Compact Astrophysical Objects Research Articles

Type Ia supernova constraints on compact object dark matter

ABSTRACT The nature of dark matter (DM) is a central question in cosmology today. While elementary particles could explain DM, compact astrophysical objects such as black holes formed in the early Universe offer a theoretically appealing alternate route. Here, we constrain the fraction of DM that can be made up of primordial black holes (PBHs) with masses $M \gtrsim 0.01\, {\rm M}_\odot$, with Type Ia supernovae. Utilizing the Dyer–Roeder distance relation, we find a maximum fractional amount of DM in compact objects (fp) of 0.50 at 95 per cent confidence level (C.L.), in the flat Lambda cold dark matter model and 0.49 when marginalizing over a constant dark energy equation of state or spatial curvature, demonstrating robustness to the cosmological model. With a prior on the homogeneity parameter, η, including values >1, we derive η = 1.08 ± 0.17, hence, fp < 0.32 at 95 per cent C.L., showing that the prior assumption of η ≤ 1 gives a conservative upper limit on fp. The Hubble constant we infer is consistent with the homogeneous case, showing that inhomogeneities like compact DM cannot account for the observed Hubble tension. In conclusion, we can exclude stellar masses PBHs as comprising all of the observed DM.

I-Love-Q relations for a generic family of neutron star equations of state

ABSTRACT Neutron stars (and quark stars) have recently been found to follow certain universality, the I-Love-Q relations. These relations can provide a greater understanding of the structural and macro properties of compact astrophysical objects with knowledge of any one of the observable. The reason behind this is the lack of sensitivity to the relations with the equation of state (EoS) of matter. In this work, we have investigated the consistency of universal relations for a generic family of equations of state. These equations of state are consistent with the prescribed limits of nuclear theory (low densities) and perturbative QCD (asymptotically high densities) and respecting recent astronomical observations. At intermediate densities, the equations of states are parametrized by the adiabatic speed of sound. Although the spread in the EoS is significant, the universal nature of the I-Love-Q trio holds relatively well up to a specific tolerance limit of 5 per cent fractional percentage error. We have compared the result first with a similar set, but not respecting the recent astrophysical constraints. A deviation from universality is seen and the tolerance exceeds the 5 per cent limit. Next, we have compared our result with a family of EoS having monotonic sound speed inside a neutron star. A slight deviation from universality is seen for this case; however, the tolerance remains within the 5 per cent limit. This shows the existence of a correlation between universal relations and observational constraints.

Propagatory dynamics of electromagnetic drift waves in inhomogeneous spin degenerate compact objects plasmas

A theoretical analysis is carried out to investigate the dynamics of electromagnetic drift modes in non-uniform spin quantum magnetoplasmas found in degenerate compact objects. General dispersion relations are derived using spin quantum hydrodynamic equations, Poisson equation, and Maxwell equation, with and without ion inertial effects. The effects of quantum spin magnetization, Bohm potential, and density inhomogeneity on the dispersion of electromagnetic drift modes are explored numerically. The advancing theoretical analysis demonstrated herein has strong correlations and consistencies with the dynamic growth of compact astrophysical objects and their surrounding atmospheres, such as black holes, white dwarfs, and dense molecular clouds.

Stability of dust-acoustic solitary waves in magnetized dusty plasmas: effect of polarization force and degenerate electron temperature

The influence of polarization force, trapped degenerate electrons, and nonthermal ions on the propagation characteristics of dust acoustic solitary waves (DASWs) in a magnetized dusty plasma is investigated. The reductive perturbation approach has been adopted to derive the extended Zakharov-Kuznetsov (eZK) equation and the solitary pulse solution is obtained to investigate the characteristics of DASWs. Pulse stability has also been analysed following the method proposed by Allen and Rowlands. Influence of different relevant plasma parameters on the growth rate of DASWs is studied. It is also found that the relevant plasma parameters have significant impact on the instability criteria of DASWs. This investigation is useful in explaining the nonlinear structures in ultra-dense degenerate plasma media which may exist in astrophysical compact objects (e.g., white dwarfs, neutron stars, etc.) and high density plasma experiments in which plasma degeneracy and relativistic effects play an important role.

СЛАБОЕ ГРАВИТАЦИОННОЕ ЛИНЗИРОВАНИЕ ЧЕРНЫМИ ДЫРАМИ С ЗАРЯДОМ

The deflection of light due to gravity was predicted by general relativity and first confirmed by observations in 1919. In the following decades, various aspects of the gravitational lensing effect were theoretically explored, including the possibility of obtaining multiple or annular images of background sources, the use of lensing as a gravitational telescope on very faint and distant objects, and the possibility of determining the Hubble constant using lensing. Gravitational lensing is the deflection of light by gravity caused by the action of compact astrophysical objects. Various objects, such as a galaxy, stars, black holes, wormholes, and others, can act as a gravitational lens. In our study, as a gravitational lens, we chose black holes described by the Gibbons-Maeda-GarfinkleHorowitz-Strominger solutions in the string frame. The two solutions in the string frame are magnetically and electrically charged black holes with an event horizon. The work will investigate the effect of electric and magnetic charges on the angle of deflection of light.

