Compact Astrophysical Objects Research Articles

Circular orbits and accretion process near a regular phantom black hole

The relativistic accretion onto astrophysical compact objects such as black holes and neutron stars is the natural phenomena for releasing energy, with up to 40% of the rest-mass energy of the matter accreting on the black hole able to be liberated. High luminosities due to accretion are observed in X-ray binaries, and in active galactic nuclei (AGN). In this paper, we investigate the geodesics motion and accretion process near a regular phantom black hole by taking an isothermal fluid with spherically symmetric black hole spacetime. The geodesic motion around the black hole during accretion provide the disc like structure. Here, we give some reasons to make circular orbits and accretion process for the considered black hole. Firstly, we discuss the motion of test particles with stabilities near the equatorial plane which make the circular orbits. Then we analyze perturbations via restoring forces and oscillations of particles around the central object. Finally, we discuss the critical speed of the fluid flow and maximum accretion rate. The physical validity of our results shows that the phantom parameter b plays an important role for the circular orbits and the maximum accretion rate.

Self-Gravitational Shock Structures in Self-Gravitating, Super-Dense, Degenerate Quantum Plasmas

The nonlinear propagation of self-gravitational shock structures (SGSSs) in a self-gravitating, super-dense, degenerate quantum plasma system (containing non-degenerate extremely heavy nuclei and ultra-relativistic degenerate electrons) has been investigated. The well-known reductive perturbation technique, which is valid in the small but finite amplitude limit, has been used to examine the nonlinear propagation of these SGSSs in such degenerate quantum plasma systems. The nonlinear dynamics of these SGSSs has been found to be governed by the Burgers equation, which is derived analytically and solved numerically in planar coordinates. These SGSSs in such plasma systems are shown to be formed due to the presence of a viscous force (which is the source of dissipation) acting on inertial extremely heavy nuclei species of the plasma system. The fundamental properties (viz., amplitude, width, speed, etc.) of these SGSSs are influenced in the ultra-relativistic limit. Our considered plasma model and the numerical analysis of the Burgers equation can be applied to astrophysical compact objects like neutron stars.

Strange stars in energy–momentum-conserved f(R,T) gravity

For the accurate understanding of compact astrophysical objects, the Tolmann–Oppenheimer–Volkoff (TOV) equation has proved to be of great use. Nowadays, it has been derived in many alternative gravity theories, yielding the prediction of different macroscopic features for such compact objects. In this work, we apply the TOV equation of the energy–momentum–conserved version of the [Formula: see text] gravity theory to strange quark stars. The [Formula: see text] theory, with [Formula: see text] being a generic function of the Ricci scalar [Formula: see text] and trace of the energy–momentum tensor [Formula: see text] to replace [Formula: see text] in the Einstein–Hilbert gravitational action, has shown to provide a very interesting alternative to the cosmological constant [Formula: see text] in a cosmological scenario, particularly in the energy–momentum conserved case (a general [Formula: see text] function does not conserve the energy–momentum tensor). Here, we impose the condition [Formula: see text] to the astrophysical case, particularly the hydrostatic equilibrium of strange stars. We solve the TOV equation by taking into account linear equations of state to describe matter inside strange stars, such as [Formula: see text] and [Formula: see text], known as the MIT bag model, with [Formula: see text] the pressure and [Formula: see text] the energy density of the star, [Formula: see text] constant and [Formula: see text] the bag constant.

Neutrino propagation hinders fast pairwise flavor conversions

Neutrino flavor conversions may dramatically affect the inner working of compact astrophysical objects as well as the synthesis of the heavier elements. We present the first sophisticated numerical solution of the neutrino flavor conversion within a (2+1+1) dimensional setup: we include the advective term in the neutrino equations of motion and track the flavor evolution in two spatial dimensions, one angular variable, and time. Notably, the advective term hinders the development of neutrino pairwise conversions, if the conditions favoring such conversions (i.e., crossings between the angular distributions of νe and ν̄e or a non-negligible flux of neutrinos traveling backward with respect to the main propagation direction) exist for time scales shorter than the typical time scale of the advective term. As a consequence, fast pairwise conversions can only occur when the conditions favoring flavor conversions are self-sustained and global, such as the ones induced by the lepton emission self-sustained asymmetry (LESA) in core-collapse supernovae. Our work highlights the major impact of the dynamical evolution of the neutrino field on the growth of flavor instabilities and the strong interplay between classical and quantum effects. Critical limitations of the linear stability analysis, used to predict neutrino flavor instabilities, are also pointed out.

