Observations Of Compact Stars Research Articles

Color-flavor locked strange stars admixed with mirror dark matter and the observations of compact stars

We investigate the structure and the tidal deformability of the color-flavor locked strange stars admixed with mirror dark matter. Assuming the stars in the GW170817 event have a mirror-dark-matter core or a mirror-dark-matter halo, the observations of the central compact object within the supernova remnant HESS J1731-347 and the compact objects in the GW190814 and GW170817 events could be explained simultaneously with a pairing gap much smaller than 200 MeV. In contrast, a pairing gap larger than about 200 MeV must be employed without the consideration of a mirror-dark-matter core (halo). More importantly, we find that for the case of the quartic coefficient a 4 < 0.589, if the mass fraction of the mirror dark matter (fD ) of the compact stars in GW170817 is in a certain range (eg., 22.8% < fD < 77.2% for a 4 = 0.55), the minimum allowed value of the pairing gap could be less than 46.5 MeV (i.e., one half of the value of the strange quark mass which is taken as 93 MeV in this paper), which leads to the result that all astrophysical observations mentioned above could be satisfied without violating the conformal bound or the recently proposed positive trace anomally bound.

Study of self-bound compact stars in f(T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathscr {T})$$\\end{document} gravity and observational constraints on the model parameters

The present study is based on f(T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathscr {T})$$\\end{document} gravity, where we impose possible observational constraints on the model parameters to obtain physically plausible features of compact stars, specifically neutron stars. To do so, as a first step, we consider the field equations in the f(T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathscr {T})$$\\end{document} gravity framework. We then solve the field equations to generate a set of new exact solutions in f(T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathscr {T})$$\\end{document} gravity where model parameters are found by using boundary conditions. A few tests are performed to asses the stability of the model and compare the obtained results with observations of compact stars, in particular with the compact binary merger event GW190814. We observe the maximum mass of the star beyond the 3M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3M_{\\odot }$$\\end{document} when the surface density is of the order 1014\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{14}$$\\end{document}gm/cm3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document} for the higher torsion parameter, which implies that an anisotropic solution in teleparallel gravity is more suitable for modeling of massive compact objects in lower mass-gap\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$mass-gap$$\\end{document}, and thus the model presented herein provides a satisfactory physical scenario with respect to the observational signature.

Confronting Strange Stars with Compact-Star Observations and New Physics

Strange stars ought to exist in the universe according to the strange quark matter hypothesis, which states that matter made of roughly equal numbers of up, down, and strange quarks could be the true ground state of baryonic matter rather than ordinary atomic nuclei. Theoretical models of strange quark matter, such as the standard MIT bag model, the density-dependent quark mass model, or the quasi-particle model, however, appear to be unable to reproduce some of the properties (masses, radii, and tidal deformabilities) of recently observed compact stars. This is different if alternative gravity theory (e.g., non-Newtonian gravity) or dark matter (e.g., mirror dark matter) are considered, which resolve these issues. The possible existence of strange stars could thus provide a clue to new physics, as discussed in this review.

Can fermion-boson stars reconcile multimessenger observations of compact stars?

Mixed fermion-boson stars are stable, horizonless, everywhere regular solutions of the coupled Einstein-(complex, massive) Klein-Gordon-Euler system. While isolated neutron stars and boson stars are uniquely determined by their central energy density, mixed configurations conform an extended parameter space that depends on the combination of the number of fermions and (ultra-light) bosons. The wider possibilities offered by fermion-boson stars could help explain the tension in the measurements of neutron star masses and radii reported in recent multi-messenger observations and nuclear-physics experiments. In this work we construct equilibrium configurations of mixed fermion-boson stars with realistic equations of state for the fermionic component and different percentages of bosonic matter. We show that our solutions are in excellent agreement with multi-messenger data, including gravitational-wave events GW170817 and GW190814 and X-ray pulsars PSR J0030+0451 and PSR J0740+6620, as well as with nuclear physics constraints from the PREX-2 experiment.

Strange stars with a mirror-dark-matter core confronting with the observations of compact stars

We investigate the structure and the tidal deformability of strange stars (SSs) with a mirror-dark-matter (MDM) core for the standard MIT bag model. We find that to explain the observations of PSR J0740+6620, PSR J0030+0451 and GW170817 simultaneously, SSs in GW170817 should have a MDM core although it is unnecessary for PSR J0740+6620 and PSR J0030+0451 to contain a MDM core. Our study leads to the result that for the standard MIT bag model, the observations of compact stars mentioned above confirm the existence of a dark-matter core inside SSs.

