Strange Matter Research Articles

Quark star matter at finite temperature

We have investigated the properties of strange quark matter and quark stars at finite temperature within the confined isospin-density-dependent mass (CIDDM) model. The newly discovered heavy compact stars PSR $\mathrm{J}0348+0432$ ($2.01\ifmmode\pm\else\textpm\fi{}0.04\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$) and MSP $\mathrm{J}0740+6620$ ($2.14{\ifmmode\pm\else\textpm\fi{}}_{0.09}^{0.10}\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ of 68.3% credibility interval and $2.14{\ifmmode\pm\else\textpm\fi{}}_{0.18}^{0.20}\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ of 95.4% credibility interval) can be well described as quark stars with large quark matter symmetry energy within the CIDDM model in this work. In particular, we also calculate the properties of the protoquark stars (PQSs) at the heating and cooling stages along the star evolution line, and we find that the tidal deformability of PQSs increases with temperature in the heating stages within the CIDDM model.

The Hypothesis of Strangelets in the Cosmic Rays

The experimental confirmation of the presence of a penetrating component in cosmic rays is presented. Unlike previous experiments, it is shown that the component is not born in the lead of the detectors, but is present in the primary radiation. In this case, the particles forming the penetrating component must be stable. Among the nuclear-active particles, there are only two stable variants. These are nuclei, among which the most penetrating are protons, and hypothetical particles of strange quark matter—strangelets (quasi-nuclei). The analysis of the muon component is not consistent with the proton variant.

Color-flavor-locked matter in a nonuniform strong magnetic field

The magnetic-color-flavor-locked quark matter is investigated in the Nambu–Jona-Lasinio model. Strong nonuniform magnetic fields are employed in order to mimic the spatial dependence of the magnetic field strength at high densities. We find that the gap solution depends strongly on the diquark coupling interaction. Based on the requirement that the parallel pressure should be an increasing function of the chemical potential, an upper limit on the central magnetic field [Formula: see text][Formula: see text]G is obtained, which increases slightly with the diquark interaction strength. Furthermore, much stronger central magnetic field could occur in the stars composed of magnetic-color-flavor matter than that of ordinary strange quark matter.

Strange Stars in the Vector Interaction Enhanced Bag Model

The vector interaction enhanced Bag model (vBag) for dense quark matter extends the commonly used thermodynamic Bag model (tdBag) by incorporating effects of dynamical chiral symmetry breaking (D χ SB) and vector repulsion. Motivated by the suggestion that the stability of strange matter is in tension with chiral symmetry breaking (D χ SB) we examine the parameter space for its stability in the vBag model in this work. Assuming the chiral transition occurs at sufficiently low density, we determine the stability region of strange matter as a function of the effective Bag constant and the vector coupling. As an astrophysical application, we construct contours of maximum mass M max and radius at maximum mass R max in this region of parameter space. We also study the stability of strange stars in the vBag model with maximum mass in the 2 M ⊙ range by computing the spectrum of radial oscillations, and comparing to results from the tdBag model, find some notable differences.

Study on charged strange stars in f(R, T) gravity

We investigate the effects of the modified f(R, T) gravity on the charged strange quark stars with the standard choice of f(R, T)=R+2χ T. Those types of stars are supposed to be made of strange quark matter (SQM) whose distribution is governed by the phenomenological MIT bag EOS as p=1/3(ρ−4B), where B is the bag constant, while the form of charge distribution is chosen to be q(r)=Q(r/ℛ)3=α r3 with α as a constant. We derive the values of the unknown parameters by matching the interior spacetime to the exterior Reissner-Nordstr{öm metric followed by the appropriate choice of the values of the parameters χ and α. Our study reveals that besides SQM, a new kind of matter distribution originates due to the interaction between the matter and the extra geometric term, while the modification of the Tolman-Oppenheimer-Volkoff (TOV) equation invokes the presence of a new force Fc. The accumulation of the electric charge distribution reaches its maximum at the surface, and the predicted values of the corresponding electric charge and electric field are of the order of 1019−20C and 1021−22 V/cm, respectively. To examine the physical validity of our solutions, we perform tests of the energy conditions, stability against the equilibrium of the forces, the adiabatic index, etc., and find that the proposed f(R, T) model survives all these critical tests. Therefore, our model can describe the non-singular charged strange stars and justify the supermassive compact stellar objects having their masses beyond the maximum mass limit for the compact stars in the standard scenario. Our model also supports the existence of several exotic astrophysical objects like super-Chandrasekhar white dwarfs, massive pulsars, and even magnetars, which remain unexplained in the framework of General Relativity (GR).

