Constraints on interquark interaction parameters with GW170817 in a binary strange star scenario

The LIGO/VIRGO detection of the gravitational waves from a binary merger system, GW170817, has put a clean and strong constraint on the tidal deformability of the merging objects. From this constraint, deep insights can be obtained in compact star equation of states, which has been one of the most puzzling problems for nuclear physicists and astrophysicists. Employing one of the most widely-used quark star EOS model, we characterize the star properties by the strange quark mass ($m_s$), an effective bag constant ($B_{\rm eff}$), the perturbative QCD correction ($a_4$), as well as the gap parameter ($\Delta$) when considering quark pairing, and investigate the dependences of the tidal deformablity on them. We find that the tidal deformability is dominated by $B_{\rm eff}$, and insensitive to $m_s$, $a_4$. We discuss the correlation between the tidal deformability and the maximum mass ($M_\mathrm{TOV}$) of a static quark star, which allows the model possibility to rule out the existence of quark stars with future gravitational wave observations and mass measurements. The current tidal deformability measurement implies $M_\mathrm{TOV} \le2.18\,M_\odot$ ($2.32\,M_\odot$ when pairing is considered) for quark stars. Combining with two-solar-mass pulsar observations, we also make constraints on the poorly known gap parameter $\Delta$ for color-flavor-locked quark matter.Comment: 6 pages; 4 figures; accepted for publication in PR

Strangeons constitute bulk strong matter-- To test using GW170817

The fundamental strong interaction determines the nature of pulsar-like compact stars which are essentially in the form of bulk strong matter. From an observational point of view, it is proposed that bulk strong matter could be composed of strangeons, i.e. quark-clusters with there-light-flavor symmetry of quarks, and therefore pulsar-like compact objects could actually be strangeon stars. The equation of state (EOS) of strangeon stars is described in a Lennard-Jones model for the purpose of constraining the EOS by both the tidal deformability $\Lambda$ of GW170817 and $M_{\rm TOV}$. It is found that the allowed parameter space is quite large as most of the Lennard-Jones EOS models satisfy the tidal deformability constraint by GW170817. The future GW detections for smaller values of $\Lambda$ and mass measurement for larger values of $M_{\rm TOV}$ will help a better constraint on the strangeon star model.Comment: Accepted by the EPJA Topical Issue "The first Neutron Star Merger Observation - Implications for Nuclear Physics

Constraining the Equation of State of Neutron Stars through GRB X-Ray Plateaus

The unknown equation of state (EoS) of neutron stars (NSs) is puzzling because of rich non-perturbative effects of strong interaction there. A method to constrain the EoS by using the detected X-ray plateaus of gamma-ray bursts (GRBs) is proposed in this paper. Observations show some GRB X-ray plateaus may be powered by strongly magnetized millisecond NSs. The properties of these NSs should then satisfy: (i) the spin-down luminosity of these NSs should be brighter than the observed luminosity of the X-ray plateaus; (ii) the total rotational energy of these NSs should be larger than the total energy of the X-ray plateaus. Through the case study of GRB 170714A, the moment of inertia of NSs is constrained as $I>1.0\times 10^{45}\left ( \frac{P_{\rm cri}}{1\;\rm ms} \right )^{2} \;\rm g\cdot cm^{2}$, where $P_{\rm cri}$ is the critical rotational period that an NS can achieve. The constraint of the radii of NSs according to GRB 080607 is shown in Table 1.Comment: 6 pages, 2 figute, The Astrophysical Journal, 886:87, 2019 December 1, https://doi.org/10.3847/1538-4357/ab490

Two types of glitches in a solid quark star model

Glitch (sudden spinup) is a common phenomenon in pulsar observations. However, the physical mechanism of glitch is still a matter of debate because it depends on the puzzle of pulsar's inner structure, i.e., the equation of state of dense matter. Some pulsars (e.g., Vela-like) show large glitches ({\Delta}{\nu}/{\nu}~10^-6) but release negligible energy, whereas the large glitches of AXPs/SGRs (anomalous X-ray pulsars/soft gamma repeaters) are usually (but not always) accompanied with detectable energy releases manifesting as X-ray bursts or outbursts. We try to understand this aspect of glitches in a starquake model of solid quark stars. There are actually two kinds of glitches in this scenario: bulk-invariable (Type I) and bulk-variable (Type II) ones. The total stellar volume changes (and then energy releases) significantly for the latter but not for the former. Therefore, glitches accompanied with X-ray bursts (e.g., that of AXP/SGRs) could originate from Type II starquakes induced probably by accretion, while the others without evident energy release (e.g., that of Vela pulsar) would be the result of Type I starquakes due to, simply, a change of stellar ellipticity.Comment: 6 pages, 2 figures, accepted for publication in MNRA

Constraint on the maximum mass of neutron stars using GW170817 event

We revisit the constraint on the maximum mass of cold spherical neutron stars coming from the observational results of GW170817. We develop a new framework for the analysis by employing both energy and angular momentum conservation laws as well as solid results of latest numerical-relativity simulations and of neutron stars in equilibrium. The new analysis shows that the maximum mass of cold spherical neutron stars can be only weakly constrained as M_{\rm max} \alt 2.3M_\odot. Our present result illustrates that the merger remnant neutron star at the onset of collapse to a black hole is not necessarily rapidly rotating and shows that we have to take into account the angular momentum conservation law to impose the constraint on the maximum mass of neutron stars.Comment: 14 pages, 5 figures, matches the version accepted by PRD for publicatio

Uniformly rotating, axisymmetric and triaxial quark stars in general relativity

Quasi-equilibrium models of uniformly rotating axisymmetric and triaxial quark stars are computed in general relativistic gravity scenario. The Isenberg-Wilson-Mathews (IWM) formulation is employed and the Compact Object CALculator (COCAL) code is extended to treat rotating stars with finite surface density and new equations of state (EOSs). Besides the MIT bag model for quark matter which is composed of de-confined quarks, we examine a new EOS proposed by Lai and Xu that is based on quark clustering and results in a stiff EOS that can support masses up to $3.3M_\odot$ in the case we considered. We perform convergence tests for our new code to evaluate the effect of finite surface density in the accuracy of our solutions and construct sequences of solutions for both small and high compactness. The onset of secular instability due to viscous dissipation is identified and possible implications are discussed. An estimate of the gravitational wave amplitude and luminosity based on quadrupole formulas is presented and comparison with neutron stars is discussed.Comment: 14 pages, 9 figures; accepted for publication in PR

Free Energy of Anisotropic Strangeon Stars

Can pulsar-like compact objects release further huge free energy besides the kinematic energy of rotation? This is actually relevant to the equation of states of cold supra-nuclear matter, which is still under hot debate. Enormous energy is surely needed to understand various observations, such as $\gamma-$ray bursts, fast radio bursts and soft $\gamma-$ray repeaters. The elastic/gravitational-free energy of solid strangeon star is revisited, with two approaches to calculate in general relativity. It is found that huge free energy (> $10^{46}$ erg) could be released via starquakes, given an extremely small anisotropy ($(p_{\rm t}-p_{\rm r})/p_{\rm r} \sim 10^{-4}$, with $p_{\rm t}$/$p_{\rm r}$ the tangential/radial pressures)