Non-Newtonian gravity in strange stars and constraints from the observations of compact stars

We study non-Newtonian gravity effects on the structure and the tidal deformability of strange stars using the standard MIT model, and compare the theory with the observations of compact stars following the approach of Yang et al. (2020). Different to Yang et al. (2020), in which the authors only employed the constraints from the mass of PSR J0740+6620 and the dimensionless tidal deformability of a 1.4M⊙ star of GW170817, here we also consider another constraint, namely, from the observations of the mass and radius of the isolated pulsar PSR J0030+0451 by NICER. We find that for the standard MIT model, strange stars can not agree with the observations of PSR J0740+6620, PSR J0030+0451 and GW170817 simultaneously. In other words, for the standard MIT model, the observations of PSR J0740+6620, PSR J0030+0451 and GW170817 rule out the possibility of the existence of strange stars even when we consider the effects of non-Newtonian gravity.

Constraints from compact star observations on non-Newtonian gravity in strange stars based on a density dependent quark mass model

Using a density dependent quark mass (QMDD) model for strange quark matter, we investigate the effects of non-Newtonian gravity on the properties of strange stars and constrain the parameters of the QMDD model by employing the mass of PSR J0740+6620 and the tidal deformability of GW170817. We find that for QMDD model these mass and tidal deformability observations would rule out the existenceof str ange stars if non-Newtonian gravity effects are ignored. For the current quark masses of $m_{u0}=2.16$ MeV, $m_{d0}=4.67$ MeV, and $m_{s0}=93$ MeV, we find that a strange star can exist for values of the non-Newtonian gravity parameter $g^{2}/\mu^{2}$ in the range of 4.58 GeV$^{-2}\leq g^{2}/\mu^{2}\leq$ 9.32 GeV$^{-2}$, and that the parameters $D$ and $C$ of the QMDD modelare restricted to 158.3 MeV$\leq D^{1/2}\leq$ 181.2 MeV and $-0.65\leq C \leq -0.12$. It is found that the largest possible maximum mass of a strange star obtained with the QMDD model is $2.42 \, M_{\odot}$, and that the secondary componentof GW190814 with a mass of 2.59_{-0.09}^{+0.08}\, M_{\dot} could not be a static strange star. We also find that forthe mass and radius of PSR J0030+0451 given by Riley et al. through the analysis of observational data of NICER, there exists a very tiny allowed parameter space for which strange stars computed for the QMDD model agree with the observations of PSR J0740+6620, GW17 0817 and PSR J0030+0451 simultaneously. However, for the mass and radius given by Miller et al., no such parameter space exist.

First-order phase transition from hypernuclear matter to deconfined quark matter obeying new constraints from compact star observations

We reconsider the problem of the hyperon puzzle and its suggested solution by quark deconfinement within the two-phase approach to hybrid compact stars with recently obtained hadronic and quark matter equations of state. For the hadronic phase we employ the hypernuclear equation of state from the lowest order constrained variational method and the quark matter phase is described by a sufficiently stiff equation of state based on a color superconducting nonlocal Nambu-Jona-Lasinio model with constant (model nlNJLA) and with density-dependent (model nlNJLB) parameters. We study the model dependence of the phase transition obtained by a Maxwell construction. Our study confirms that also with the present set of equations of state quark deconfinement presents a viable solution of the hyperon puzzle even for the new constraint on the lower limit of the maximum mass from PSR J0740+6620. In this work we provide with model nlNJLB for the first time a hybrid star EoS with an intermediate hypernuclear matter phase between the nuclear and color superconducting quark matter phases, for which the maximum mass of the compact star reaches $2.2~M_\odot$, in accordance with most recent constraints. In model nlNJLA such a phase cannot be realised because the phase transition onset is at low densities, before the hyperon threshold density is passed. We discuss possible consequences of the hybrid equation of state for the deconfinement phase transition in symmetric matter as it will be probed in future heavy-ion collisions at FAIR, NICA and corresponding energy scan programs at the CERN and RHIC facilities.

Compact Star Properties from an Extended Linear Sigma Model

The equation of state provided by effective models of strongly interacting matter should comply with the restrictions imposed by current astrophysical observations of compact stars. Using the equation of state given by the (axial-)vector meson extended linear sigma model, we determine the mass–radius relation and study whether these restrictions are satisfied under the assumption that most of the star is filled with quark matter. We also compare the mass–radius sequence with those given by the equations of state of somewhat simpler models.