Magnetic field quantization in pulsars

Magnetic field quantization is an important issue for degenerate environments such as neutron stars, radio pulsars and magnetars etc., due to the fact that these stars have a magnetic field higher than the quantum critical field strength of the order of $4.4\times 10^{13}~\text{G}$ , accordingly, the cyclotron energy may be equal to or even much more than the Fermi energy of degenerate particles. We shall formulate here the exotic physics of strongly magnetized neutron stars, known as pulsars, specifically focusing on the outcomes of the quantized magnetic pressure. In this scenario, while following the modified quantum hydrodynamic model, we shall investigate both linear and nonlinear fast magnetosonic waves in a strongly magnetized, weakly ionized degenerate plasma consisting of neutrons and an electron–ion plasma in the atmosphere of a pulsar. Here, linear analysis depicts that sufficiently long, fast magnetosonic waves may exist in a weakly dispersive pulsar having finite phase speed at cutoff. To investigate one-dimensional nonlinear fast magnetosonic waves, a neutron density expression as a function of both the electron magnetic and neutron degenerate pressures, is derived with the aid of Riemann’s wave solution. Consequently, a modified Korteweg–de Vries equation is derived, having a rarefractive solitary wave solution. It is found that the basic properties such as amplitude, width and phase speed of the fast magnetoacoustic waves are significantly altered by the electron magnetic and the neutron degenerate pressures. The results of this theoretical investigation may be useful for understanding the formation and features of the solitary structures in astrophysical compact objects such as pulsars, magnetars and white dwarfs etc.

The MUSE-Faint survey

Aims. It has been shown that the ultra-faint dwarf galaxy Eridanus 2 may host a stellar cluster in its centre. If this cluster is shown to exist, it can be used to set constraints on the mass and abundance of massive astrophysical compact halo objects (MACHOs) as a form of dark matter. Previous research has shown promising expectations in the mass range of 10−100 M⊙, but lacked spectroscopic measurements of the cluster. We aim to provide spectroscopic evidence regarding the nature of the putative star cluster in Eridanus 2 and to place constraints on MACHOs as a constituent of dark matter. Methods. We present spectroscopic observations of the central square arcminute of Eridanus 2 from MUSE-Faint, a survey of ultra-faint dwarf galaxies with the Multi Unit Spectroscopic Explorer on the Very Large Telescope. We derived line-of-sight velocities for possible member stars of the putative cluster and for stars in the centre of Eridanus 2. We discuss the existence of the cluster and determine new constraints for MACHOs using the Fokker–Planck diffusion approximation. Results. Out of 182 extracted spectra, we identify 26 member stars of Eridanus 2, seven of which are possible cluster members. We find intrinsic mean line-of-sight velocities of 79.7+3.1−3.8 km s−1 and 76.0+3.2−3.7 km s−1 for the cluster and the bulk of Eridanus 2, respectively, as well as intrinsic velocity dispersions of < 7.6 km s−1 (68% upper limit) and 10.3+3.9−3.2 km s−1, respectively. This indicates that the cluster most likely exists as a distinct dynamical population hosted by Eridanus 2 and that it does not have a surplus of dark matter over the background distribution. Among the member stars in the bulk of Eridanus 2, we find possible carbon stars, alluding to the existence of an intermediate-age population. We derived constraints on the fraction of dark matter that can consist of MACHOs with a given mass between 1 and 105 M⊙. For dark matter consisting purely of MACHOs, the mass of the MACHOs must be less than ∼7.6 M⊙ and ∼44 M⊙ at a 68- and 95% confidence level, respectively.