Time‐dependent charged particle stopping in quantum plasmas: testing the G1–G2 scheme for quasi‐one‐dimensional systems

AbstractWarm dense matter—an exotic, highly compressed state on the border between solid and plasma phases is of high current interest, in particular for compact astrophysical objects, high‐pressure laboratory systems, and inertial confinement fusion. For many applications, the interaction of quantum plasmas with energetic particles is crucial. Moreover, often the system is driven far out of equilibrium. In that case, there is high interest in time‐dependent simulations to understand the physics, in particular, during thermalization. Recently a novel many‐particle technique, the G1–G2 scheme was presented with reference to the study by Schlünzen et al. (2020) which allows for first‐principle simulations of the time evolution of interacting quantum systems. Here we apply this scheme to a spatially uniform dense quantum plasma (jellium) and explore its performance. To this end, the G1–G2 scheme is transformed into momentum representation, and first results are presented for a quasi‐one‐dimensional model system.

Curvature and dynamical spacetimes: can we peer into the quantum regime?

Stationary compact astrophysical objects such as black holes and neutron stars behave as classical systems from the gravitational point of view. Their (observable) curvature is everywhere ‘small’. Here we investigate whether mergers of such objects, or other strongly dynamical spacetimes such as collapsing configurations, may probe the strong-curvature regime of general relativity. Our results indicate that dynamical black hole spacetimes always result in a modest increase in the Kretschmann scalar, relative to the stationary state. In contrast, we find that the Kretschmann scalar can dynamically increase by orders of magnitude, during the gravitational collapse of scalar fields, and that the (normalized) peak curvature does not correspond to that of the critical solution. Nevertheless, without fine tuning of initial data, this increase lies far below that needed to render quantum-gravity corrections important.

Anisotropic strange quark star in Finch-Skea geometry and its maximum mass for non-zero strange quark mass (m s ≠ 0)* *BD and KBG thank the CSIR for providing the fellowship vide No 09/1219(0005)/2019-EMR-I and 09/1219(0004)/2019-EMR-I, respectively.

A class of relativistic astrophysical compact objects is analyzed in the modified Finch-Skea geometry described by the MIT bag model equation of state of interior matter, , where B is known as the bag constant. B plays an important role in determining the physical features and structure of strange stars. We consider the finite mass of the strange quark () and study its effects on the stability of quark matter inside a star. We note that the inclusion of strange quark mass affects the gross properties of the stellar configuration, such as maximum mass, surface red-shift, and the radius of strange quark stars. To apply our model physically, we consider three compact objects, namely, (i) VELA X-1, (ii) 4U 1820-30, and (iii) PSR J 1903+327, which are thought to be strange stars. The range of B is restricted from 57.55 to (), for which strange matter might be stable relative to iron (). However, we also observe that metastable and unstable strange matter depend on B and . All energy conditions hold well in this approach. Stability in terms of the Lagrangian perturbation of radial pressure is studied in this paper.

Spectral Line VLBI Studies Using the ngEHT

Spectroscopy in the mm/sub-mm wavelength range is a powerful tool to study the gaseous medium in various astrophysical environments. The next generation Event Horizon Telescope (ngEHT) equipped with a wide-bandwidth backend system has great potential for science using high angular resolution spectroscopy. Spectral line VLBI studies using the ngEHT will enable us to scrutinize compact astrophysical objects obscured by an optically thick medium on unprecedented angular scales. However, the capabilities of ngEHT for spectroscopy and specific scientific applications have not been properly envisioned. In this white paper, we briefly address science cases newly achievable via spectral line VLBI observations in the mm/sub-mm wavelength ranges, and suggest technical requirements to facilitate spectral line VLBI studies in the ngEHT era.

