Compact Objects Research Articles

Detecting the third family of compact stars with normalizing flows

Detecting the third family of compact stars with normalizing flows

Detailed analysis of the relativistic configuration of Bardeen anisotropic spheres in modified f(G) gravity

The aim of this work is to investigate anisotropic compact objects within the framework of f(G) modified theory of gravity. For our present work, we utilize Krori–Barua metrics, i.e., λ(r) = Xr 2 + Y and β(r) = Zr 2. We use some matching conditions of spherically symmetric spacetime with Bardeen’s model as an exterior geometry. Further, we establish some expressions of energy density and pressure components to analyze the stellar configuration of Bardeen compact stars by assuming viable f(G) models. We examine the energy conditions for different stellar structures to verify the viability of our considered models. Moreover, we also investigate some other physical features, such as equilibrium condition, equation of state parameters, adiabatic index, stability analysis, mass function, surface redshift, and compactness factor, respectively. It is worthwhile to mention here for the current study that our stellar structure in the background of Bardeen's model is more viable and stable.

The Parallel Compact Object CALculator: An Efficient General Relativistic Initial Data Solver for Compact Objects

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.

Imaging a semiclassical horizonless compact object with strong redshift

Imaging a semiclassical horizonless compact object with strong redshift

Estimating wind speeds in tornadoes using debris trajectories of large compact objects

Abstract Currently, the Enhanced Fujita scale does not consider the wind-induced movement of various large compact objects such as vehicles, construction equipment, farming equipment / haybales, etc. that are often found in post-event damage surveys. One reason for this is that modelling debris in tornadoes comes with considerable uncertainties since there are many parameters to determine, leading to difficulties in using trajectories to analyze wind speeds of tornadoes. This paper aims to develop a forensic tool using analytical tornado models to estimate lofting wind speeds based on trajectories of large compact objects. This is accomplished by implementing a Monte Carlo simulation to randomly select the parameters and plotting cumulative distribution functions showing the likelihood of lofting at each wind speed. After analyzing the debris lofting from several documented tornadoes in Canada, the results indicate that the method provides threshold lofting wind speeds that are similar to the estimated speeds given by other methods. However, the introduction of trajectories produces estimated lofting wind speeds that are higher than the EF-scale rating given from the ground survey assessment based on structural damage. Further studies will be required to better understand these differences.

Anisotropic stellar structures admitting Karmakar condition in f(R, φ, χ) theory

This article employs the Karmarkar condition to study the geometry of compact stars experiencing anisotropy in the context of f(R, φ, χ) gravity. The unknown constants involved in Karmarkar condition are calculated by matching the internal and external regimes at the hypersurface. We investigate the acceptable behavior of matter variables and anisotropy. The viability of all the resulting models is checked using energy bounds. We also discuss some important factors such as mass, compactness and redshift parameters. Finally, we investigate the stable aspect of considered stars through causality condition and Herrera cracking technique. We conclude that all the discussed stellar structures in this theory satisfy the required constraints.

An Approximate Kerr-Newman-like Metric Endowed with a Magnetic Dipole and Mass Quadrupole

Abstract Approximate all-terrain spacetimes for astrophysical applications are presented. The metrics possess five relativistic multipole moments,
namely mass, rotation, mass quadrupole, charge, and magnetic dipole moment. All these spacetimes approximately satisfy the Einstein-
Maxwell field equations. The first metric is generated by means of the Hoenselaers-Perjés method from given relativistic multipoles. The
second metric is a perturbation of the Kerr-Newman metric, which makes it a relevant approximation for astrophysical calculations. The
last metric is an extension of the Hartle-Thorne metric that is important for obtaining internal models of compact objects perturbatively.
The electromagnetic field is calculated using Cartan forms for locally nonrotating observers. These spacetimes are relevant to infer properties of compact objects from astrophysical observations. Furthermore, the numerical implementations of these metrics are straightforward, making them versatile for simulating the potential astrophysical applications.

Black holes, white holes, and near-horizon physics

Black and white holes play remarkably contrasting roles in general relativity versus observational astrophysics. While there is observational evidence for the existence of compact objects that are “cold, dark, and heavy”, which thereby are natural candidates for black holes, the theoretically viable time-reversed variants — the “white holes” — have nowhere near the same level of observational support. Herein we shall explore the theoretical possibility that the connection between black and white holes is much more intimate than commonly appreciated. We shall first construct “horizon penetrating” coordinate systems that differ from the standard curvature coordinates only in a small near-horizon region, thereby emphasizing that ultimately the distinction between black and white horizons depends only on near-horizon physics. We shall then construct an explicit model for a “black-to-white transition” where all of the nontrivial physics is confined to a compact region of spacetime — a finite-duration finite-thickness, (in principle arbitrarily small), region straddling the naïve horizon. Moreover we shall show that it is possible to arrange the “black-to-white transition” to have zero action — so that it will not be subject to destructive interference in the Feynman path integral. This then raises the very intriguing possibility that astrophysical black holes might be interpretable in terms of a quantum superposition of black and white horizons — a “gray” horizon.