Kinetic Alfvén soliton structure with exchange-correlation potential in quantum plasma

Solitary kinetic Alfvén waves are investigated in low-β electron-ion quantum plasma by considering the exchange-correlation effects of degenerate electrons by using the quantum hydrodynamic fluid model. It is found that both the width and amplitude of kinetic Alfvén solitons are modified in the presence of exchange-correlation effects. Moreover, the kinetic Alfvén soliton with hump profile moves with sub-Alfvénic wave speed. Our results may be helpful where quantum effects and plasma can coexist especially in solid-state objects (i.e. metallic, semiconductors and their nano-objects), inertial confinement fusion system and astrophysical compact objects (i.e. the interior of white dwarf and atmosphere of neutron stars).

Analytical model of dark energy stars

In this paper, we study the structure and stability of compact astrophysical objects which are ruled by the dark energy equation of state (EoS). The existence of dark energy is important for explaining the current accelerated expansion of the universe. Exact solutions to Einstein field equations (EFE) have been found by considering particularized metric potential, Finch and Skea ansatz. 1 The obtained solutions are relevant to the explanation of compact fluid sphere. Further, we have observed at the junction interface that the interior solution is matched with the Schwarzschild’s exterior vacuum solution. Based on that, we have noticed the obtained solutions are well in agreement with the observed maximum mass bound of [Formula: see text], namely, PSR J1416-2230, Vela X-1, 4U 1608-52, Her X-1 and PSR J1903+327, whose predictable masses and radii are not compatible with the standard neutron star models. Also, the stability of the stellar configuration has been discussed briefly, by considering the energy conditions, surface redshift, compactness, mass-radius relation in terms of the state parameter [Formula: see text]. Finally, we demonstrate that the features so obtained are physically acceptable and consistent with the observed/reported data.[Formula: see text] Thus, the present dark energy equation of state appears talented regarding the presence of several exotic astrophysical matters.

Nonlinear coupling of electromagnetic and electron acoustic waves in multi-species degenerate astrophysical plasma

Nonlinear wave-coupling is studied in a multispecies degenerate astrophysical plasma consisting of two electron species (at different temperatures): a highly degenerate main component plus a smaller classical relativistic flow immersed in a static neutralizing ion background. It is shown that the high frequency electromagnetic waves through their strong nonlinear interactions with the electron-acoustic waves [sustained by a multielectron component (degenerate) plasma surrounding a compact astrophysical object] can scatter to lower frequencies so that the radiation observed faraway will be spectrally shifted downward. It is also shown that, under definite conditions, the electromagnetic waves could settle into stationary solitonic states. It is expected that the effects of such structures may persist as detectable signatures in forms of modulated micropulses in the radiation observed far away from the accreting compact object. Both these effects will advance our abilities to interpret the radiation coming out of the compact objects.

Complexity factor for a class of compact stars in $f(R,T)$ gravity

We investigate the concept of complexity factor for a class of compact star in the framework of modified $f(R,T)$ gravity. We obtain a generic form of hydrostatic equilibrium equation, express the Einstein field equations, mass function and also physical observation for linear form of function $f(R,T)=R+2\lambda T$, where $\lambda $ is the coupling parameter, $R$ is Ricci scalar and $T$ is trace of energy momentum tensor. We have analyzed the properties of compact astrophysical objects like energy density and anisotropic pressure are affected by changing the values of coupling parameter $\lambda $. We obtained numerical outputs of some physical variables for different chosen values of coupling parameter $\lambda $ to observe the effect of $\lambda $ on these quantities and show these in tabular form for different compact stars $4U 1820\mbox{-}30$, $\mathit{Her} X\mbox{-}1$, $\mathit{SAX} J 1808.4\mbox{-}3658$ and $\mathit{VelaX}\mbox{-}12$ with radii 10, 7.7, 7.07 and 9.99 respectively. We determine structure scalars with orthogonal splitting of the Riemann tensor and with the help of these scalars the complexity factor can be determined. Furthermore, we have checked some astrophysical sources for vanishing complexity factor.

