objects has greatly increased, and their nature as efficient electron and proton accelerators has been observed, it remains to be clarified whether these objects are the main contributors to galactic cosmic rays. Here, we review the observations of γ -ray emission from SNRs and the perspectives for the future. [ABSTRACT FROM AUTHOR]

The Landau states of electrons with orbital angular momentum in magnetic fields are important in the quantum theories of metals and of synchrotron radiation at storage rings, in relativistic astrophysics of neutron stars, and in many other areas. In realistic scenarios, electrons are often born inside the field or injected from a field-free region, requiring nonstationary quantum states to account for boundary or initial conditions. This study presents nonstationary Laguerre–Gaussian (NSLG) states in a longitudinal magnetic field, characterizing vortex electrons after their transfer from vacuum to the field. Comparisons with Landau states and calculations of observables such as mean energy and root-mean-square (r.m.s.) radius show that the r.m.s. radius of the electron packet in the NSLG state oscillates in time around a significantly larger value than that of the Landau state. This quantum effect of oscillations is due to boundary conditions and can potentially be observed in various problems, particularly when using magnetic lenses of electron microscopes and linear accelerators. Analogies are drawn between a quantum wave packet and a classical beam of many particles in phase space, including the calculation of mean emittance of the NSLG state as a measure of its quantum nature. [ABSTRACT FROM AUTHOR]

In this paper, we study the relativistic correction to Bekenstein–Hawking entropy in the canonical ensemble and isothermal–isobaric ensemble and apply it to the cases of non-rotating BTZ and AdS-Schwarzschild black holes. This is realized by generalizing the equations obtained using Boltzmann–Gibbs (BG) statistics with its relativistic generalization, Kaniadakis statistics, or κ -statistics. The relativistic corrections are found to be logarithmic in nature and it is observed that their effect becomes appreciable in the high-temperature limit suggesting that the entropy corrections must include these relativistically corrected terms while taking the aforementioned limit. The non-relativistic corrections are recovered in the κ → 0 limit. [ABSTRACT FROM AUTHOR]

In this study, we investigate the literature on anisotropic stellar spheres in f (R , ϕ) gravity utilizing Tolman–Kuchowicz (TK) spacetime. Due to the existence of anisotropic matter distribution, we derive the equations of motion for spherically symmetric spacetime by taking into account physically accurate formulations of the metric potentials. For this purpose, we develop the matching conditions by comparing them with the exterior solution to determine the TK constants. We have also discussed the graphical analysis of energy density, pressure components, equation of state components, equilibrium condition, and stability analysis. Moreover, other properties of a compact star, like the mass–radius function, compactness, and redshift function, have also been investigated. It is also noticed that energy conditions are satisfied, which justifies that our considered stars are viable. The validity of our proposed model has been proven by studying the development of various physical characteristics, and it is determined that our system is physically stable and free of any singularities. [ABSTRACT FROM AUTHOR]

We implement general relativistic hydrodynamics in the moving-mesh code arepo. We also couple a solver for the Einstein field equations employing the conformal flatness approximation. The implementation is validated by evolving isolated static neutron stars using a fixed metric or a dynamical space–time. In both tests, the frequencies of the radial oscillation mode match those of independent calculations. We run the first moving-mesh simulation of a neutron star merger. The simulation includes a scheme to adaptively refine or derefine cells and thereby adjusting the local resolution dynamically. The general dynamics are in agreement with independent smoothed particle hydrodynamics and static-mesh simulations of neutron star mergers. Coarsely comparing, we find that dynamical features like the post-merger double-core structure or the quasi-radial oscillation mode persist on longer time scales, possibly reflecting a low numerical diffusivity of our method. Similarly, the post-merger gravitational wave emission shows the same features as observed in simulations with other codes. In particular, the main frequency of the post-merger phase is found to be in good agreement with independent results for the same binary system, while, in comparison, the amplitude of the post-merger gravitational wave signal falls off slower, i.e. the post-merger oscillations are less damped. The successful implementation of general relativistic hydrodynamics in the moving-mesh arepo code, including a dynamical space–time evolution, provides a fundamentally new tool to simulate general relativistic problems in astrophysics. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop the isotropic and anisotropic quark stars configurations in the context of f (T ,) gravity in the static spherically symmetric background. To explore the combined effects of torsion scalar T and the trace of energy–momentum tensor (EMT) on relativistic astrophysics, we use diagonal as well as non-diagonal tetrad fields. By considering the conformal Killing vectors along with the MIT bag model, the interior solutions of the field equations corresponding to the linear f (T ,) = α T (r) + β (r) + ϕ model (in which α , β are the constants and ϕ indicates the cosmological constant) are calculated. The feasibility of the obtained solutions is confirmed by implementing several physical tests. The model parameters are constrained subject to the existence and stability of the quark star models. We formulate the energy constraints, stability equations, mass function, compactness and redshift factor, and present the graphical analysis of all physical quantities. It is found that the derived solutions for both diagonal and non-diagonal tetrad exhibit well-behaved profiles in the framework of modified teleparallel gravity. [ABSTRACT FROM AUTHOR]

