Maximum Mass Of Neutron Stars Research Articles

Uniqueness of the Inflationary Higgs Scalar for Neutron Stars and Failure of Non-Inflationary Approximations

Neutron stars are perfect candidates to investigate the effects of a modified gravity theory, since the curvature effects are significant and more importantly, potentially testable. In most cases studied in the literature in the context of massive scalar-tensor theories, inflationary models were examined. The most important of scalar-tensor models is the Higgs model, which, depending on the values of the scalar field, can be approximated by different scalar potentials, one of which is the inflationary. Since it is not certain how large the values of the scalar field will be at the near vicinity and inside a neutron star, in this work we will answer the question, which potential form of the Higgs model is more appropriate in order for it to describe consistently a static neutron star. As we will show numerically, the non-inflationary Higgs potential, which is valid for certain values of the scalar field in the Jordan frame, leads to extremely large maximum neutron star masses; however, the model is not self-consistent, because the scalar field approximation used for the derivation of the potential, is violated both at the center and at the surface of the star. These results shows the uniqueness of the inflationary Higgs potential, since it is the only approximation for the Higgs model, that provides self-consistent results.

Hadron-quark Pasta Phase in Massive Neutron Stars

Abstract The structured hadron-quark mixed phase, known as the pasta phase, is expected to appear in the core of massive neutron stars. Motivated by the recent advances in astrophysical observations, we explore the possibility of the appearance of quarks inside neutron stars and check its compatibility with current constraints. We investigate the properties of the hadron-quark pasta phases and their influences on the equation of state (EOS) for neutron stars. In this work, we extend the energy minimization (EM) method to describe the hadron-quark pasta phase, where the surface and Coulomb contributions are included in the minimization procedure. By allowing different electron densities in the hadronic and quark matter phases, the total electron chemical potential with the electric potential remains constant, and local β equilibrium is achieved inside the Wigner–Seitz cell. The mixed phase described in the EM method shows the features lying between the Gibbs and Maxwell constructions, which is helpful for understanding the transition from the Gibbs construction to the Maxwell construction with increasing surface tension. We employ the relativistic mean-field model to describe the hadronic matter, while the quark matter is described by the MIT bag model with vector interactions. It is found that the vector interactions among quarks can significantly stiffen the EOS at high densities and help enhance the maximum mass of neutron stars. Other parameters like the bag constant can also affect the deconfinement phase transition in neutron stars. Our results show that hadron-quark pasta phases may appear in the core of massive neutron stars that can be compatible with current observational constraints.

Maximum baryon masses for static neutron stars in f(R) gravity

We investigate the upper mass limit predictions of the baryonic mass for static neutron stars in the context of f(R) gravity. We use the most popular f(R) gravity model, namely the R 2 gravity, and calculate the maximum baryon mass of static neutron stars adopting several realistic equations of state and one ideal equation of state, namely that of causal limit. Our motivation is based on the fact that neutron stars with baryon masses larger than the maximum mass for static neutron star configurations inevitably collapse to black holes. Thus with our analysis, we want further to enlighten the predictions for the maximum baryon masses of static neutron stars in R 2 gravity, which, in turn, further strengthens our understanding of the mysterious mass gap region. As we show, the baryon masses of most of the equations of states studied in this paper lie in the lower limits of the mass gap region –, but intriguingly enough, the highest value of the maximum baryon masses we found is of the order of . This upper mass limit also appears as a maximum static neutron star gravitational mass limit in other contexts. Combining the two results which refer to baryon and gravitational masses, we point out that the gravitational mass of static neutron stars cannot be larger than three solar masses, while based on maximum baryon masses results of the present work, we can conspicuously state that it is highly likely the lower mass limits of astrophysical black holes in the range of –. This, in turn, implies that maximum neutron star masses in the context of R 2 gravity are likely to be in the lower limits of the range of –. Hence our work further supports the General Relativity claim that neutron stars cannot have gravitational masses larger than and then, to explain observations comparable or over this limit, we need alternative extensions of General Relativity, other than f(R) gravity.