Relativistic polytropic models of charged anisotropic compact objects

In this paper, we introduce new viable solutions to the Einstein-Maxwell field equations by incorporating the features of anisotropic matter distributions within the realm of the general theory of relativity (). To obtain these solutions, we employed the Finch-Skea spacetime, along with a generalized polytropic equation of state (). We constructed various models of generalized polytropes by assuming different values of the polytropic index, i.e., , and . Next, numerous physical characteristics of these considered models were studied via graphical analysis, and they were found to obey all the essential conditions for astrophysical compact objects. Furthermore, such outcomes of charged anisotropic compact star models could be reproduced in various other cases including linear, quadratic, and polytropic

The new discontinuous Galerkin methods based numerical relativity program Nmesh

Interpreting gravitational wave observations and understanding the physics of astrophysical compact objects such as black holes or neutron stars requires accurate theoretical models. Here, we present a new numerical relativity computer program, called Nmesh, that has the design goal to become a next generation program for the simulation of challenging relativistic astrophysics problems such as binary black hole or neutron star mergers. In order to efficiently run on large supercomputers, Nmesh uses a discontinuous Galerkin method together with a domain decomposition and mesh refinement that parallelizes and scales well. In this work, we discuss the various numerical methods we use. We also present results of test problems such as the evolution of scalar waves, single black holes and neutron stars, as well as shock tubes. In addition, we introduce a new positivity limiter that allows us to stably evolve single neutron stars without an additional artificial atmosphere, or other more traditional limiters.

Neutrino fast flavor pendulum. II. Collisional damping

In compact astrophysical objects, the neutrino density can be so high that neutrino-neutrino refraction can lead to fast flavor conversion of the kind $\nu_e \bar\nu_e \leftrightarrow \nu_x \bar\nu_x$ with $x=\mu,\tau$, depending on the neutrino angle distribution. Previously, we have shown that in a homogeneous, axisymmetric two-flavor system, these collective solutions evolve in analogy to a gyroscopic pendulum. In flavor space, its deviation from the weak-interaction direction is quantified by a variable $\cos\vartheta$ that moves between $+1$ and $\cos\vartheta_{\rm min}$, the latter following from a linear mode analysis. As a next step, we include collisional damping of flavor coherence, assuming a common damping rate $\Gamma$ for all modes. Empirically we find that the damped pendular motion reaches an asymptotic level of pair conversion $f=A+(1-A)\cos\vartheta_{\rm min}$ (numerically $A\simeq 0.370$) that does not depend on details of the angular distribution (except for fixing $\cos\vartheta_{\rm min}$), the initial seed, nor $\Gamma$. On the other hand, even a small asymmetry between the neutrino and antineutrino damping rates strongly changes this picture and can even enable flavor instabilities in otherwise stable systems.

Electromagnetic solitons and their stability in relativistic degenerate dense plasmas with two electron species

The evolution of electromagnetic (EM) solitons due to nonlinear coupling of circularly polarized intense laser pulses with low-frequency electron-acoustic perturbations is studied in relativistic degenerate dense astrophysical plasmas with two groups of electrons: a sparse population of classical relativistic electrons and a dense population of relativistic degenerate electron gas. Different forms of localized stationary solutions are obtained and their properties are analyzed. Using the Vakhitov-Kolokolov stability criterion, the conditions for the existence and stability of a moving EM soliton are also studied. It is noted that the stable and unstable regions shift around the plane of soliton eigenfrequency and the soliton velocity due to the effects of relativistic degeneracy, the fraction of classical to degenerate electrons and the EM wave frequency. Furthermore, while the standing solitons exhibit stable profiles for a longer time, the moving solitons, however, can be stable or unstable depending on the degree of electron degeneracy, the soliton eigenfrequency and the soliton velocity. The latter with an enhanced value can eventually lead to a soliton collapse. The results should be useful for understanding the formation of solitons in the coupling of highly intense laser pulses with slow response of degenerate dense plasmas in the next generation laser-plasma interaction experiments as well as the novel features of x-ray and γ-ray pulses that originate from compact astrophysical objects.

Kinetics of Degenerate Electron–Positron Plasmas

Relativistic plasma can be formed in strong electromagnetic or gravitational fields. Such conditions exist in compact astrophysical objects, such as white dwarfs and neutron stars, as well as in accretion discs around neutron stars and black holes. Relativistic plasma may also be produced in the laboratory during interactions of ultra-intense lasers with solid targets or laser beams between themselves. The process of thermalization in relativistic plasma can be affected by quantum degeneracy, as reaction rates are either suppressed by Pauli blocking or intensified by Bose enhancement. In addition, specific quantum phenomena, such as Bose–Einstein condensation, may occur in such a plasma. In this review, the process of plasma thermalization is discussed and illustrated with several examples. The conditions for quantum condensation of photons are formulated. Similarly, the conditions for thermalization delay due to the quantum degeneracy of fermions are analyzed. Finally, the process of formation of such relativistic plasma originating from an overcritical electric field is discussed. All these results are relevant for relativistic astrophysics as well as for laboratory experiments with ultra-intense lasers.