Ultrashort electron beams with femtosecond to picosecond bunch durations offer unique opportunities to explore active research areas ranging from ultrafast structural dynamics to ultra‐high dose‐rate radiobiological studies. It presents a straightforward method to generate relativistic electron beams in ambient air via the tight focusing of a few‐cycle, mJ‐class femtosecond infrared laser. It demonstrates experimentally that electrons can reach up to 1.4 MeV at a dose‐rate of 0.15 Gy/s, providing enough dose rate for radiation therapy applications. 3D Particle‐In‐Cell simulations confirm that the acceleration mechanism is based on the relativistic ponderomotive force and show theoretical agreement with the measured electron energies and divergence. Relativistic peak intensities up to 1019 Wcm−2 are reached in ambient air due to a very low B‐integral accumulation during focusing, which prevents intensity clamping. Furthermore, it discusses the scalability of this method with the continuing development of mJ‐class high average power lasers, and providing a promising approach for FLASH radiation therapy. [ABSTRACT FROM AUTHOR]

This paper is devoted to some aspects of the study of relativistic effects (geodetic precession and geodetic nutation, which together constitute geodetic rotation) in the rotation of celestial bodies of the Solar System using the example of Jupiter and its Galilean satellites (Io, Europa, Ganymede, and Callisto). The difference in the angular velocity vectors of geodetic rotation is shown depending on the choice of reference frame. Thus, the absolute value of the angular velocity vector of the geodetic rotation of the satellite under study relative to the barycenter of the Solar System will not coincide with the absolute value of a similar vector of the satellite under study relative to the barycenter of the planet's satellite system. As a result, the most significant secular and periodic terms of geodetic rotation were determined for the first time: a) of Jupiter and its Galilean satellites in terms of the Euler angles, in the perturbing terms of physical libration and in the absolute value of the angular rotation vector of the geodetic rotation of the body under study relative to the barycenter of the Solar System and the plane of the mean orbit of Jupiter of the epoch J2000.0; b) of Galilean satellites of Jupiter in the perturbing terms of physical libration and the absolute value of the angular rotation vector of the geodetic rotation of the studied body relative to the barycenter of the Galilean satellite system of Jupiter and the mean orbit plane of the studied Galilean satellite of the epoch J2000.0. The obtained analytical values of the geodetic rotation of the studied objects can be used for a numerical study of their rotation in the relativistic approximation, and also used to evaluate the influence of relativistic effects on the orbital–rotational dynamics of bodies of exoplanetary systems. [ABSTRACT FROM AUTHOR]

Reproducibility of results is a cornerstone of the scientific method. Scientific computing encounters two challenges when aiming for this goal. Firstly, reproducibility should not depend on details of the runtime environment, such as the compiler version or computing environment, so results are verifiable by third-parties. Secondly, different versions of software code executed in the same runtime environment should produce consistent numerical results for physical quantities. In this manuscript, we test the feasibility of reproducing scientific results obtained using the IllinoisGRMHD code that is part of an open-source community software for simulation in relativistic astrophysics, the Einstein Toolkit. We verify that numerical results of simulating a single isolated neutron star with IllinoisGRMHD can be reproduced, and compare them to results reported by the code authors in 2015. We use two different supercomputers: Expanse at SDSC, and Stampede2 at TACC. By compiling the source code archived along with the paper on both Expanse and Stampede2, we find that IllinoisGRMHD reproduces results published in its announcement paper up to errors comparable to round-off level changes in initial data parameters. We also verify that a current version of IllinoisGRMHD reproduces these results once we account for bug fixes which have occurred since the original publication. [ABSTRACT FROM AUTHOR]

We investigate the influence of parametric magnetic field configurations of a hypermassive neutron star (HMNS) on the outflow properties, nucleosynthesis yields, and kilonova light curves. We perform three-dimensional dynamical space–time general-relativistic magnetohydrodynamic simulations, including a neutrino leakage scheme, microphysical finite-temperature equation of state, and an initial poloidal magnetic field. We find that varying the magnetic field strength and falloff impacts the formation of magnetized winds or mildly relativistic jetted outflows, which in turn has profound effects on the outflow properties. All of the evolved configurations collapse to a black hole ∼38–40 ms after coalescence, where the ones forming jetted outflows seem more effective at redistributing angular momentum, which result in earlier collapse times. Larger mass ejecta rates and radial velocities of unbound material characterize the systems that form jetted outflows. The bolometric light curves of the kilonovae and r -process yields that are produced by the post-merger remnant system change considerably with different magnetic field parameters. We conclude that the magnetic field strength and falloff have robust effects on the outflow properties and electromagnetic observables. This can be particularly important as the total ejecta mass from our simulations (≃10−3 M⊙) makes the ejecta from HMNS a compelling source to power kilonova through radioactive decay of r -process elements. [ABSTRACT FROM AUTHOR]

The aim of the current paper is to apply Bambi's method (Bambi, 2015) to a source which contains two or more simultaneous triads of variability components. The joint x variable that can be composed in this case, unlike some previous studies, allows the goodness of the fit to be tested. It appears that a good fit requires one of the observation groups to be disregarded. Even then, the model's prediction for the mass of the neutron star in the accreting millisecond pulsar IGR J17511-3057 is way too high to be accepted. [ABSTRACT FROM AUTHOR]