The Maximum Accreted Mass of Recycled Pulsars

Abstract The maximum mass of neutron stars (NSs) is of great importance for constraining equations of state of NSs and understanding the mass gap between NSs and stellar-mass black holes. NSs in X-ray binaries increase in mass by accreting material from their companions (known as the recycling process), and the uncertainties in the accretion process make studying the NS mass at birth a challenge. In this work, we investigate the NS accreted mass while considering the effect of NS spin evolution and provide the maximum accreted mass for NSs in the recycling process. By exploring a series of binary evolution calculations, we obtain the final NS mass and the maximum accreted mass for a given birth mass of an NS and a mass transfer efficiency. Our results show that NSs can accrete relatively more material for binary systems with donor masses in the range of 1.8 ∼ 2.4 M ⊙, NSs accrete relatively more mass when the remnant WD mass is in the range of ∼ 0.25–0.30 M ⊙, and the maximum accreted mass is positively correlated with the initial NS mass. For a 1.4 M ⊙ NS at birth with a moderate mass transfer efficiency of 0.3, the maximum accreted mass could be 0.27 M ⊙. The results can be used to estimate the minimum birth mass for systems with massive NSs in observations.

Nuclear Physics Multimessenger Astrophysics Constraints on the Neutron Star Equation of State: Adding NICER’s PSR J0740+6620 Measurement

Abstract In the past few years, new observations of neutron stars (NSs) and NS mergers have provided a wealth of data that allow one to constrain the equation of state (EOS) of nuclear matter at densities above nuclear saturation density. However, most observations were based on NSs with masses of about 1.4 M ⊙, probing densities up to ∼three to four times the nuclear saturation density. Even higher densities are probed inside massive NSs such as PSR J0740+6620. Very recently, new radio observations provided an update to the mass estimate for PSR J0740+6620, and X-ray observations by the NICER and XMM telescopes constrained its radius. Based on these new measurements, we revisit our previous nuclear physics multimessenger astrophysics constraints and derive updated constraints on the EOS describing the NS interior. By combining astrophysical observations of two radio pulsars, two NICER measurements, the two gravitational-wave detections GW170817 and GW190425, detailed modeling of the kilonova AT 2017gfo, and the gamma-ray burst GRB 170817A, we are able to estimate the radius of a typical 1.4 M ⊙ NS to be 11.94 − 0.87 + 0.76 km at 90% confidence. Our analysis allows us to revisit the upper bound on the maximum mass of NSs and disfavors the presence of a strong first-order phase transition from nuclear matter to exotic forms of matter, such as quark matter, inside NSs.

The Gravitational-wave physics II: Progress

It has been a half-decade since the first direct detection of gravitational waves, which signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave cosmology. The increasing number of the detected gravitational-wave events has revealed the promising capability of constraining various aspects of cosmology, astronomy, and gravity. Due to the limited space in this review article, we will briefly summarize the recent progress over the past five years, but with a special focus on some of our own work for the Key Project ``Physics associated with the gravitational waves'' supported by the National Natural Science Foundation of China. In particular, (1) we have presented the mechanism of the gravitational-wave production during some physical processes of the early Universe, such as inflation, preheating and phase transition, and the cosmological implications of gravitational-wave measurements; (2) we have put constraints on the neutron star maximum mass according to GW170817 observations; (3) we have developed a numerical relativity algorithm based on the finite element method and a waveform model for the binary black hole coalescence along an eccentric orbit.