Contraction of cold neutron star due to in the presence a quark core

Motivated by importance of the existence of quark matter on structure of neutron star. For this purpose, we use a suitable equation of state (EoS) which include three different parts: (i) a layer of hadronic matter, (ii) a mixed phase of quarks and hadrons, and, (iii) a strange quark matter in the core. For this system, in order to do more investigation of the EoS, we evaluate energy, Le Chatelier’s principle and stability conditions. Our results show that the EoS satisfies these conditions. Considering this EoS, we study the effect of quark matter on the structure of neutron stars such as maximum mass and the corresponding radius, average density, compactness, Kretschmann scalar, Schwarzschild radius, gravitational redshift and dynamical stability. Also, considering the mentioned EoS in this paper, we find that the maximum mass of hybrid stars is a little smaller than that of the corresponding pure neutron star. Indeed the maximum mass of hybrid stars can be quite close to the pure neutron stars. Our calculations about the dynamical stability show that these stars are stable against the radial adiabatic infinitesimal perturbations. In addition, our analyze indicates that neutron stars are under a contraction due to the existence of quark core.

Strange stars with MIT bag model in the Rastall theory of gravity

The aim of this paper is to study the charged anisotropic strange stars in the Rastall framework. Basic formulation of field equations in this framework is presented in the presence of charged anisotropic source. To obtain the solutions of the Rastall field equations in spherically symmetric Karori and Barua (KB) type space-time, we have considered a linear equation of state of strange matter, using the MIT bag model. The constraints on the Rastall dimensionless parameter [Formula: see text] are also discussed to obtain the physically reasonable solution. We explore some physical features of the presented model like energy conditions, stability and hydrostatic equilibrium, which are necessary to check the physical viability of the model. We also sought for the influence of the Rastall dimensionless parameter on the behavior of the physical features of obtained solution. We plot the graphs of matter variables for different chosen values of the parameter [Formula: see text] to inspect more details of analytical investigations and predict the numerical values of these variables exhibited in the tabular form. For this analysis, we choose four different arbitrary models of strange stars with compactness [Formula: see text] 0.25, 0.30, 0.35 and 0.40. We observed that all the necessary physical conditions are satisfied and the presented model is quite reasonable to study the strange stars.

Study of anisotropic strange stars in f(R,T) gravity: An embedding approach under the simplest linear functional of the matter-geometry coupling

The present work is focused on the investigation of the existence of compact structures describing anisotropic matter distributions within the framework of modified gravity theories, specifically f(R,$\mathcal{T}$) gravity theory. Additionally, we have taken f(R,$\mathcal{T}$) as a linear function of the Ricci scalar $R$ and the trace of the energy-momentum tensor $\mathcal{T}$ as $f(R,\mathcal{T})$=$R+2\chi\mathcal{T}$,where $\chi$ is a dimensionless coupling parameter, and the Lagrangian matter $\mathcal{L}_m=-\frac{1}{3}\left(2p_{t}+p_{r}\right)$, to describe the complete set of field equations for the anisotropic matter distribution. We follow the embedding class one procedure using Eisland condition to obtain a full space-time description inside the stellar configuration. Once the space-time geometry is specified we determine the complete solution of the modified Einstein equations by using the MIT bag model equation of state $p_{r}=\frac{1}{3}\left(\rho-4B\right)$ that describes the strange quark matter (SQM) distribution inside the stellar system, where $B$ denotes a bag constant. The physical validity of our anisotropic solution is confirmed by executing several physical tests. It is worth mentioning that with the help of the observed mass values for the various strange star candidates we have predicted the exact radii by taking different values for $\chi$ and $B$. These predicted radii show monotonic decreasing nature as the parameter $\chi$ is moved from $-0.8$ to $0.8$ progressively. In this case, our anisotropic stellar system becomes more massive and transforms into more dense compact stars.