We make use of the condition of vanishing complexity, based on the current definition proposed by Herrera (Phys Rev D 97:044010, 2018), to find exact interior solutions to the Einstein equations for describing compact stellar objects. In the framework of general relativity, the complexity factor is an outcome of the orthogonal splitting of the Riemann tensor from which structure scalars are obtained. By using the Vaidya–Tikekar (V–T) metric ansatz (J Astrophys Astron 3:325, 1982) for the spacetime of a static spherically symmetric matter distribution, we model superdense, relativistic stars. The interior spacetime is matched to the exterior Schwarzschild solution across the boundary of the star where the radial pressure vanishes. The physical viability of the model has been tested following the current data corresponding to the pulsar 4 U 1820 - 30 . The stability of the model fulfilled the given criteria, namely the Tolman–Oppenheimer–Volkoff equation, the adiabatic index and the causality conditions. [ABSTRACT FROM AUTHOR]

The numerical study of relativistic magnetohydrodynamics (MHD) plays a crucial role in high-energy astrophysics but unfortunately is computationally demanding, given the complex physics involved (high Lorentz factor flows, extreme magnetization, and curved spacetimes near compact objects) and the large variety of spatial scales needed to resolve turbulent motions. A great benefit comes from the porting of existing codes running on standard processors to GPU-based platforms. However, this usually requires a drastic rewriting of the original code, the use of specific languages like CUDA, and a complex analysis of data management and optimization of parallel processes. Here, we describe the porting of the ECHO code for special and general relativistic MHD to accelerated devices, simply based on native Fortran language built-in constructs, especially do concurrent loops, few OpenACC directives, and straightforward data management provided by the Unified Memory option of NVIDIA compilers. Thanks to these very minor modifications to the original code, the new version of ECHO runs at least 16 times faster on GPU platforms as compared to CPU-based ones. The chosen benchmark is the 3D propagation of a relativistic MHD Alfvén wave, for which strong and weak scaling tests performed on the LEONARDO pre-exascale supercomputer at CINECA are provided (using up to 256 nodes corresponding to 1024 GPUs, and over 14 billion cells). Finally, an example of high-resolution relativistic MHD Alfvénic turbulence simulation is shown, demonstrating the potential for astrophysical plasmas of the new GPU-based version of ECHO. [ABSTRACT FROM AUTHOR]

By using the minimal extension of the standard model and considering the general relativistic effects near the surface of a static neutron star in the presence of quintessence fields, we drive analytical expressions for the cross-section and energy deposition rate of the process ( ν e + ν ¯ e → (W , Z , γ) → e − + e + ). We have also obtained the ratio of energy deposition to total Newtonian energy deposition ( Q ̇ ∕ Q ̇ Newt ). We have displayed that the contributions of the electron neutrino's charge radius are more effective than the dipole moment in this process. The obtained solutions indicate that the contribution of the electron neutrino charge radius changes the results approximately (1–5)% depending on the R M value of the star. [ABSTRACT FROM AUTHOR]

This is a review on recent results on the equations of motion for light rays in a non-magnetised pressure-free plasma on a general-relativistic spacetime. Under the assumption that both the spacetime metric and the plasma density are stationary and axisymmetric, the necessary and sufficient conditions are given for the existence of a (generalised) Carter constant. It is shown how, if these conditions are satisfied, the shadows of a Kerr black hole and of a Schwarzschild black hole can be calculated in a plasma. An example is given for an axisymmetric and static plasma on the Schwarzschild spacetime where circular light rays off the equatorial plane exist. [ABSTRACT FROM AUTHOR]

In this paper, we investigated anisotropic solution for a self-bound compact star model. The solution is obtained via Vaidya–Tikekar metric function 1 / W (r) = K + C r 2 K (1 + C r 2) and the vanishing complexity factor condition. The physical analysis of the solution has been discussed in detail which includes mass-radius ratio, and stability analysis via adiabatic index for a secondary component of the GW190814 event for different values K. Furthermore, we predicted the radii and obtained the compactness factor of different self-bound compact objects for different K. Moreover, the hydrostatic equilibrium condition of the solution is also checked. The physical result shows that the solution so obtained is well behaved and may be useful for modeling the self-bound compact objects as far as observational data is concerned. [ABSTRACT FROM AUTHOR]

We revisit the precessing black hole binary model, a candidate to explain the bizarre quasi-periodic optical flares in OJ-287's light curve, from first principles. We deviate from existing work in three significant ways: (1) Including crucial aspects of relativistic dynamics related to the accretion disc's gravitational moments. (2) Adopting a model-agnostic prescription for the disc's density and scale height. (3) Using Markov chain Monte Carlo methods to recover reliable system parameters and uncertainties. We showcase our model's predictive power by timing the 2019 Eddington flare within 40 h of the observed epoch, exclusively using data available prior to it. Additionally, we obtain a novel direct measurement of OJ-287's disc mass and quadrupole moment exclusively from the optical flare timings. Our improved methodology can uncover previously unstated correlations in the parameter posteriors and patterns in the flare timing uncertainties. According to the model, the 26th optical flare is expected to occur on the 21st of August 2023 ± 32 d, shifted by approximately a year with respect to previous expectations. [ABSTRACT FROM AUTHOR]