Neutron-mirror neutron mixing and neutron stars

The oscillation of neutron n into mirror neutron n', its mass degenerate partner from dark mirror sector, can gradually transform the neutron stars into the mixed stars consisting in part of mirror dark matter. In quark stars n-n' transitions are suppressed. We study the structure of mixed stars and derive the mass-radius scaling relations between the configurations of purely neutron star and maximally mixed star (MMS) containing equal amounts of ordinary and mirror components. In particular, we show that the MMS masses can be at most M^{mathrm{max}}_{NS}/sqrt{2}, where M^mathrm{max}_{NS} is a maximum mass of a pure neutron star allowed by a given equation of state. We evaluate n-n' transition rate in neutron stars, and show that various astrophysical limits on pulsar properties exclude the transition times in a wide range 10^{5},text {year}< tau _varepsilon < 10^{15},text {year}. For short transition times, tau _varepsilon < 10^5 year, the different mixed stars of the same mass can have different radii, depending on their age, which possibility can be tested by the NICER measurements. We also discuss subtleties related with the possible existence of mixed quark stars, and possible implications for the gravitational waves from the neutron star mergers and associated electromagnetic signals.

Systematics on the high-density nuclear equation of state from relativistic Hartree-Fock theory with Brown-Rho scaling

Using the Brown-Rho (BR) scaling law, several new relativistic Hartree-Fock (RHF) models with chiral limit are thoroughly investigated. The high-density nuclear equation of state (EOS) of RHF with the BR scaling become softer than one without the BR scaling. The EOS of RHF with BR scaling is consistent with the constraint extracted from collective flows and kaon production in heavy-ion collisions. It is found that, with a sizable strength parameter of the mass drop $x=0.126$, the symmetry energy is almost flat at density above 3 times the saturation density ${\ensuremath{\rho}}_{0}$, and even decreases slightly. This is caused from the fact that the decline of the potential part of symmetry energy is faster than the increase of the kinetic part of symmetry energy. The decrease of potential part is mainly because the neutron mass ${M}_{n}^{*}$ and the proton mass ${M}_{p}^{*}$ are close to each other when the density gradually approaches the critical density of chiral limit. For a mass drop of $x=0.092$, since the critical density of chiral limit is higher than one of $x=0.126$, the symmetry energy becomes flat at density above $5{\ensuremath{\rho}}_{0}$. While, since the critical density of chiral limit is very high for a small mass drop $x=0.053$, the symmetry energy of RHFs with $x=0.053$ always increases at the entire density domain of this work (below 6 ${\ensuremath{\rho}}_{0}$). The maximum mass of neutron star (NS) obtained with present models can satisfy $M=2.08\ifmmode\pm\else\textpm\fi{}0.07{M}_{\ensuremath{\bigodot}}$. However, the radius of 1.4 ${M}_{\ensuremath{\bigodot}}$ with the mass drop of $x=0.126$ will surpass the upper limit (13.7 km) extracted from the tidal deformability parameter of coalescence of a NS binary system and the radius of $\text{J0030}+0451$ being $12.{71}_{\ensuremath{-}1.19}^{+1.14}$ km. The radius of 1.4 ${M}_{\ensuremath{\bigodot}}$ with the mass drop of $x=0.092$ is 13.6 km, closing to 13.7 km and the radius of $\text{J0030}+0451$ being $12.{71}_{\ensuremath{-}1.19}^{+1.14}$ km. Therefore, the RHF model with BR scaling prefers the mass drop of $x\ensuremath{\lesssim}0.092$.

Radius and equation of state constraints from massive neutron stars and GW190814

Motivated by the unknown nature of the $2.50-2.67\,M_\odot$ compact object in the binary merger event GW190814, we study the maximum neutron star mass based on constraints from low-energy nuclear physics, neutron star tidal deformabilities from GW170817, and simultaneous mass-radius measurements of PSR J0030+045 from NICER. Our prior distribution is based on a combination of nuclear modeling valid in the vicinity of normal nuclear densities together with the assumption of a maximally stiff equation of state at high densities, a choice that enables us to probe the connection between observed heavy neutron stars and the transition density at which conventional nuclear physics models must break down. We demonstrate that a modification of the highly uncertain supra-saturation density equation of state beyond 2.64 times normal nuclear density is required in order for chiral effective field theory models to be consistent with current neutron star observations and the existence of $2.6\,M_\odot$ neutron stars. We also show that the existence of very massive neutron stars strongly impacts the radii of $\sim 2.0\,M_\odot$ neutron stars (but not necessarily the radii of $1.4\,M_\odot$ neutron stars), which further motivates future NICER radius measurements of PSR J1614-2230 and PSR J0740+6620.

