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Register a new userBayesian inference of strange star equation of state using the GW170817 and GW190425 data
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The observations of compact star inspirals from LIGO/Virgo provide a valuable tool to study the highly uncertain equation of state (EOS) of dense matter at the densities in which the compact stars reside. It is not clear whether the merging stars are neutron stars or quark stars containing self-bound quark matter. In this work, we explore the allowed bag-model-like EOSs by assuming the merging stars are strange quark stars (SQSs) from a Bayesian analysis employing the tidal deformability observational data of the GW170817 and GW190425 binary mergers. We consider two extreme states of strange quark matter, either in nonsuperfluid or color-flavor locked (CFL) and find the results in these two cases essentially reconcile. In particular, our results indicate that the sound speed in the SQS matter is approximately a constant close to the conformal limit of $c/sqrt{3}$. The universal relations between the mass, the tidal deformability and the compactness are provided for the SQSs. The most probable values of the maximum mass are found to be $M_{rm TOV}=2.10_{-0.12}^{+0.12}~(2.15_{-0.14}^{+0.16}),M_{odot}$ for normal (CFL) SQSs at a $90%$ confidence level. The corresponding radius and tidal deformability for a $1.4,M_{odot}$ star are $R_{rm 1.4}= 11.50_{-0.55}^{+0.52}~({11.42}_{-0.44}^{+0.52})~rm km$ and $Lambda_{1.4}= {650}_{-190}^{+230}~({630}_{-150}^{+220})$, respectively. We also investigate the possibility of GW190814s secondary component $m_2$ of mass $2.59_{-0.09}^{+0.08},M_{odot}$ being an SQS, and find that it could be a CFL SQS with the pairing gap $Delta$ larger than $244~rm MeV$ and the effective bag parameter $B_{rm eff}^{1/4}$ in the range of $170$ to $192$ MeV, at a $90%$ confidence level.
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One of the key goals of observing neutron stars is to infer the equation of state (EoS) of the cold, ultradense matter in their interiors. We present here a Bayesian statistical method of inferring the pressures at five fixed densities, from a sample of mock neutron star masses and radii. We show that while five polytropic segments are needed for maximum flexibility in the absence of any prior knowledge of the EoS, regularizers are also necessary to ensure that simple underlying EoS are not over-parametrized. For ideal data with small measurement uncertainties, we show that the pressure at roughly twice the nuclear saturation density, rho_sat, can be inferred to within 0.3 dex for many realizations of potential sources of uncertainties. The pressures of more complicated EoS with significant phase transitions can also be inferred to within ~30%. We also find that marginalizing the multi-dimensional parameter space of pressure to infer a mass-radius relation can lead to biases of nearly 1 km in radius, towards larger radii. Using the full, five-dimensional posterior likelihoods avoids this bias.
Gravitational-wave observations of binary neutron star coalescences constrain the neutron-star equation of state by enabling measurement of the tidal deformation of each neutron star. This deformation is determined by the tidal deformability parameter $Lambda$, which was constrained using the first binary neutron star gravitational-wave observation, GW170817. Now, with the measurement of the second binary neutron star, GW190425, we can combine different gravitational-wave measurements to obtain tighter constraints on the neutron-star equation of state. In this paper, we combine data from GW170817 and GW190425 to place constraints on the neutron-star equation of state. To facilitate this calculation, we derive interpolated marginalized likelihoods for each event using a machine learning algorithm. These likelihoods, which we make publicly available, allow for results from multiple gravitational-wave signals to be easily combined. Using these new data products, we find that the radius of a fiducial 1.4 $M_odot$ neutron star is constrained to $11.6^{+1.6}_{-0.9}$ km at 90% confidence and the pressure at twice the nuclear saturation density is constrained to $3.1^{+3.1}_{-1.3}times10^{34}$ dyne/cm$^2$ at 90% confidence. This result is dominated by GW170817 and is consistent with findings from other works.
We investigate the possibility that GW170817 has not been the merger of two conventional neutron stars (NS) but involved at least one if not two hybrid stars with a quark matter core which might even belong to a third family of compact stars. To this end, we develop a Bayesian analysis method for selecting the most probable equation of state (EoS) under a set of constraints from compact star physics, which now also include the tidal deformability from GW170817 and the first result for the mass and radius determination for PSR J0030+0451 by NICER. We apply this method for the first time to a two-parameter family of hybrid EoS based on the DD2 model with nucleonic excluded volume for hadronic matter and the color superconducting generalized nlNJL model for quark matter. The model has a variable onset of deconfinement and can mimic the effects of pasta phases with the possibility of a third family of hybrid stars in the mass-radius ($M-R$) diagram. The main findings of this study are that: 1) the presence of multiple configurations for a given mass (twins) corresponds to a set of disconnected lines in the diagram of tidal deformabilities for binary mergers, so that merger events from the same mass range may result in a probability landscape with different peak positions; 2) the Bayesian analysis with the above observational constraints favors an early onset of the deconfinement transition, at masses of $M_{rm onset}le 0.8~M_odot$ with a $M-R$ relationship that in the range of observed neutron star masses is almost indistinguishable from that of a soft hadronic APR EoS; 3) a few yet fictitious measurements of the NICER experiment with a $1sigma$ range that is half of the present value and different mass and radius would change the posterior likelihood so that hybrid EoS with a phase transition onset in the range $M_{rm onset} = 1.1 - 1.6~M_odot $ would be favored.
The LIGO/Virgo detection of gravitational waves originating from a neutron-star merger, GW170817, has recently provided new stringent limits on the tidal deformabilities of the stars involved in the collision. Combining this measurement with the existence of two-solar-mass stars, we generate a generic family of neutron-star-matter Equations of State (EoSs) that interpolate between state-of-the-art theoretical results at low and high baryon density. Comparing the results to ones obtained without the tidal-deformability constraint, we witness a dramatic reduction in the family of allowed EoSs. Based on our analysis, we conclude that the maximal radius of a 1.4-solar-mass neutron star is 13.6 km, and that smallest allowed tidal deformability of a similar-mass star is $Lambda(1.4 M_odot) = 120$.
Using the solutions of the gap equations of the magnetic-color-flavor-locked (MCFL) phase of paired quark matter in a magnetic field, and taking into consideration the separation between the longitudinal and transverse pressures due to the field-induced breaking of the spatial rotational symmetry, the equation of state (EoS) of the MCFL phase is self-consistently determined. This result is then used to investigate the possibility of absolute stability, which turns out to require a field-dependent bag constant to hold. That is, only if the bag constant varies with the magnetic field, there exists a window in the magnetic field vs. bag constant plane for absolute stability of strange matter. Implications for stellar models of magnetized (self-bound) strange stars and hybrid (MCFL core) stars are calculated and discussed.
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