Type II Supernova 1987A (SN 1987A), observed in 1987, released an energy of Q ≈ 3 × 10 53 erg. This huge energy is essentially the magnitude of gravitational potential or self-gravitational energy (PE) of a new born cold neutron star having a gravitational compactness or redshift z b ≈ 0.15 . One may wonder what could be the upper limit on the amount of energy that might be released with the formation of a cold Ultra Compact Object (UCO) with an arbitrary high z b . Accordingly, here, for the first time, we obtain an analytical expression for the PE of a homogeneous general relativistic UCO assuming it to be cold and static. It is found that the PE of a homogeneous UCO of mass M may exceed Mc2 and be as large as 1.34 Mc2. This result, though surprising, follows from an exact and correct analytical calculation based on the standard General Theory of Relativity (GTR). Further, UCOs supported by tangential stresses may be inhomogeneous and much more massive than neutron stars with PE ∼ 2.1 Mc2 Thus, in principle, formation of an UCO of a few solar masses ( M ⊙ ) might release an energy Q ∼ 10 55 erg. [ABSTRACT FROM AUTHOR]

Testing gravity theories combining (massive and massless) scalar & electrodynamic fields become the most important issue in relativistic astrophysics using data from, black hole observations. In the present work, we first show a spherically symmetric black hole solution in general relativity coupling to generic-type nonlinear electrodynamics (NED) together with the quintessential field. We also obtain possible values for the parameters of the quintessential field and NED charge in the black hole environment for different values of degree of nonlinearity. Also, event horizon properties and scalar invariants of the black hole spacetime are studied. We investigate the equatorial motion of test particles around the regular-Kiselev black holes and study the combined effects of quintessential field and the NED charge of the black hole on particle angular momentum together with its energy at their circular orbits as well as their innermost circular stable orbits (ISCOs) and compared the obtained results with Reissner-Nordström black hole (RN BH) case. Moreover, we study particle oscillations along the orbits above than ISCO and applications to quasiperiodic oscillations (QPOs) where we obtain constrain values for the quintessential parameter and black hole mass charge parameters using observational QPO data from microquasars. [ABSTRACT FROM AUTHOR]

A set of exact spherically symmetric solutions for describing the interior of a relativistic star in the context of f(R,T)$f(\mathcal {R}, T)$ modified theory of gravity is presented. The existence of a Conformal Killing Vectors (CKV) and Bardeen's model to find the boundary conditions as an exterior space‐time geometry to achieve the metric potentials for current study is used. Moreover, matching conditions have been employed to obtain a set of free parameters for the physical analysis. Furthermore, some physically valid attributes like the behavior of energy density, pressure, equilibrium conditions, the adiabatic index, mass‐radius relation, surface red‐shifts, and compactness factor for the realistic stars such as PSRJ1614−2230$PSR\nobreakspace J1614-2230$, PSRJ1903+327$PSR\nobreakspace J1903+327$, SMCX−1$SMC\nobreakspace X-1$, CenX−3$Cen\nobreakspace X-3$, LMCX−4$LMC\nobreakspace X-4$ and EXO1785−248$EXO\nobreakspace 1785-248$ are investigated. It is observed that for the current investigation, the derived compact star solutions are physically acceptable and authentic in the presence of charge with conformal motion. A set of exact spherically symmetric solutions for describing the interior of a relativistic star in the context of f(R,T)$f(\mathcal {R}, T)$ modified theory of gravity is presented. The existence of a Conformal Killing Vectors (CKV) and Bardeen's model to find the boundary conditions as an exterior space‐time geometry to achieve the metric potentials for current study is used. Moreover, matching conditions have been employed to obtain a set of free parameters for the physical analysis. Furthermore, some physically valid attributes like the behavior of energy density, pressure, equilibrium conditions, the adiabatic index, mass‐radius relation, surface red‐shifts, and compactness factor for the realistic stars such as PSRJ1614−2230$\textit{PSR}\nobreakspace J1614- 2230$, PSRJ1903+327$\textit{PSR}\nobreakspace J1903+327$, SMCX−1$\textit{SMC}\nobreakspace X-1$, CenX−3$\textit{Cen}\nobreakspace X-3$, LMCX−4$\textit{LMC}\nobreakspace X-4$ and EXO1785−248$\textit{EXO}\nobreakspace 1785- 248$ are investigated. It is observed that for the current investigation, the derived compact star solutions are physically acceptable and authentic in the presence of charge with conformal motion. [ABSTRACT FROM AUTHOR]

We discuss possible existence of a magnetodisk around the exoplanet HAT–P–11b. We used available observations to determine properties of the exoplanet and the stellar wind passing by it and obtained a rough estimate of the size of the planet's magnetosphere. Comparing our estimate to published results of computations in a 3D electromagnetic relativistic and collisionless particle-in-cell model of the magnetosphere, we found a discrepancy in the magnetosphere size estimated using these two techniques. A possible interpretation of the discrepancy is suggested. [ABSTRACT FROM AUTHOR]

This note aims at investigating two different situations where the classical general relativistic dynamics compete with the evolution driven by Hawking evaporation. We focus, in particular, on binary systems of black holes emitting gravitational waves and gravitons, and on the cosmological evolution when black holes are immersed in their own radiation bath. Several non-trivial features are underlined in both cases. [ABSTRACT FROM AUTHOR]