Two Novel Approaches to the Hadron-Quark Mixed Phase in Compact Stars

First-order phase transitions, such as the liquid-gas transition, proceed via formation of structures, such as bubbles and droplets. In strongly interacting compact star matter, at the crust-core transition but also the hadron-quark transition in the core, these structures form different shapes dubbed “pasta phases”. We describe two methods to obtain one-parameter families of hybrid equations of state (EoS) substituting the Maxwell construction that mimic the thermodynamic behaviour of pasta phase in between a low-density hadron and a high-density quark matter phase without explicitly computing geometrical structures. Both methods reproduce the Maxwell construction as a limiting case. The first method replaces the behaviour of pressure against chemical potential in a finite region around the critical pressure of the Maxwell construction by a polynomial interpolation. The second method uses extrapolations of the hadronic and quark matter EoS beyond the Maxwell point to define a mixing of both with weight functions bounded by finite limits around the Maxwell point. We apply both methods to the case of a hybrid EoS with a strong first order transition that entails the formation of a third family of compact stars and the corresponding mass twin phenomenon. For both models, we investigate the robustness of this phenomenon against variation of the single parameter: the pressure increment at the critical chemical potential that quantifies the deviation from the Maxwell construction. We also show sets of results for compact star observables other than mass and radius, namely the moment of inertia and the baryon mass.

Effect of quantum fluctuations in the high-energy cold nuclear equation of state and in compact star observables

We present a novel technique to obtain exact equation of state (EoS) by the Functional Renormalization Group (FRG) method, using the expansion of the effective potential in a base of harmonic functions at finite chemical potential. Within this theoretical framework we determined the equation of state and the phase diagram of a simple model of massless fermions coupled to scalars through Yukawa-coupling at the zero-temperature limit. We compared our results to the 1-loop and the mean field approximation of the same model and other high-density nuclear matter equation of states. We found a $10-20\%$ difference between these approximations. As an application, we used our exact, FRG-based equation of states to test the effect of the quantum fluctuations in superdense nuclear matter of a compact astrophysical object for the first time. We calculated the mass-radius relation for a compact star using the Tolmann-Oppenheimer-Volkov equation and observed a $\sim 5\%$ effect in compact star observables due to quantum fluctuations.

The Effect of Quantum Fluctuations in Compact Star Observables

AbstractAstrophysical measurements regarding compact stars are just ahead of a big evolution jump, since the NICER experiment deployed on ISS on 2017 June 14. This will provide soon data that would enable the determination of compact star radius with less than 10% error. This can be further constrained by the new observation of gravitational waves originated from merging neutron stars, GW170817. This poses new challenges to nuclear models aiming to explain the structure of super dense nuclear matter found in neutron stars. Detailed studies of the QCD phase diagram show the importance of bosonic quantum fluctuations in the cold dense matter equation of state. Here we used a demonstrative model with one bosonic and one fermionic degree of freedom coupled by Yukawa coupling, we show the effect of bosonic quantum fluctuations on compact star observables such as mass, radius, and compactness. We have also calculated the difference in the value of compressibility which is caused by quantum fluctuations. The above-mentioned quantities are calculated in the mean field, one-loop, and in high order many loop approximation. The results show that the magnitude of these effects is in the range of 4–5%, which place it into the region where modern measurements may detect it. This forms a base for further investigations that how these results carry over to more complicated models.

The stifness of the supranuclear equation of state (once again)

We revisit the present status of the stiffness of the supranuclear equations of state, particularly the solutions that increase the stiffness in the presence of hyperons, the putative transition to a quark matter phase and the robustness of massive compact star observations.

Probing dense matter in compact star cores with radio pulsar data

Astrophysical observations of compact stars provide, in addition to collider experiments, the other big source of information on matter under extreme conditions. The largest and most precise data set about neutron stars is the timing data of radio pulsars. We show how this unique data can be used to learn about the ultra-dense matter in the compact star interior. The method relies on astro-seismology based on special global oscillation modes (r-modes) that emit gravitational waves. They would prevent pulsars from spinning with their observed high frequencies, unless the damping of these modes, determined by the microscopic properties of matter, can prevent this. We show that for each form of matter there is a distinct region in a frequency/spindown-rate diagram where r-modes can be present. We find that stars containing ungapped quark matter are consistent with both the observed radio and x-ray data, whereas, even when taking into account the considerable uncertainties, neutron star models with standard viscous damping are inconsistent with both data sets and additional damping mechanisms would be required.

Do lattice data constrain the vector interaction strength of QCD?