Head on interaction of magnetoacoustic solitons in a spin-1/2 dense plasma with geometrical effects

The head on interaction due to two magnetoacoustic solitons is studied in a dense electron-ion magnetoplasma with geometrical and electron spin effects. For this purpose, the extended Poincaré-Lighthill-Kuo (PLK) technique and quantum two fluid equations are utilized to derive a set of nonplanar Korteweg–de Vries equations. The PLK method incorporates the post collision trajectories and the phase shifts encountered by the magnetoacoustic solitons after collision. The interaction is numerically analyzed by choosing some typical plasma parameters from compact astrophysical objects. It is observed that the spin-1/2 effects, statistical pressure, displacement current and geometry of the system significantly modify the phase shifts encountered by the solitons. It is noticed that the enhancement in the number density reduces the phase shift of the colliding solitons. Furthermore, the cylindrical geometry is observed to decrease phase shift by comparison with the planar geometry. The present investigation provides a deeper insight of head on collision caused by the magnetoacoustic solitons in dense magnetoplasmas.

Self‐gravito‐acoustic waves and their instabilities in a self‐gravitating degenerate quantum plasma system

A self‐gravitating degenerate quantum plasma (SGDQP) system containing degenerate electron and light nucleus species along with extremely low‐dense heavy‐nucleus species is considered. The existence of new degenerate pressure‐driven self‐gravito‐acoustic (DPDSGA) waves in this SGDQP system is found, and their dispersion properties along with stable and unstable parametric regimes are identified. The DPDSGA waves emit from this SGDQP system due to the compression and rarefaction (and vice‐versa) of the perturbed state of it. Its compression is due to the inward poll of degenerate electron and light nucleus species by the self‐gravitational attractive pressures, whereas its rarefaction is due to the outward degenerate pressures exerted by the degenerate electron and light nucleus species. The DPDSGA waves are new because they completely disappear if the electron and light nucleus degeneracies are neglected. The DPDSGA waves exist in the SGDQP system that occurs in astrophysical compact objects like white dwarfs [H. M. Van Horn, Science 252, 384 (1991); D. Koester, Astron. Astrophys. Rev. 11, 33 (2002)].

Nonlinear excitations in a degenerate relativistic magneto-rotating quantum plasma

An investigation of heavy nucleus-acoustic (HNA) excitations in a degenerate relativistic magnetorotating quantum plasma system comprising relativistically degenerate light nuclei/electrons and inertial nondegenerate heavy nuclei has been presented. The Zakharov-Kuznetsov-Burgers (ZKB) equation has been derived by employing the reductive perturbation method. The solution of the ZKB equation supports only positive potential monotonic and oscillatory HNA shock waves in congruence with the space observations. It is observed that the heavy nucleus viscosity is a source of dissipation and is responsible for the formation of HNA monotonic and oscillatory shock structures. Bifurcation analysis is also examined in the absence of dissipation. It is shown that the combined effects of external magnetic field strength, rotational frequency, and obliqueness significantly modify the basic properties of different HNA nonlinear structures. The results should be utilitarian to understand the characteristics of nonlinear excitations in degenerate relativistic magnetorotating quantum plasma which is present in astrophysical compact objects especially in white dwarfs and neutron stars.

Probing boson stars with extreme mass ratio inspirals

We propose to use gravitational waves from extreme mass ratio inspirals (EMRI), composed of a boson star and a supermassive black hole in the center of galaxies, as a new method to search for boson stars. Gravitational waves from EMRI have the advantage of being long-lasting within the frequency band of future space-based interferometer gravitational wave detectors and can accumulate large signal-to-noise ratio (SNR) for very sub-solar mass boson stars. Compared to gravitational waves from boson star binaries, which fall within the LIGO band, we find that much larger ranges of the mass and compactness of boson stars, as well as the underlying particle physics parameter space, can be probed by EMRI. We take tidal disruption of the boson stars into account and distinguish those which dissolve before the inner-most-stable-circular-orbit (ISCO) and those which dissolve after it. Due to the large number of cycles recorded, EMRIs can lead to a very precise mass determination of the boson star and distinguish it from standard astrophysical compact objects in the event of a signal. Tidal effects in inspiralling binary systems, as well as possible correlated electromagnetic signals, can also serve as potential discriminants.