Tidal disruption events from three-body scatterings and eccentricity pumping in the disks of active galactic nuclei

Abstract Tidal Disruption Events (TDEs) are routinely observed in quiescent galaxies, as stars from the nuclear star cluster are scattered into the loss cone of the central supermassive black hole (SMBH). TDEs are also expected to occur in Active Galactic Nuclei (AGN), due to scattering or orbital eccentricity pumping of stars embedded in the innermost regions of the AGN accretion disk. Encounters with embedded stellar-mass black holes (BH) can result in AGN μTDEs. AGN TDEs and μTDEs could therefore account for a fraction of observed AGN variability. Here, by performing scattering experiments with the few-body code SpaceHub, we compute the probability of AGN TDEs and μTDEs as a result of 3-body interactions between stars and binary BHs. We find that AGN TDEs are more probable during the early life of the AGNs, when rates are $\sim (6\times 10^{-5}-5 \times 10^{-2}) (f_\bullet /0.01)\, \rm {AGN}^{-1}$ yr−1 (where f• is the ratio between the number density of BHs and stars), generally higher than in quiescent galactic nuclei. By contrast, μTDEs should occur throughout the AGN lifetime at a rate of $\sim (1\times 10^{-4} - 4\times 10^{-2})(f_\bullet /0.01)\, \rm {AGN}^{-1}$ yr−1. Detection and characterization of AGN TDEs and μ AGN TDEs with future surveys using Rubin and Roman will help constrain the populations of stars and compact objects embedded in AGN disks, a key input for the LVK AGN channel.

Evolutionary nature of puffed-up stripped star binaries and their occurrence in stellar populations

The majority of massive stars are formed in multiple systems, and at some point during their life, they interact with their companions via mass transfer. This interaction typically leads to the primary shedding its outer envelope, resulting in the formation of a "stripped star". Classically, stripped stars are expected to quickly contract to become hot and UV-bright helium stars. Surprisingly, recent optical spectroscopic surveys have unveiled many stripped stars that are much larger and cooler, appearing "puffed up" and overlapping with the Main Sequence (MS) in the Hertzsprung-Russell diagram. Here, we study the evolutionary nature of puffed-up stripped (PS) stars and the duration of this enigmatic phase using the stellar-evolution code MESA. We computed grids of binary models at four metallicities: Solar (Z$_ odot $ = 0.017), Large Magellanic Cloud (LMC, Z=0.0068), Small Magellanic Cloud (SMC, Z = 0.0034), and Z = 0.1Z$_ Contrary to previous assumptions, we find that stripped stars regain thermal equilibrium shortly after the end of mass transfer and maintain it during most of the PS phase. Their further contraction towards hot and compact He stars is determined by the rate at which the residual H-rich envelope is depleted, with the main agents being H-shell burning (dominant for M lesssim 50 and mass-loss in winds. The duration of the PS star phase is approximately 10<!PCT!> of the core-He burning lifetime (1<!PCT!> total lifetime) and up to 100 times more than the thermal timescale. It decreases with increasing mass and luminosity and increases with metallicity. We explored several factors influencing PS star lifetimes: orbital period, mass ratio, winds, and semiconvection. We further carried out a simple binary population synthesis estimation, finding that sim 0.5 – 0.7<!PCT!> of all the stars with log (L/L$_ > 3.7 may, in fact, be PS stars. Our results indicate that tens to hundreds of PS stars in post-interaction binaries may be hiding in the MS population, disguised as 'normal' stars: sim 100 (sim 280) in the SMC (LMC) and sim 1500 in the Milky Way . Their true nature may be revealed by spectroscopically measured low surface gravities, high N enrichment, and likely slow rotation rates.