Black hole X-ray binaries are ideal environments to study the accretion phenomena in strong gravitational potentials. These systems undergo dramatic accretion state transitions and analysis of the X-ray spectra is used to probe the properties of the accretion disc and its evolution. In this work, we present a systematic investigation of ∼1800 spectra obtained by Rossi X-Ray Timing Explorer Proportional Counter Array observations of GRO J1655−40 and LMC X-3 to explore the nature of the accretion disc via non-relativistic and relativistic disc models describing the thermal emission in black hole X-ray binaries. We demonstrate that the non-relativistic modelling throughout an outburst with the phenomenological multicolour disc model DISKBB yields significantly lower and often unphysical inner disc radii and correspondingly higher (∼50–60 per cent) disc temperatures compared to its relativistic counterparts KYNBB and KERRBB. We obtained the dimensionless spin parameters of a * = 0.774 ± 0.069 and a * = 0.752 ± 0.061 for GRO J1655−40 with KERRBB and KYNBB , respectively. We report a spin value of a * = 0.098 ± 0.063 for LMC X-3 using the updated black hole mass of 6.98 M⊙. Both measurements are consistent with the previous studies. Using our results, we highlight the importance of self-consistent modelling of the thermal emission, especially when estimating the spin with the continuum-fitting method which assumes the disc terminates at the innermost stable circular orbit at all times. [ABSTRACT FROM AUTHOR]

This article shows that the version of the special theory of relativity (STR) presented in all physics textbooks is incorrect, since relativistic formulas obtained therein are incorrect. They are incorrectly explained using the wrong principle of non-exceeding the speed of light and have led to incorrect conclusions about the physical unreality of imaginary numbers and the existence in nature of only our visible universe. This version of the STR proved to be in demand only because its authors were not able to explain physical sense of imaginary numbers. The article provides three proofs of physical reality of imaginary numbers and explains their physical sense in the theory of linear electric circuits, relativistic physics and astrophysics. This made it possible to obtain corrected relativistic formulas, from which appropriate conclusions were drawn. [ABSTRACT FROM AUTHOR]

We developed a numerical methodology to compute the fully relativistic propagation time of photons emitted by a pulsar in orbit around a massive compact object, like the supermassive black hole Sagittarius A* in the Galactic centre, whose gravitational field is described by a generic spherically symmetric space–time. Pulsars at the Galactic centre are usually regarded as the next major precision probe for theories of gravity, filling the current experimental gap between horizon-scale gravity tests and those at larger scales. We retain a completely general approach, which allows us to apply our code to the Schwarzschild space–time (by which we successfully validate our methodology) and to three different well-motivated alternatives to the standard black hole paradigm. The results of our calculations highlight departures spanning several orders of magnitudes in timing residuals, that are supposed to be detectable with future observing facilities like the Square Kilometer Array. [ABSTRACT FROM AUTHOR]

Gravitational lensing is the relativistic effect generated by massive bodies, which bend the space-time surrounding them. It is a deeply investigated topic in astrophysics and allows validating theoretical relativistic results and studying faint astrophysical objects that would not be visible otherwise. In recent years, machine learning methods have been applied to support the analysis of the gravitational lensing phenomena by detecting lensing effects in datasets consisting of images associated with brightness variation time series. However, the state-of-the-art approaches either consider only images and neglect time-series data or achieve relatively low accuracy on the most difficult datasets. This paper introduces DeepGraviLens, a novel multi-modal network that classifies spatio-temporal data belonging to one non-lensed system type and three lensed system types. It surpasses the current state-of-the-art accuracy results by ≈ 3 % to ≈ 11 % , depending on the considered data set. Such an improvement will enable the acceleration of the analysis of lensed objects in upcoming astrophysical surveys, which will exploit the petabytes of data collected, e.g., from the Vera C. Rubin Observatory. [ABSTRACT FROM AUTHOR]

Tolman VII solution (Tolman in Phys Rev 55(4):364, 1939) is an exact analytic solution to the Einstein field equations describing the space-time of a static spherically symmetric distribution of matter. The solution has been shown to be capable of describing the interior of compact objects like neutron stars. Generalized (Raghoonundun and Hobill in Phys Rev D 92(12):124005, 2015) and modified (Jiang and Yagi in Phys Rev D 99(12):124029, 2019) versions of the solution are also available in the literature, which have been subsequently developed to accommodate a wide range of neutron star EOS. The stability of the modified Tolman VII solution has recently been analyzed (Posada et al in Phys Rev D 103(10):104067, 2021), which provides a critical value of the adiabatic index above which the stellar configuration becomes unstable against radial oscillations. In this paper, making use of the generalized version of the Tolman VII solution, we prescribe an upper bound on the compactness (M/R) beyond which the star becomes unstable against radial oscillations (Chandrasekhar in Phys Rev Lett 12(4):114, 1964). Our study brings to attention the role of model parameters in the generalized Tolman VII solution. The analysis also provides new insight into the role of inhomogeneity of the matter distribution vis-a-vis equation of state (EOS) on the compactness of a relativistic star. [ABSTRACT FROM AUTHOR]