Unified Equation of State for Neutron Stars Based on the Gogny Interaction

The effective Gogny interactions of the D1 family were established by D. Gogny more than forty years ago with the aim to describe simultaneously the mean field and the pairing field corresponding to the nuclear interaction. The most popular Gogny parametrizations, namely D1S, D1N and D1M, describe accurately the ground-state properties of spherical and deformed finite nuclei all across the mass table obtained with Hartree–Fock–Bogoliubov (HFB) calculations. However, these forces produce a rather soft equation of state (EoS) in neutron matter, which leads to predict maximum masses of neutron stars well below the observed value of two solar masses. To remove this limitation, we built new Gogny parametrizations by modifying the density dependence of the symmetry energy predicted by the force in such a way that they can be applied to the neutron star domain and can also reproduce the properties of finite nuclei as good as their predecessors. These new parametrizations allow us to obtain stiffer EoS’s based on the Gogny interactions, which predict maximum masses of neutron stars around two solar masses. Moreover, other global properties of the star, such as the moment of inertia and the tidal deformability, are in harmony with those obtained with other well tested EoSs based on the SLy4 Skyrme force or the Barcelona–Catania–Paris–Madrid (BCPM) energy density functional. Properties of the core-crust transition predicted by these Gogny EoSs are also analyzed. Using these new Gogny forces, the EoS in the inner crust is obtained with the Wigner–Seitz approximation in the Variational Wigner–Kirkwood approach along with the Strutinsky integral method, which allows one to estimate in a perturbative way the proton shell and pairing corrections. For the outer crust, the EoS is determined basically by the nuclear masses, which are taken from the experiments, wherever they are available, or by HFB calculations performed with these new forces if the experimental masses are not known.

Constraints on high density equation of state from maximum neutron star mass

The low density nuclear matter equation of state is strongly constrained by nuclear properties; however, for constraining the high density equation of state it is necessary to resort to indirect information obtained from the observation of neutron stars, compact objects that may have a central density several times nuclear matter saturation density, ${n}_{0}$. Taking a metamodeling approach to generate a huge set of equations of state that satisfy nuclear matter properties close to ${n}_{0}$ and that do not contain a first-order phase transition, the possibility of constraining the high density equation of state (EOS) was investigated. The entire information obtained from the GW170817 event for the probability distribution of $\stackrel{\texttildelow{}}{\mathrm{\ensuremath{\Lambda}}}$ was used to make a probabilistic inference of the EOS, which goes beyond the constraints imposed by nuclear matter properties. Nuclear matter properties close to saturation, below $2{n}_{0}$, do not allow us to distinguish between equations of state that predict different neutron star (NS) maximum masses. This is, however, not true if the equation of state is constrained at low densities by the tidal deformability of the NS merger associated to GW170817. Above $3{n}_{0}$, differences may be large, for both approaches, and, in particular, the pressure and speed of sound of the sets studied do not overlap, showing that the knowledge of the NS maximum mass may give important information on the high density EOS. Narrowing the maximum mass uncertainty interval will have a sizable effect on constraining the high density EOS.