Simulating Binary Neutron Stars with Hybrid Equation of States: Gravitational Waves, Electromagnetic Signatures and Challenges for Numerical Relativity

The gravitational wave and electromagnetic signatures connected to the merger of two neutron stars allow us to test the nature of matter at supranuclear densities. Since the Equation of State governing the interior of neutron stars is only loosely constrained, there is even the possibility that strange quark matter exists inside the core of neutron stars. We investigate how strange quark matter cores affect the binary neutron star coalescence by performing numerical relativity simulations. Interestingly, the strong phase transition can cause a reduction of the convergence order of the numerical schemes to first order if the numerical resolution is not high enough. Therefore, an additional challenge is added in producing high-quality gravitational wave templates for Equation of States with a strong phase transition. Focusing on one particular configuration of an equal mass configuration consistent with GW170817, we compute and discuss the associated gravitational wave signal and some of the electromagnetic counterparts connected to the merger of the two stars. We find that existing waveform approximants employed for the analysis of GW170817 allow describing this kind of systems within the numerical uncertainties, which, however, are several times larger than for pure hadronic Equation of States, which means that even higher resolutions have been employed for an accurate gravitational wave model comparison. We also show that for the chosen Equation of State, quasi-universal relations describing the gravitational wave emission after the moment of merger seem to hold and that the electromagnetic signatures connected to our chosen setup would not be bright enough to explain the kilonova associated to GW170817.

Relativistic strange stars in Tolman–Kuchowicz spacetime

In this article we propose a relativistic model of a static spherically symmetric anisotropic strange star with the help of Tolman–Kuchowicz (TK) metric potentials (Tolman, 1939 and Kuchowicz, 1968). The form of the potentials are λ(r)=ln(1+ar2+br4) and ν(r)=Br2+2lnC where a, b, B and C are constants which we have to evaluate using boundary conditions. We also consider the simplest form of the phenomenological MIT bag equation of state (EOS) to represent the strange quark matter (SQM) distribution inside the stellar system. Here, the radial pressure pr relates with the density profile ρ as follows, pr(r)=13[ρ(r)−4Bg], where Bg is the Bag constant. To check the physical acceptability and stability of the stellar system based on the obtained solutions, we have performed various physical tests. It is shown that the model satisfies all the stability criteria, including nonsingular nature of the density and pressure, implies stable nature. Here, the Bag constant for different strange star candidates are found to be (68–70) MeV/fm3 which satisfies all the acceptability criteria and remains in the experimental range.

A possible solution to the Hubble constant discrepancy: Cosmology where the local volume expansion is driven by the domain average density

Abstract The Hubble constant problem is the discrepancy between different measurements of the Hubble constant on different scales. We show that this problem can be resolved within the general relativistic framework of the perturbation theory in the inhomogeneous universe, with the help of a spatial averaging procedure over a finite local domain in the $t={\rm{const.}}$ hypersurface. The idea presented in this paper is unique in the sense that it has all of the following properties. a) It is based on the general relativistic perturbation theory, with ordinary dust matter only. No strange matter nor energy components are required. b) The employment of the spatially invariant averaging procedure on the finite domain is essential. c) The key is the first-order effect of the inhomogeneities in the linear perturbation theory. No nonlinear effects are required.

Maximum Mass of Differentially Rotating Strange Quark Stars

Abstract We present the first fully relativistic numerical calculations of differentially rotating strange quark stars models for broad ranges of the maximum density and of the degree of differential rotation. Our simulations are performed with the very accurate and stable multi-domain spectral code FlatStar and use the MIT Bag model for describing strange quark matter. Our calculations, based on a thorough exploration of the solution space, show that the maximum mass of strange stars depends on both the degree of differential rotation and a type of solution, similar to neutron stars. The highest increase of the maximum mass (compared to the value for a non-rotating star) is obtained for models with a low degree of differential rotation. This highest mass is over four times larger than that of the equivalent non-rotating configuration. Comparing our results with calculations done for realistic models of neutron stars, we conclude that for small degrees of differential rotation, strange stars can sustain masses much larger than stars made from nuclear matter, which reinforces the hope of demonstrating, or of ruling out, the existence of strange matter through observation of the gravitational waves, gamma-rays, or neutrinos of the massive material object born from the merger of a compact binary system or during some supernova events.