Impact of the hot inner crust on compact stars at finite temperature

We conducted a study on the thermal properties of stellar matter with the nuclear energy density functional BCPM. This functional is based on microscopic Brueckner-Hartree-Fock calculations and has demonstrated success in describing cold neutron stars. To enhance its applicability in astrophysics, we extended the BCPM equation of state to finite temperature for beta -stable neutrino-free matter, taking into consideration the hot inner crust. Such an equation of state holds significant importance for hot compact objects, particularly those resulting from a binary neutron star merger event. Our exploration has shown that with increasing temperature, there is a fast decrease in the crust-core transition density, suggesting that for hot stars it is not realistic to assume a fixed value of this density. The microscopic calculations also reveal that the presence of nuclear clusters persists up to $T=7.21$\,MeV, identified as the limiting temperature of the crust. Above this threshold, the manifestation of clusters is not anticipated. Below this temperature, clusters within the inner crust are surrounded by uniform matter with varying densities, allowing for the distinction between the upper and lower transition density branches. Moreover, we computed mass--radius relations of neutron stars, assuming an isothermal profile for beta -stable neutron star matter at various temperature values. Our findings highlight the significant influence of the hot inner crust on the mass--radius relationship, leading to the formation of larger and more inflated neutron stars. Consequently, under our prescription, the final outcome is a unified equation of state at finite temperature.

Black holes immersed in dark matter: Energy condition and sound speed

In this work, we study the impact of the environment around a black hole in detail. We introduce nonvanishing radial pressure in a manner analogous to compact stars. We examine both isotropic and anisotropic fluid configurations with and without radial pressure respectively. Our focus extends beyond just dark matter density to the vital role of the energy condition and sound speed in the spacetime of a black hole immersed in matter. In cases of anisotropic pressure with vanishing radial pressure, all profiles violate the dominant energy condition near the BH, and the tangential sound speed exceeds light speed for all dark matter profiles. In our second approach, without assuming vanishing radial pressure, we observe similar violations and superluminal sound speeds. To rectify this, we introduce a hard cutoff for the sound speed, ensuring it remains subluminal. As a consequence, the energy condition is also satisfied. However, this results in increased density and pressure near the BH. This raises questions about the sound speed and its impact on the density structure, as well as questions about the validity of the model itself. With the matter distribution, we also compute the metric for different configurations. It reveals sensitivity to the profile structure. The metric components point toward the horizon structure. Published by the American Physical Society 2024

Investigating the equation-of-state, stability and mass–radius relationship of anisotropic and massive neutron stars embedded in f(R,T) modified gravity

In this study, our main focus is to investigate the mass–radius relation and several important properties of massive neutron stars to realize the nature, behavior and evolution of these kinds of compact objects at present time. Also, we want to understand the equation-of-state of the core nuclear matter precisely with their stable equilibrium configuration. We have chosen a few massive binary pulsars and investigated on maximum attainable mass and lowest possible radius of them. We have considered Einstein–Hilbert action as [Formula: see text], where [Formula: see text] is the Ricci scalar and [Formula: see text], the trace of energy–momentum tensor with [Formula: see text] as the coupling parameter and also used modified TOV equations. The interior space-time of the spherical neutron star is matched to the exterior Schwarzschild line element at the surface of the star. The [Formula: see text]–[Formula: see text] curve can predict the maximum achievable mass is about [Formula: see text] with lowest possible radius of around [Formula: see text] km for the massive compact stars under stable equilibrium. We have obtained a clear picture of structural evolution of massive neutron stars through accelerating space-time and can put some constraints on several quantities related to them. It is depicted from our present investigation that all the derived outcomes are compatible with physically adopted regimes which reveal the viability of our current model in the context of [Formula: see text] modified gravity.

Observational signatures of charged rotating traversable wormhole: shadows and light rings with different accretions

In this work, we investigate the optical appearance of qualitatively new observational features of accretion disk images around the charged rotating traversable wormhole (TWH) space-time for different spin, throat, and charge values. To accomplish this, we first consider the Hamilton–Jacobi method to derive the geodesic equations for the motion of photons and study the effects of parameters on the photon orbit in the observer’s sky. We found that each parameter affects the size and shape of the wormhole (WH) shadow and flatness is observed in the shadow because of spin and other parameters. To produce shadow images of sufficient visual quality but within manageable computational times, we adopt the ray-trace procedure and characterize the significant features of light trajectories on the observer’s screen, depending on the interaction between the space-time structure and the accretion disk. In addition, we consider the static spherically symmetric accretion flow model to observe the specific intensity around the traversable WH space-time geometry. It is found that the intensity and positions of the photon ring vary with respect to the involved parameters. In future observation, this type of study may provide a fertile playground to test the nature of compact objects, specifically the WH in the strong-field regime.

Isotropic compact stars admitting Heintzmann solution in Rastall gravity

This paper is devoted to observe the physical attributes of static spherically symmetric isotropic compact stellar candidates in the context of Rastall theory of gravity. In order to inspect the structural composition of compact objects, the Heintzmann ansatz is taken into account. The unknown parameters associated with Heintzmann ansatz are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition, stellar equation and adiabatic index for several choices of Rastall parameter. It is observed that the stars under consideration manifest stable structures corresponding to Heintzmann ansatz in this framework. Moreover, it is shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved.