Tensor and Coulomb-exchange terms in the relativistic mean-field model with δ -meson and isoscalar-isovector coupling

Tensor couplings and Coulomb-exchange terms in the relativistic mean-field (RMF) model impact the neutron skin thickness of nuclei [N. Liliani, A. M. Nugraha, J. P. Diningrum, and A. Sulaksono, Phys. Rev. C 93, 054322 (2016)], and a significantly thicker skin of $^{208}\mathrm{Pb}$ predicted by PREX-II leads to the existence of a phase transition in the interior of neutron stars (NSs) [F. J. Fattoyev, J. Piekarewicz, and C. J. Horowitz, Phys. Rev. Lett. 120, 172702 (2018)]. In this work, we revisit the effects of tensors and Coulomb-exchange terms within the RMF model with the $\ensuremath{\delta}$-meson and isovector-isoscalar coupling on finite nuclei. We also perform a systematic investigation of the implicit effect of both terms on the nuclear matter and NS properties. We confirmed in our previous work [N. Liliani, A. M. Nugraha, J. P. Diningrum, and A. Sulaksono, Phys. Rev. C 93, 054322 (2016)] that the roles of tensor couplings, Coulomb exchange, and isovector-isoscalar coupling on the bulk properties of finite nuclei are crucial. Conversely, the $\ensuremath{\delta}$-meson term's inclusion yields no significant effect on the bulk properties of finite nuclei. The tensor couplings and Coulomb-exchange terms implicitly influence the isoscalar and isovector parts of nuclear matter. By contrast, the $\ensuremath{\delta}$-meson and isovector-isoscalar coupling significantly impact the isovector parts of nuclear matter predictions. Except for the binding energy and the equation of state of pure neutron matter in low-density cases, the nuclear matter property predictions of all parameter sets used are relatively compatible with experimental data and the chiral effective field theory result. Moreover, the tensor couplings, Coulomb-exchange, $\ensuremath{\delta}$-meson, and isovector-isoscalar coupling terms play a significant role in predicting the canonical NS radius but not in the maximum mass of NS. For relatively large ${\ensuremath{\eta}}_{2\ensuremath{\rho}}$, the compatibility of the canonical NS radius predicted by the model with all of the observation results used in this work is better than that with the tensor couplings and Coulomb exchange excluded. Moreover, the presence of tensor couplings, Coulomb exchange, and various isovector-isoscalar nonlinear coupling terms influence the relation between NS tidal deformation and neutron skin thickness of $^{208}\mathrm{Pb}$. However, tensor couplings and Coulomb-exchange terms play a crucial role in making the NS tidal deformability prediction compatible with the constraints from observations and experimental data. In conclusion, tensor couplings and Coulomb-exchange RMF models have an essential role in finite nuclei predictions and implicitly influence the nuclear matter and NS property predictions.

Neutron Stars and the Nuclear Matter Equation of State

Neutron stars provide a window into the properties of dense nuclear matter. Several recent observational and theoretical developments provide powerful constraints on their structure and internal composition. Among these are the first observed binary neutron star merger, GW170817, whose gravitational radiation was accompanied by electromagnetic radiation from a short γ-ray burst and an optical afterglow believed to be due to the radioactive decay of newly minted heavy r-process nuclei. These observations give important constraints on the radii of typical neutron stars and on the upper limit to the neutron star maximum mass and complement recent pulsar observations that established a lower limit. Pulse-profile observations by the Neutron Star Interior Composition Explorer (NICER) X-ray telescope provide an independent, consistent measure of the neutron star radius. Theoretical many-body studies of neutron matter reinforce these estimates of neutron star radii. Studies using parameterized dense matter equations of state (EOSs) reveal several EOS-independent relations connecting global neutron star properties.

Tight multimessenger constraints on the neutron star equation of state from GW170817 and a forward model for kilonova light-curve synthesis