Secondary cameras onboard the Mini-EUSO experiment: Control software and calibration

Mini-EUSO is a space experiment selected to be installed inside the International Space Station. It has a compact telescope with a large field of view (44 × 44 sq. deg.) focusing light on an array of photo-multipliers tubes in order to observe UV emission coming from Earth’s atmosphere. Observations will be complemented with data recorded by some ancillary detectors. In particular, the Mini-EUSO Additional Data Acquisition System (ADS) is composed by two cameras, which will allow us to obtain data in the near infrared, and in the visible range. These will be used to monitor the observation conditions, and to acquire useful information on several scientific topics to be studied with the main instrument, such as the physics of atmosphere, meteors, and strange quark matter. Here we present the ADS control software developed to stream cameras together with the UV main instrument, in order to grab images in an automated and independent way, and we also describe the calibration activities performed on these two ancillary cameras before flight.

Puzzle of muons in extensive air showers

In order to examine a muon excess observed by the Pierre Auger Observatory, detailed Monte Carlo simulations were carried out for primary protons, iron nuclei and strangelets (hypothetical stable lumps of strange quark matter). We obtained a rough agreement between the simulations and the data for ordinary nuclei without any contribution of strangelets in primary flux of cosmic rays. Our simulations suggest that the shower observables are dominated by details of hadronic interaction models.

Structure of Hot Strange Quark Stars: an NJL Model Approach at Finite Temperature

In this paper, we investigated the thermodynamic properties of strange quark matter using Nambu-Jona-Lasinio (NJL) model at finite temperatures where we considered the dynamical mass as the effective interaction between quarks. By considering the pressure of strange quark matter (SQM) at finite temperatures, we showed that the equation of state of this system gets stiffer by increasing temperature. In addition, we investigated the energy conditions and stability of the equation of state and showed that the equation of state of SQM satisfy the conditions of stability. Finally, we computed the structure properties of hot strange quark stars (SQS) including the gravitational mass, radius, Schwarzschild radius, average density, compactness and gravitational redshift. Our calculations showed that in this model, the maximum mass and radius of SQS increase by increasing temperature. Furthermore it was shown that the average density of SQS is greater than the normal nuclear density, and it is an increasing function of temperature. We also discussed the temperature dependence of the maximum gravitational mass calculated from different methods.

Strange matter and kaon to pion ratio in the SU(3) Polyakov–Nambu–Jona-Lasinio model

The behavior of strange matter in the frame of the SU(3) Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model including the ${U}_{A}(1)$ anomaly is considered. We discuss the appearance of a peak in the ratio of the number of strange mesons to nonstrange mesons known as the ``horn.'' The PNJL model gives a schematic description of the chiral phase transition and meson properties at finite temperature and density. By using the model, we can show that the splitting of kaon and antikaon masses appears as a result of the introduction of density. This may explain the difference in the ${K}^{+}/{\ensuremath{\pi}}^{+}$ ratio and the ${K}^{\ensuremath{-}}/{\ensuremath{\pi}}^{\ensuremath{-}}$ ratio at low $\sqrt{{s}_{NN}}$ and their tendency to the same value at high $\sqrt{{s}_{NN}}$. We also show that the rise in the ratio ${K}^{+}/{\ensuremath{\pi}}^{+}$ appears near the critical endpoint when we build the ${K}^{+}/{\ensuremath{\pi}}^{+}$ ratio along the phase-transition diagram and it can be considered as a critical region signal.