In this paper, we review recent and ongoing work by our group on numerical simulations of relativistic jets. Relativistic outflows in astrophysics are related to dilute, high energy plasmas, with physical conditions out of the reach of current laboratory capabilities. Simulations are thus imperative for the study of these objects. We present a number of such scenarios that have been studied by our group at the Universitat de València. In particular, we have focused on the evolution of extragalactic outflows through galactic and intergalactic environments, deceleration by interaction with stars or clouds or the propagation of jets in X-ray binaries and interaction with stellar winds from massive companions. All also share their role as particle acceleration sites and production of non-thermal radiation throughout the electromagnetic spectrum. Therefore, our work is not only aimed at understanding the impact of outflows on their environments and thus their role in galaxy and cluster evolution, but also the nature and capabilities of these sites as generators of high- and very-high-energy radiation and cosmic rays. [ABSTRACT FROM AUTHOR]

Testing gravity theories is an important and interesting issue in relativistic astrophysics using astrophysical observations. In recent years, Event horizon telescope collaboration provided valuable data from shadow of supermassive black holes located at the center of the galaxies Milky Way and Meissner 87, which may help to get information about the spacetime around the black holes. In this work, first, we consider rotating black hole solution for charged black hole with the NUT charge in the presence of clouds of strings, and study the event horizon properties of spacetime around rotating twisting and charged black holes in the cloud of strings and quintessential fields. The null geodesics of the spacetime is derived and the shadow case, radius of the shadow of the black and its distortion, due to the effects of the black hole spin, also analyzed. It is obtained that gravitational effects of black hole charge and quintessential field compensate each other and the rotating black hole's shadow can be circular. We obtain constraints on charge and spin parameters of supermassive black holes Sgr A ∗ and M87 ∗ using observational data from their shadow (size and observational angle) for various values of the NUT charge, cloud of strings and quintessential field parameters. Moreover, we also investigate gravitational capture cross-section for electromagnetic waves and emission rate of the black hole radiation energy through Hawking evaporation. [ABSTRACT FROM AUTHOR]

Aims. We extend the classical formulation of the dynamical friction effect on a test star by Chandrasekhar to the case of relativistic velocities and velocity distributions, also accounting for post-Newtonian corrections to the gravitational force. Methods. The original kinetic framework was revised and used to construct a special-relativistic dynamical friction formula where the relative velocity changes in subsequent encounters are added up with Lorentz transformation, and the velocity distribution of the field stars accounts for relativistic velocities. Furthermore, a simple expression is obtained for systems where the post-Newtonian correction on the gravitational forces become relevant even at non-relativistic particle velocities. Finally, using a linearized Lagrangian we derived another expression for the dynamical friction expression in a more compact form than previously used. Results. Comparing our formulation with the classical one, we observe that a given test particle undergoes a slightly stronger drag when moving through a distribution of field stars with relativistic velocity distribution. Vice versa, a purely classical treatment of a system where post-Newtonian (PN) corrections should be included, overestimates the effect of dynamical friction at low test particle velocity, regardless of the form of velocity distribution. Finally, a first-order PN dynamical friction covariant formulation is weaker its classical counterpart at small velocities, but much higher for large velocities over a broad range of mass ratios. [ABSTRACT FROM AUTHOR]

This work explores the spectral behavior of interacting many-body systems — gravitating dust solutions (galaxy formations and black hole clusters) and Brownian fluids. The eigenvalue dynamics of these systems are then represented by the two-dimensional Yang–Mills field (i.e. spectral projection). The interacting particles in the many-body systems are associated with random matrices of dimensions, N. The Painlevé II dynamical system is shown to surface at large N (N → ∞); when the mentioned Yang–Mills field is configured in a specific way. Critical phenomena (Douglas–Kazakov phase transition) of the interacting many-body systems at large N were attained via the spectral projection of the mentioned physical systems. In addition, the existence of instantons (spectral Dirac monopoles) in the strong coupling phase was shown during phase transition. [ABSTRACT FROM AUTHOR]

General relativistic instability supernovae (GRSNe) at 10 ≲ z ≲ 15 are predicted to be observed as red faint point sources, and they can be detected only in the reddest filters in James Webb Space Telescope (JWST)/NIRCam (F444W and F356W). They should be observed as persistent point sources with little flux variations for a couple of decades because of time dilation. We search for static point sources detected only in the F444W filter or only in the F444W and F356W filters in the early JWST deep field data. No real point source of such kind is identified. Therefore, the GRSN rate at 10 ≲ z ≲ 15 is constrained to be less than ∼8 × 10−7 Mpc−3 yr−1 for the first time. [ABSTRACT FROM AUTHOR]

Relativistic magnetic reconnection is a nonideal plasma process that is a source of nonthermal particle acceleration in many high-energy astrophysical systems. Particle-in-cell (PIC) methods are commonly used for simulating reconnection from first principles. While much progress has been made in understanding the physics of reconnection, especially in 2D, the adoption of advanced algorithms and numerical techniques for efficiently modeling such systems has been limited. With the GPU-accelerated PIC code WarpX, we explore the accuracy and potential performance benefits of two advanced Maxwell solver algorithms: a nonstandard finite-difference scheme (CKC) and an ultrahigh-order pseudo-spectral method (PSATD). We find that, for the relativistic reconnection problem, CKC and PSATD qualitatively and quantitatively match the standard Yee-grid finite-difference method. CKC and PSATD both admit a time step that is 40% longer than that of Yee, resulting in a ∼40% faster time to solution for CKC, but no performance benefit for PSATD when using a current deposition scheme that satisfies Gauss's law. Relaxing this constraint maintains accuracy and yields a 30% speedup. Unlike Yee and CKC, PSATD is numerically stable at any time step, allowing for a larger time step than with the finite-difference methods. We found that increasing the time step 2.4–3 times over the standard Yee step still yields accurate results, but it only translates to modest performance improvements over CKC, due to the current deposition scheme used with PSATD. Further optimization of this scheme will likely improve the effective performance of PSATD. [ABSTRACT FROM AUTHOR]