ABSTRACT We present a rapid analytic framework for predicting kilonova light curves following neutron star (NS) mergers, where the main input parameters are binary-based properties measurable by gravitational wave detectors (chirp mass and mass ratio, orbital inclination) and properties dependent on the nuclear equation of state (tidal deformability, maximum NS mass). This enables synthesis of a kilonova sample for any NS source population, or determination of the observing depth needed to detect a live kilonova given gravitational wave source parameters in low latency. We validate this code, implemented in the public mosfit package, by fitting it to GW170817. A Bayes factor analysis overwhelmingly (B &amp;gt; 1010) favours the inclusion of an additional luminosity source in addition to lanthanide-poor dynamical ejecta during the first day. This is well fit by a shock-heated cocoon model, though differences in the ejecta structure, opacity or nuclear heating rate cannot be ruled out as alternatives. The emission thereafter is dominated by a lanthanide-rich viscous wind. We find the mass ratio of the binary is q = 0.92 ± 0.07 (90 per cent credible interval). We place tight constraints on the maximum stable NS mass, MTOV $=2.17^{+0.08}_{-0.11}$ M⊙. For a uniform prior in tidal deformability, the radius of a 1.4-M⊙ NS is R1.4 ∼ 10.7 km. Re-weighting with a prior based on equations of state that support our credible range in MTOV, we derive a final measurement R1.4 $=11.06^{+1.01}_{-0.98}$ km. Applying our code to the second gravitationally detected NS merger, GW190425, we estimate that an associated kilonova would have been fainter (by ∼0.7 mag at 1 d post-merger) and declined faster than GW170817, underlining the importance of tuning follow-up strategies individually for each GW-detected NS merger.

Equation of state and radial oscillations of neutron stars

We investigate radial oscillations of pure neutron stars and hybrid stars, employing equations of state of nuclear matter from Brueckner-Hartree-Fock theory, and of quark matter from the Dyson-Schwinger quark model, performing a Gibbs construction for the mixed phase in hybrid stars. We calculate the eigenfrequencies and corresponding oscillation functions. Our results for the zero points of the first-order radial oscillation frequencies give the maximum mass of stable neutron stars, consistent with the common criterion $dM/d\rho_c=0$. Possible observations of the radial oscillation frequencies could help to learn more about the equation of state, predict the maximum mass of neutron stars more precisely, and indicate the presence of quark matter.

Constraints on the Maximum Mass of Neutron Stars with a Quark Core from GW170817 and NICER PSR J0030+0451 Data

Abstract We perform a Bayesian analysis of the maximum mass M TOV of neutron stars with a quark core, incorporating the observational data from tidal deformability of the GW170817 binary neutron star merger as detected by LIGO/Virgo and the mass and radius of PSR J0030+0451 as detected by the Neutron Star Interior Composition Explorer. The analysis is performed under the assumption that the hadron–quark phase transition is of first order, where the low-density hadronic matter described in a unified manner by the soft QMF or the stiff DD2 equation of state (EOS) transforms into a high-density phase of quark matter modeled by the generic “constant-sound-speed” parameterization. The mass distribution measured for the 2.14 M ⊙ pulsar MSP J0740+6620 is used as the lower limit on M TOV. We find the most probable values of the hybrid star maximum mass are M TOV = 2.36 − 0.26 + 0.49 M ⊙ ( 2.39 − 0.28 + 0.47 M ⊙ ) for QMF (DD2), with an absolute upper bound around 2.85 M ⊙, to the 90% posterior credible level. Such results appear robust with respect to the uncertainties in the hadronic EOS. We also discuss astrophysical implications of this result, especially on the postmerger product of GW170817, short gamma-ray bursts, and other likely binary neutron star mergers.

Relativistic stars in 4D Einstein-Gauss-Bonnet gravity

In the present paper we investigate the structure of relativistic stars in 4D Einstein-Gauss-Bonnet gravity. The mass-radius relations are obtained for realistic hadronic and for strange quark star equations of state, and for a wide range of the Gauss-Bonnet coupling parameter α. Even though the deviations from general relativity for nonzero values of α can be large enough, they are still comparable with the variations due to different modern realistic equations of state if we restrict ourselves to moderate values of α. That is why the current observations of the neutron star masses and radii alone can not impose stringent constraints on the value of the parameter α. Nevertheless some rough constraints on α can be put. The existence of stable stellar mass black holes imposes √(α) ≲ 2.6 km for α > 0 while the requirement that the maximum neutron star mass should be greater than two solar masses gives √(|α|) ≲ 3.9 km for α < 0. We also present an exact solution describing the structure of relativistic stars with uniform energy density in 4D Einstein-Gauss-Bonnet gravity.