Chiral symmetry breaking, color superconductivity, and equation of state for magnetized strange quark matter

We investigate the vacuum structure of dense quark matter in strong magnetic fields at finite temperature and densities in a 3 flavor Nambu Jona Lasinio (NJL) model including the Kobayashi-Maskawa-t'Hooft (KMT) determinant term using a variational method. The method uses an explicit structure for the `ground' state in terms of quark-antiquark condensates as well as diquark condensates. The mass gap equations and the superconducting gap equations are solved self consistently and are used to compute the thermodynamic potential along with the charge neutrality conditions. We also derive the equation of state for the charge neutral strange quark matter in the presence of strong magnetic fields which could be relevant for neutron stars.

Exploring physical features of anisotropic strange stars beyond standard maximum mass limit in $f\left(R,\mathcal {T}\right)$ gravity

We study a specific model of anisotropic strange stars in the modified $f\left(R,\mathcal{T}\right)$-type gravity by deriving solutions to the modified Einstein field equations representing a spherically symmetric anisotropic stellar object. We take a standard assumption that $f(R,\mathcal{T})=R+2\chi\mathcal{T}$, where $R$ is Ricci scalar, $\mathcal{T}$ is the trace of the energy-momentum tensor of matter, and $\chi$ is a coupling constant. To obtain our solution to the modified Einstein equations, we successfully apply the `embedding class 1' techniques. We also consider the case when the strange quark matter (SQM) distribution is governed by the simplified MIT bag model equation of state given by $p_r=\frac{1}{3}\left(\rho-4B\right)$, where $B$ is bag constant. We calculate the radius of the strange star candidates by directly solving the modified TOV equation with the observed values of the mass and some parametric values of $B$ and $\chi$. The physical acceptability of our solutions is verified by performing several physical tests. Interestingly, besides the SQM, another type of matter distribution originates due to the effect of coupling between the matter and curvature terms in the $f\left(R,\mathcal{T}\right)$ gravity theory. Our study shows that with decreasing the value of $\chi$, the stellar systems under investigations become gradually massive and larger in size, turning them into less dense compact objects. It also reveals that for $\chi<0$ the $f\left(R,\mathcal{T}\right)$ gravity emerges as a suitable theory for explaining the observed massive stellar objects like massive pulsars, super-Chandrasekhar stars and magnetars, etc., which remain obscure in the standard framework of General Relativity (GR).

Isospin properties in quark matter and quark stars within isospin-dependent quark mass models

We investigate the isospin properties of the strange quark matter (SQM) and quark stars (QSs) in the framework of the isospin-dependent confining quark matter (ICQM) model and confined-isospin and density-dependent mass (CIDDM) model. Within these two isospin-dependent quark mass phenomenological models, we study the quark matter symmetry energy, the stability of strange quark matter, the quark fractions, the isospin asymmetry, and the quark mass asymmetry in SQM, and the mass-radius relation of quark stars. We find that including isospin dependence of the quark mass can significantly influence the isospin properties of the quark matter. Recently, the LIGO-Virgo collaboration reported their detection of gravitational wave (GW) signals GW170817, which are originating from a binary compact star merger. Using the ICQM model and CIDDM model, we describe the compact stars with the new maximum mass limits $2.{01}_{\ensuremath{-}0.04}^{+0.04}\ensuremath{\le}M/{M}_{\ensuremath{\bigodot}}\ensuremath{\le}2.{16}_{\ensuremath{-}0.15}^{+0.17}$ as quark stars, and the dimensionless tidal deformabilities of QSs are also investigated in this work.

The r-mode instability windows of strange stars

With the constraint from gravitational wave emission of a binary merger system (GW170817) and two-solar-mass pulsar observations, we investigate the r-mode instability windows of strange stars with unpaired and color-flavor-locked phase strange quark matter. Shear viscosities due to surface rubbing and electron-electron scattering are taken into account in this work. The results show that the effects of the equation of state of unpaired strange quark matter are only dominant at low temperature, but do not have significant effects on strange stars in the color-flavor-locked phase. A color-flavor-locked phase strange star, which is surrounded by an insulating nuclear crust, seems to be consistent with observational data of young pulsars. We find that an additional enhanced dissipation mechanisms might exist in SAX J1808.4–3658. Fast spinning young pulsar PSR J0537–6910 is a primary source for detecting gravitational waves from a rotating strange star, and young pulsars might be strange stars with color-flavor-locked phase strange quark matter.