Vacuum Polarization Instead of “Dark Matter” in a Galaxy

We considered a vacuum polarization inside a galaxy in the eikonal approximation and found that two possible types of polarization exist. The first type is described by the equation of state p=ρ/3, similar to radiation. Using the conformally unimodular metric allows us to construct a non-singular solution for this vacuum “substance” if a compact astrophysical object exists in the galaxy’s center. As a result, a “dark” galactical halo appears that increases the rotation velocity of a test particle as a function of the distance from a galactic center. The second type of vacuum polarization has a more complicated equation of state. As a static physical effect, it produces the renormalization of the gravitational constant, thus, causing no static halo. However, a non-stationary polarization of the second type, resulting from an exponential increase (or decrease) of the galactic nuclei mass with time in some hypothetical time-dependent process, produces a gravitational potential, appearing similar to a dark matter halo.

New exact models of ideal gas in 5D EGB using curvature coordinates

Exploiting the use of curvature coordinates (also called Schwarzschild coordinates), new classes of exact solutions are discovered. By prescribing the timelike potential two new solutions are found one of which displays all the necessary qualitative features demanded of closed compact astrophysical objects. Since the analysis reduces to a single first order nonlinear differential equation, the single integration constant is obtained in terms of the mass and radius through matching of the interior and exterior metrics. Invoking the second matching condition results in constraining the value of the Gauss–Bonnet coupling constant in terms of the mass and radius of the star. Stability, causality and energy conditions are satisfied for a suitable choice of parameter space. In attempting to specify the radially oriented potential it was only possible to find a defective spacetime and to regain the interior Schwarzschild metric for an incompressible fluid sphere.

Propagation and energy of the dressed solitons in the Thomas–Fermi magnetoplasma

A theoretical investigation is presented for dust-acoustic (DA) waves in a collisionless Thomas–Fermi magnetoplasma. The plasma system consists of electrons, ions, and negatively charged dust grains, all existing in a quantizing magnetic field. The Korteweg–de Vries (KdV) and KdV type equations are derived by using the reductive perturbation method. The solutions of these evolved equations are obtained. The contribution of higher-order corrections to the DA is investigated. The electric field and the soliton energy were also derived. The K-dV and dressed soliton energies are depleted as the dust temperature and magnetic field increase. But they magnify as obliqueness increases. The present results are beneficial in understanding the waves propagating in Thomas–Fermi magnetoplasma that are applicable for high-intensity laser–solid matter interaction experiments and astrophysical compact objects such as white dwarfs.

Linear and Nonlinear Spherically Symmetric Perturbation in Astrophysical Compact Objects

A degenerate quantum plasma system (DQPS) consisting of arbitrary number of degenerate light (electron, proton, neutron, etc.) and heavy (light nuclei/element, heavy nuclei/element, etc.) particle species is considered to investigate the spherically symmetric perturbation (both linear and nonlinear) of self-gravitational field. The fundamental characteristics (amplitude, width, etc.) of small amplitude self-gravitational potential structures, which are found to be formed in the considered plasma system, are examined by the reductive perturbation technique. The derivation and numerical analysis of the modified Korteweg-de Vries (mKdV) equation are obtained. The analytical results are applied in astrophysical compact objects like white dwarfs and neutron stars, which are the examples of DQPS. Journal of Engineering Science 13(1), 2022, 97-104

Multipolar structure of rotating boson stars

The relativistic multipole moments provide a key ingredient to characterize the gravitational field around compact astrophysical objects. They play a crucial role in the description of the orbital evolution of coalescing binary systems and encode valuable information on the nature of the binary's components, which leaves a measurable imprint in their gravitational-wave emission. We present a new study on the multipolar structure of a class of arbitrarily spinning boson stars with quartic self-interactions in the large coupling limit, where these solutions are expected to be stable. Our results strengthen and extend previous numerical analyses, showing that even for the most compact configurations the multipolar structure deviates significantly from that of a Kerr black hole. We provide accurate data for the multipole moments as functions of the object's mass and spin, which can be directly used to construct inspiral waveform approximants, and to perform parameter estimations and searches for boson star binaries.

Analytic Approximations for the Velocity Suppression of Dark Matter Capture

Compact astrophysical objects have been considered in the literature as dark matter (DM) probes, via the observational effects of annihilating captured DM. In this paper we investigate the role of stellar velocity on multiscatter-capture rates and find that the capture rates of DM by a star moving with respect to the DM halo rest frame are suppressed by a predictable amount. We develop and validate an analytical expression for the capture rate suppression factor. This suppression factor can be used to directly reevaluate projected bounds on the DM–nucleon cross section, for any given stellar velocity, as we explicitly show using Population III stars as DM probes. These objects (Population III stars) are particularly interesting candidates, since they form at high redshifts, in very high DM-density environments. We find that previous results, obtained under the assumption of a star at rest with respect to the DM rest frame, are essentially unchanged when considering the possible orbital velocities for those central stars.