Distinguishability of binary extreme-mass-ratio inspirals in low frequency band

The inspiral of compact stellar objects into massive black holes are one of the main astrophysical sources for the Laser Interferometer Space Antenna (LISA) and Taiji. These extreme-mass-ratio inspirals (EMRIs) have great potential for cosmology and fundamental physics. A binary extreme-mass-ratio inspiral (b-EMRI) describes the case where binary black holes (BBHs) are captured by a supermassive black hole. The b-EMRIs serve as multi-band gravitational wave sources and provide insights into the dynamics of nuclei and tests of general relativity. However, if the b-EMRIs can be distinguished from the normal EMRIs or not is still not clear. In this work, with a few of assumptions, and using the Teukolsky equation, we calculate the approximate gravitational waves of b-EMRIs and assess their detectability by space-based detectors. We also decouple the secondary object information from the Teukolsky equation, enabling us to calculate the energy fluxes and generate the waveforms more conveniently. Variations in the quadrupole of the binary result in small but non-negligible changes in energy fluxes and waveforms, making it possible to distinguish b-EMRI signals with data analysis. This opens up the potential of using b-EMRIs to test gravity theories and for further astrophysical studies.

Anisotropic compact star in linear f(Q)-action

In this paper, a significant leap forward in understanding compact stellar systems and the modified f(Q) gravity theory is achieved. The pivotal discovery lies in the successful derivation of an exact solution that fulfils the static geometry and spherical symmetry criteria, permitting the study of compact stellar configurations with an anisotropic fluid. The model is rigorously tested and satisfies the vital physical conditions within the stellar fluid, guaranteeing its viability. The numerical values of constant parameters have been calculated by using the observational data of the compact star, namely, Her X-1. The equi-mass contours highlight an impressive correlation between the f(Q) gravity parameters. Boosting α while keeping β fixed and concurrently boosting R leads to a significant global boost in mass distribution. This can be ascribed to the enhanced coupling arising from a higher α, which broadens the mass distribution. In addition, the larger object size arising from the rise in R allows for more mass accommodation. Therefore, raising both R and α leads to an exaggerated mass distribution, proving the combined influence of coupling strength and object size on total mass. Altogether, this investigation advances our knowledge of compact stellar systems and supports the evolution of the modified f(Q) theory of gravity, opening the way for more breakthroughs in this field.

Proto-strange quark stars from density-dependent quark mass model

In this paper, we investigate the evolution of strange quark stars (SQS) from birth as proto-strange quark stars to maturity as stable SQSs at a zero temperature. We assume that self-bound free quarks form entirely the compact star and study its evolution through a series of snapshots using a density-dependent quark mass model. We consider β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta $$\\end{document}-equilibrated stellar matter at two major stages of the star’s evolution: neutrino trapped regime and neutrino transparent regime during the deleptonization and cooling processes of the star. We fix the entropy density per baryon and the lepton fraction to investigate the nuclear equation of state (EoS), particle distribution, temperature profile inside the star, sound velocity, polytropic index, and the structure of the star. Our results show that stars with higher neutrino concentrations are slightly more massive than the neutrino-poor ones along the evolution lines of the SQS. We obtain EoSs in agreement with the conformal boundary set through sound velocity, and also the 2 M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_\\odot $$\\end{document} mass constraint for NSs was satisfied at all stages of the star’s evolution.

Swift X-ray and UV observations of six Gaia binaries supposedly containing a neutron star

Recent observations have led to the discovery of numerous optically selected binaries containing an undetected component with mass consistent with a compact object (neutron star (NS) or white dwarf). Using the Neil Gehrels Swift Observatory, we carried out X-ray and UV observations of a small sample of these binaries. Four systems are wide (with an orbital period P$>300 d$), and were chosen because of their small distance (d$<$250 pc) and because the mass of the collapsed component favors a NS. Two others are compact systems (P$<0.9 d$) that show strong signs of containing a NS. The source 2MASS J15274848$+$3536572 was detected in the X-ray band with a flux of $5 $ erg cm$^ $ s$^ $ and a spectrum well fitted by a power law or a thermal plasma emission model. This source also shows a UV (2200 AA ) excess, which might indicate the presence of mass accretion. For the other targets, we derived X-ray flux upper limits of the order of $10^ $ erg cm$^ $ s$^ $ . These results are consistent with the hypothesis that the collapsed component in these six systems is a NS.

Recent Outbursts of the High-mass X-Ray Binary CI Camelopardalis

Recent Outbursts of the High-mass X-Ray Binary CI Camelopardalis