To visualize what is seen by an observer in curved spacetime, both general relativistic effects and special relativistic effects need to be considered. In this study, the author analytically proved that the zenith view seen by an observer falling freely into a Schwarzschild black hole (that is, the observer's view of the point on the opposite side of the black hole) becomes enlarged, "redder," and darker as the observer falls from an arbitrary initial rest position. An equation expressing the magnification factor of the zenith view was also derived. The derived factor characterizes the brightness and frequency of light emanated from the zenith view. Using these results, the author derived visualizations of the zenith views for various scenarios. As expressed by the derivations presented here, the moment when the observer sees the zenith view appear twice as large compared to the initial rest position coincides with the crossing of the event horizon; that is, a freely falling observer can detect the horizon through the simple procedure of continuing to observe the zenith view. This result implies that the effect of aberration dominates the effect of light deflection while the observer is falling. [ABSTRACT FROM AUTHOR]

In this work, we present an anisotropic generalization of the Buchdahl static stellar model by implementing the method of extended gravitational decoupling and further requirement of vanishing complexity (Herrera, Phys Rev D 97:044010,2018). Starting off with a general spherically symmetric static metric with two unknown gravitational potentials, we impose the condition of vanishing complexity which then reduces the problem to a single‐generating metric function [Contreras and Stuchlik, Eur. Phys. J. C (2022) 82:706]. The Buchdahl ansatz is then employed to obtain the complete gravitational behavior of the isotropic seed solution. The method of extended gravitational decoupling is thereafter utilized to obtain an anisotropic counterpart of the Buchdahl perfect gravitating sphere. We subject our solution to rigorous physical tests to ensure that it serves as a viable candidate for compact objects such as neutron stars as well as pulsars. We show that the decoupling parameter plays a crucial role in determining the stability and regularity of several key physical features of the entire stellar model. A novel finding of our investigation is the energy exchange between the seed solution and the secondary source which we show is sensitive to the decoupling parameter. In addition, it has also been shown that the direction of energy flow depends on the radial distance of the shell from the center of the stellar configuration. [ABSTRACT FROM AUTHOR]

In order to predict the cosmological abundance of dark matter, an estimation of particle rates in an expanding thermal environment is needed. For thermal dark matter, the non-relativistic regime sets the stage for the freeze-out of the dark matter energy density. We compute transition widths and annihilation, bound-state formation, and dissociation cross sections of dark matter fermion pairs in the unifying framework of non-relativistic effective field theories at finite temperature, with the thermal bath modeling the thermodynamical behaviour of the early universe. We reproduce and extend some known results for the paradigmatic case of a dark fermion species coupled to dark gauge bosons. The effective field theory framework allows to highlight their range of validity and consistency, and to identify some possible improvements. [ABSTRACT FROM AUTHOR]

This work is a study of the nature of neutrinos, which are fundamental building blocks of the universe. Understanding their properties (mass, interaction, ...) is vital to piecing together the evolution of the cosmos to the present day and the events leading up to our existence. We obtained the z n r (the redshfit at which a neutrino of mass m ν will become non-relativistic) using neutrino coupled to the scalar field and relation: z n r = (m ν 5.28 × 10 − 4 ev) − 1 . We first estimate the total neutrino's mass Σ m υ Using the coupling between the scalar field and the neutrinos, then we obtained z n r . Observational data used in this article are P a n t h e o n and C M B , B A O , and C C . We found neutrino's mass is Σ m υ < 0.26 eV (95 % confidence level) and z n r = 462 for Pantheon catalog and Σ m υ < 0.16 eV (95 % confidence level) and z n r = 340 for C M B + B A O + C C + P a n t h e o n . [ABSTRACT FROM AUTHOR]

A brief outline of the development of ideas and a review of some achievements in modern celestial mechanics are presented. The emphasis is on the fact that the classical definition of this science given by Laplace does not fully reflect the content of modern celestial mechanics, and the term dynamic astronomy is more capacious. Dynamic astronomy studies almost everything that moves and rotates in space: from dust particles to comets and asteroids, from artificial satellites, planets, and their satellites to stars and galaxies. This complex science includes problems of both classical and relativistic celestial mechanics; it includes the theory of equilibrium figures and various numerical and computer simulation methods. Qualitative methods are of great importance, the culmination of which was the creation of the KAM theory. The development of celestial mechanics went through the practice of various applications, and the range of problems in it is exceptionally wide. A striking stimulus for the development of dynamical astronomy was the discovery of exoplanets around other stars. The article traces a chain of ideas from Keplerian orbits to osculating Lagrangian ellipses, from two-body problems to many-body problems, and from Gaussian rings to models built on the basis of precessing analogues of these rings. [ABSTRACT FROM AUTHOR]