Impacts of nucleon-nucleon short-range correlations on neutron stars

Nucleon-nucleon short-range correlations (SRCs) are inevitable consequences of realistic nucleon-nucleon interactions. The corresponding SRC pairs, characterized by high relative momenta, have been verified experimentally to be important in finite nuclei. In this work, we study the impacts of nucleon-nucleon SRCs on the physical properties of neutron stars. Considering the small center-of-mass momenta of SRC pairs, new nucleon momentum distributions with high-momentum tails (HMTs) are proposed by extrapolating well-established experimental results of SRCs in finite nuclei to neutron star matter. HMTs with different shapes are adopted to estimate theoretical uncertainties. It is found that nucleon-nucleon SRCs generally stiffen the equation of state of neutron star matter and increase the maximum mass of neutron stars.

Thermal evolution of relativistic hyperonic compact stars with calibrated equations of state

A set of unified relativistic mean-field equations of state for hyperonic compact stars recently built in [M. Fortin, Ad. R. Raduta, S. Avancini, and C. Providencia, Phys. Rev. D {\bf 101}, 034017 (2020)] is used to study the thermal evolution of non-magnetized and non-rotating spherically-symmetric isolated and accreting neutron stars under different hypothesis concerning proton $S$-wave superfluidity. These equations of state have been obtained in the following way: the slope of the symmetry energy is in agreement with experimental data; the coupling constants of $\Lambda$ and $\Xi$-hyperons are determined from experimental hypernuclear data; uncertainties in the nucleon-$\Sigma$ interaction potential are accounted for; current constraints on the lower bound of the maximum neutron star mass are satisfied. Within the considered set of equations of state, the presence of hyperons is essential for the description of the cooling/heating curves. One of the conclusions we reach is that the criterion of best agreement with observational data leads to different equations of states and proton $S$-wave superfluidity gaps when applied separately for isolated neutron stars and accreting neutron stars in quiescence. This means that at least in one situation the traditional simulation framework that we employ is not complete and/or the equations of state are inappropriate. Another result is that, considering equations of state which do not allow for nucleonic dUrca or allow for it only in very massive NS, the low luminosity of SAX J1808 requires a repulsive $\Sigma$-hyperon potential in symmetric nuclear matter in the range $U_\Sigma^{(N)}\approx 10-30$ MeV. This range of values for $U_\Sigma^{(N)} $ is also supported by the criterion of best agreement with all available data from INS and XRT.

Causal limit of neutron star maximum mass in f(R) gravity in view of GW190814

We investigate the causal limit of maximum mass for stars in the framework of f(R) gravity. We choose a causal equation of state, with variable speed of sound, and with the transition density and pressure corresponding to the SLy equation of state. The transition density is chosen to be equal to twice the saturation density ρt∼2ρ0, and also the analysis is performed for the transition density, chosen to be equal to the saturation density ρt∼ρ0. We examine numerically the combined effect of the stiff causal equation of state and of the sound speed on the maximum mass of static neutron stars, in the context of Jordan frame of f(R) gravity. This yields the most extreme upper bound for neutron star masses in the context of extended gravity. As we will evince for the case of R2 model, the upper causal mass limit lies within, but not deeply in, the mass-gap region, and is marginally the same with the general relativistic causal maximum mass, indicating that the ∼3M⊙ general relativistic limit is respected. In view of the modified gravity perspective for the secondary component of the GW190814 event, we also discuss the strange star possibility. Using several well established facts for neutron star physics and the Occam's razor approach, although the strange star is exciting, for the moment, it remains a possibility for describing the secondary component of GW190814. We underpin the fact that the secondary component of the compact binary GW190814 is probably a neutron star, a black hole or even a rapidly rotating neutron star, but not a strange star. We also discuss, in general, the potential role of the extended gravity description for the binary merging.