We show how repulsive interactions of deconfined quarks as well as confined hadrons have an influence on the baryon number susceptibilities and the curvature of the chiral pseudo-critical line in effective models of QCD. We discuss implications and constraints for the vector interaction strength from comparisons to lattice QCD and comment on earlier constraints, extracted from the curvature of the transition line of QCD and compact star observables. Our results clearly point to a strong vector repulsion in the hadronic phase and near-zero repulsion in the deconfined phase.

Compact stars in alternative theories of gravity: Einstein-Dilaton-Gauss-Bonnet gravity

We develop a theoretical framework to study slowly rotating compact stars in a rather general class of alternative theories of gravity, with the ultimate goal of investigating constraints on alternative theories from electromagnetic and gravitational-wave observations of compact stars. Our Lagrangian includes as special cases scalar-tensor theories (and indirectly $f(R)$ theories) as well as models with a scalar field coupled to quadratic curvature invariants. As a first application of the formalism, we discuss (for the first time in the literature) compact stars in Einstein-Dilaton-Gauss-Bonnet gravity. We show that compact objects with central densities typical of neutron stars cannot exist for certain values of the coupling constants of the theory. In fact, the existence and stability of compact stars sets more stringent constraints on the theory than the existence of black hole solutions. This work is a first step in a program to systematically rule out (possibly using Bayesian model selection) theories that are incompatible with astrophysical observations of compact stars.

Constraining asymmetric dark matter through observations of compact stars

We put constraints on asymmetric dark matter candidates with spin-dependent interactions based on the simple existence of white dwarfs and neutron stars in globular clusters. For a wide range of the parameters (WIMP mass and WIMP-nucleon cross section), weakly interacting massive particles (WIMPs) can be trapped in progenitors in large numbers and once the original star collapses to a white dwarf or a neutron star, these WIMPs might self-gravitate and eventually collapse forming a mini--black hole that eventually destroys the star. We impose constraints competitive to direct dark matter search experiments, for WIMPs with masses down to the TeV scale.

Vacuum revealed: The final state of vacuum instabilities in compact stars

Quantum fields in compact stars can be amplified due to a semiclassical instability, which may affect the structure of relativistic stars. We show that a classical counterpart to this instability occurs in exactly the same region of the parameter space. For negative coupling parameter $\ensuremath{\xi}$, the instability is related to the well-known ``spontaneous scalarization'' effect. The plausible end state is a static, asymptotically flat configuration with a nonzero scalar field, which is compatible with weak-field experiments and energetically favored over stellar solutions in general relativity. For $\ensuremath{\xi}&gt;0$ the new configurations are energetically disfavored, and the end state of the instability remains an open issue. The vacuum instability may lead to new experimental opportunities to probe the nature of vacuum energy via astrophysical observations of compact stars.

Electron–positron pair oscillation in spatially inhomogeneous electric fields and radiation

It is known that strong electric fields produce electron and positron pairs from the vacuum, and due to the back-reaction these pairs oscillate back and forth coherently with the alternating electric fields in time. We study this phenomenon in spatially inhomogeneous and bound electric fields by integrating the equations of energy–momentum and particle-number conservations and Maxwell equations. The space and time evolutions of the pair-induced electric field, electric charge- and current-densities are calculated. The results show non-vanishing electric charge-density and the propagation of pair-induced electric fields, that are different from the case of homogeneous and unbound electric fields. The space and time variations of pair-induced electric charges and currents emit an electromagnetic radiation. We obtain the narrow spectrum and intensity of this radiation, whose peak ωpeak locates in the region around 4 keV for electric field strength ∼Ec. We discuss their relevances to both the laboratory experiments for electron and positron pair-productions and the astrophysical observations of compact stars with an electromagnetic structure.

Cold quark matter

We perform an O(alpha_s^2) perturbative calculation of the equation of state of cold but dense QCD matter with two massless and one massive quark flavor, finding that perturbation theory converges reasonably well for quark chemical potentials above 1 GeV. Using a running coupling constant and strange quark mass, and allowing for further non-perturbative effects, our results point to a narrow range where absolutely stable strange quark matter may exist. Absent stable strange quark matter, our findings suggest that quark matter in compact star cores becomes confined to hadrons only slightly above the density of atomic nuclei. Finally, we show that equations of state including quark matter lead to hybrid star masses up to M~2M_solar, in agreement with current observations. For strange stars, we find maximal masses of M~2.75M_solar and conclude that confirmed observations of compact stars with M>2M_solar would strongly favor the existence of stable strange quark matter.