Testing gravity theories and their parameters using observations is an important issue in relativistic astrophysics. In this context, we investigate the motion of test particles and their harmonic oscillations in the spacetime of non-rotating hairy black holes (BHs) in Hordeski gravity, together with astrophysical applications of quasiperiodic oscillations (QPOs). We show possible values of upper and lower frequencies of twin-peak QPOs which may occur in the orbits from innermost stable circular orbits to infinity for various values of the Horndeski parameter q in relativistic precession, warped disk models, and three different sub-models of the epicyclic resonant model. We also study the behaviour of the QPO orbits and their position relative to innermost stable circular orbits (ISCOs) with respect to different values of the parameter q. It is obtained that at a critical value of the Horndeski parameter ISCO radius takes 6M which has been in the pure Schwarzschild case. Finally, we obtain mass constraints of the central BH of microquasars GRS 1915+105 and XTE 1550-564 at the GR limit and the possible value of the Horndeski parameter in the frame of the above-mentioned QPO models. The analysis of orbits of twin peak QPOs with the ratio of upper and lower frequencies 3:2, around the BHs in the frame of relativistic precession (RP) and epicyclic resonance (ER4) QPO models have shown that the orbits locate close to the ISCO. It is obtained that the distance between QPO orbits and ISCO is less than the error of the observations. [ABSTRACT FROM AUTHOR]

A hydrodynamic model for small acoustic oscillations in a cloud of stars is built, taking into account the self-consistent gravitational field in equilibrium with a non-zero second correlation moment. It is assumed that the momentum flux density tensor should include the analog of the anisotropic pressure tensor and the second correlation moment of both longitudinal and transverse gravitational field strength. The non-relativistic temporal equation for the second correlation moment of the gravitational field strength is derived from the Einstein equations using the first-order post-Newtonian approximation. One longitudinal and two transverse branches of acoustic oscillations are found in a homogeneous and isotropic star cloud. The requirement for the velocity of transverse oscillations to be zero provides the boundary condition for the stability of the cloud. The critical radius of the spherical cloud of stars is obtained, which is precisely consistent with the virial theorem. [ABSTRACT FROM AUTHOR]

We present the derivation of hydrodynamical equations for a perfect fluid in General Relativity, within the 3 + 1 decomposition of spacetime framework, using only primitive variables. Primitive variables are opposed to conserved variables, as defined in the widely used Valencia formulation of the same hydrodynamical equations. The equations are derived in a covariant way, so that they can be used to describe any configuration of the perfect fluid. Once derived, the equations are tested numerically. We implement them in an evolution code for spherically symmetric self-gravitating compact objects. The code uses pseudospectral methods for both the metric and the hydrodynamics. First, convergence tests are performed, then the frequencies of radial modes of polytropes are recovered with and without the Cowling approximation, and finally the performance of our code in the black hole collapse and migration tests are described. The results of the tests and the comparison with a reference core-collapse and neutron star oscillations code suggests that not only our code can handle very strong gravitational fields, but also that this new formulation helps gaining a significant amount of computational time in hydrodynamical simulations of smooth flows in General Relativity. [ABSTRACT FROM AUTHOR]

The Global Navigation Satellite Systems (GNSSs) apply conventional relativistic corrections to correct apparent clock offsets. However, with the continuous improvements made to time synchronization approaches and onboard atomic clocks, this model has obviously limits the accuracy improvements of BDS-3 intersatellite time synchronization, and also limited the apparent performance of BDS-3 onboard atomic clocks. This study demonstrates that the amplitude of GNSS conventional relativistic periodic errors can reach 0.08 ns for BDS-3 MEO and IGSO satellites within a day, and 0.40 ns within a month. The frequency stability of the errors' signal can reach 1.33E-14 (approximately10000 s) and 1.78E-15 (approximately1 day), and the absolute influences on the predicted clock offset accuracy can reach 0.05 ns (1 h), 1.08 ns (1 day). Notably, the above signals caused by the model errors are found in the results of current high-accuracy intersatellite time synchronization based on BDS-3 intersatellite links (ISLs). After adopting a higher-accuracy relativistic correction model, the frequency instability of BDS-3 intersatellite clock offsets is obviously reduced, and the RMS of 24 h fitting residual can be reduced by approximately 60% from 0.12 ns to 0.05 ns. The average 2 h 95% predicted error decreases by approximately 17% from 0.38 ns to 0.32 ns. These results indicate that this high-accuracy relativistic correction model can improve both the BDS-3 satellites' time synchronization accuracy and the BDS-3 apparent clock offset prediction capability. This study, thus contributes to time synchronization research and signal-in-space accuracy improvements for BDS-3. [ABSTRACT FROM AUTHOR]

Puffy disc is a numerical model, expected to capture the properties of the accretion flow in X‐ray black hole (BH) binaries in the luminous, mildly sub‐Eddington state. We fit the kerrbb and kynbb spectral models in XSPEC to synthetic spectra of puffy accretion discs, obtained in general relativistic radiative magnetohydrodynamic simulations, to see if they correctly recover the BH spin and mass accretion rate assumed in the numerical simulation. We conclude that neither of the two models is capable of correctly recovering the puffy disc parameters, which highlights a necessity to develop new, more accurate spectral models for the luminous regime of accretion in X‐ray BH binaries. We propose that such spectral models should be based on the results of numerical simulations of accretion. [ABSTRACT FROM AUTHOR]