A conservative energy-momentum tensor in the f(R,T) gravity and its implications for the phenomenology of neutron stars

The solutions for the Tolmann-Oppenheimer-Volkoff (TOV) equation bring valuable informations about the macroscopical features of compact astrophysical objects as neutron stars. They are sensitive to both the equation of state considered for nuclear matter and the background gravitational theory. In this work we construct the TOV equation for a conservative version of the $f(R,T)$ gravity. While the non-vanishing of the covariant derivative of the $f(R,T)$ energy-momentum tensor yields, in a cosmological perspective, the prediction of creation of matter throughout the universe evolution as shown by T. Harko, in the analysis of the hydrostatic equilibrium of compact astrophysical objects, this property still lacks a convincing physical explanation. The imposition of $\nabla^{\mu}T_{\mu\nu}=0$ demands a particular form for the function $h(T)$ in $f(R,T)=R+h(T)$, which is here derived. Therefore, the choice of a specific equation of state for the star matter demands a unique form of $h(T)$, manifesting a strong connection between conserved $f(R,T)$ gravity and the star matter constitution. We construct and solve the TOV equation for the general equation of state for $p=k\rho^{\Gamma}$, with $k$ being the EoS parameter, $\rho$ {\it the energy density} and $\Gamma$ is the adiabatic index. We also derive the macroscopical properties of neutron stars ($\Gamma=5/3$) within this approach.

Quantum Rayleigh-Taylor instability: the exchange and correlation effects of electron

Using quantum magneto-hydrodynamic (Q-MHD) model, a set of governing equations is presented for a Rayleigh–Taylor (RT) instability in an extremely dense plasma. A local dispersion relation is derived for the RT mode in a magnetized plasma, where the effects of exchange and correlation for electrons are taken into account. The dispersion relation is then analyzed both analytically as well as numerically. It is found that inclusion of the exchange and correlation effects modifies the growth rate of the RT instability and gives the quantum RT mode a stabilizing effect. The linear growth rate of the mode is found to be localized due to Bohm potential and the potential associated with the exchange and correlation. It is estimated that results might be useful in interpreting the dynamical properties of a quantum nonuniform magneto-plasma system on industrial phenomena such as ultra-small electronic devices as well as natural phenomena such as astrophysical compact objects (white dwarfs/neutron stars).

One-fluid Equations of General Relativistic Two-fluid Plasma with the Landau–Lifshitz Radiation Reaction Force in Curved Space

Incorporating the radiation reaction force into two-fluid plasma in curved space, we get a set of one-fluid general relativistic magnetohydrodynamics (GRMHD) equations with the Landau-Lifshitz radiation reaction force. We analyze the importance of the radiation reaction acting on plasma around an astrophysical compact object.

Compact objects in conformal nonlinear electrodynamics

In this paper we consider a special case of vacuum nonlinear electrodynamics with a stress–energy tensor conformal to the Maxwell theory. Distinctive features of this model are the absence of a dimensional parameter for the nonlinearity description and a very simple form of the dominant energy condition, which can easily be verified in an arbitrary pseudo-Riemannian space-time with the consequent constraints on the model parameters. In this paper we analyze some properties of astrophysical compact objects coupled to conformal vacuum nonlinear electrodynamics.

Kinetic Alfven waves in dense quantum plasmas with effect of spin magnetization

Using Sagdeev potential approach, propagation characteristics of kinetic Alfven waves (KAWs) are investigated in a collisionless low-β quantum plasma, taking into account the electrons spin magnetization effects. The numerical analysis illustrates that the dynamics of linear and nonlinear structures are significantly modified due to the change in spin magnetization effects. Moreover, It is found that only density hump structures are formed in the sub-Alfvenic region. Our study may offer a unique opportunity in understanding the structural and electrodynamical properties of astrophysical compact objects (i.e. neutron stars, pulsars, magnetars and interior of white dwarf etc.)

Modeling the dynamics of black holes through balanced equations of motion

We present a general approach for the formulation of equations of motion for astrophysical compact objects in general relativistic theories. The objects are modeled as relativistic particles, which are assumed to be moving in a geometric background which in turn is asymptotically flat. The background can be affected by the compact object; so that our approach can be applied to binary systems; by concentrating on the main contributions coming from back reaction effects due to gravitational radiation. Our premises are different from the traditional post-Newtonian and self-force approaches since we make strong use of the asymptotic properties of the self geometry of the particle; as is described below. In particular, we use a center of mass setting, and relativistic individual dynamical times for each object. We expect our model to complement the other approaches in different regimes.