arXiv subjects

Fridolin Weber

Publications and source records attributed to Fridolin Weber.

At least 37 records · Page 2Linked to original sources

arXiv2023

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.

astro-ph.HE↗

arXiv2023

Deducing Neutron Star Equation of State Parameters Directly From Telescope Spectra with Uncertainty-Aware Machine Learning

Neutron stars provide a unique laboratory for studying matter at extreme pressures and densities. While there is no direct way to explore their interior structure, X-rays emitted from these stars can indirectly provide clues to the equation of state (EOS) of superdense nuclear matter through the inference of the star's mass and radius. However, inference of EOS directly from a star's X-ray spectra is extremely challenging and is complicated by systematic uncertainties. The current state of the art is to use simulation-based likelihoods in a piece-wise method, which first infer the star's mass and radius to reduce the dimensionality of the problem, and from those quantities infer the EOS. We demonstrate a series of enhancements to the state of the art, in terms of realistic uncertainty quantification and improved regression of physical properties with machine learning. We also demonstrate novel inference of the EOS directly from the high-dimensional spectra of observed stars, avoiding the intermediate mass-radius step. Our network is conditioned on the sources of uncertainty of each star, allowing for natural and complete propagation of uncertainties to the EOS.

astro-ph.HE↗

arXiv2022

Massive relativistic compact stars from SU(3) symmetric quark models

We construct a set of hyperonic equations of state (EoS) by assuming SU(3) symmetry within the baryon octet and by using a covariant density functional (CDF) theory approach. The low-density regions of our EoS are constrained by terrestrial experiments, while the high-density regime is modeled by systematically varying the nuclear matter skewness coefficient $Q_{\rm sat}$ and the symmetry energy slope $L_{\rm sym}$. The sensitivity of the EoS predictions is explored in terms of $z$ parameter of the SU(3) symmetric model that modifies the meson-hyperon coupling constants away from their SU(6) symmetric values. Our results show that model EoS based on our approach can support static Tolman-Oppenheimer-Volkof (TOV) masses in the range $2.3$-$2.5\,M_{\odot}$ in the large-$Q_{\rm sat}$ and small-$z$ regime, however, such stars contain only a trace amount of hyperons compared to SU(6) models. We also construct uniformly rotating Keplerian configurations for our model EoS for which the masses of stellar sequences may reach up to $3.0\,M_{\odot}$. These results are used to explore the systematic dependence of the ratio of maximum masses of rotating and static stars, the lower bound on the rotational frequency of the models that will allow secondary masses in the gravitational waves events to be compact stars with $M_2 \lesssim 3.0\,M_{\odot}$ and the strangeness fraction on the model parameters. We conclude that very massive stellar models can be, in principle, constructed within the SU(3) symmetric model, however, they are nucleonic-like as their strangeness fraction drops below 3\%.

nucl-th↗

arXiv2022

Hyperonization in compact stars

We review the covariant density functional approach to the equation of state of the dense nuclear matter in compact stars. The main emphasis is on the hyperonization of the dense matter, and the role played by the $Δ$-resonances. The implications of hyperonization for the astrophysics of compact stars, including the equation of state, composition, and stellar parameters are examined. The mass-radius relation and tidal deformabilities of static and rapidly rotating (Keplerian) configurations are discussed in some detail. We briefly touch upon some other recent developments involving hyperonization in hot hypernuclear matter at high- and low-densities.

nucl-th↗

arXiv2021

Anisotropic magnetized white dwarfs: Unifying under- and over-luminous peculiar and standard type Ia supernovae

Ever since the observation of peculiar over-luminous type Ia supernovae (SNeIa), exploring possible violations of the canonical Chandrasekhar mass-limit (CML) has become a pressing research area of modern astrophysics. Since its first detection in 2003, more than a dozen of peculiar over-luminous SNeIa has been detected, but the true nature of the underlying progenitors is still under dispute. Furthermore, there are also under-luminous SNeIa whose progenitor masses appear to be well below the CML (sub-Chandrasekhar progenitors). These observations call into question how sacrosanct the CML is. We have shown recently in Paper I that the presence of a strong magnetic field, the anisotropy of dense matter, as well as the orientation of the magnetic field itself significantly influence the properties of neutron and quark stars. Here, we study these effects for white dwarfs, which shows that their properties are also severely impacted. Most importantly, we arrive at a variety of mass-radius relations of white dwarfs which accommodate sub- to super-Chandrasekhar mass limits. This urges caution when using white dwarfs associated with SNeIa as standard candles.

astro-ph.HE↗

arXiv2021

Mass limits of the extremely fast-spinning white dwarf CTCV J2056-3014

CTCV J2056--3014 is a nearby cataclysmic variable with an orbital period of approximately $1.76$ hours at a distance of about $853$ light-years from the Earth. Its recently reported X-ray properties suggest that J2056-3014 is an unusual accretion-powered intermediate polar that harbors a fast-spinning white dwarf (WD) with a spin period of $29.6$ s. The low X-ray luminosity and the relatively modest accretion rate per unit area suggest that the shock is not occurring near the WD surface. It has been argued that, under these conditions, the maximum temperature of the shock cannot be directly used to determine the mass of the WD (which, under the abovementioned assumptions, would be around $0.46$ $M_\odot$). Here, we explore the stability of this rapidly rotating WD using a modern equation of state (EoS) that accounts for electron--ion, electron--electron, and ion--ion interactions. For this EoS, we determine the mass density thresholds for the onset of pycnonuclear fusion reactions and study the impact of microscopic stability and rapid rotation on the structure and stability of WDs, considering them with helium, carbon, oxygen, and neon. From this analysis, we obtain a minimum mass for CTCV J2056--3014 of $0.56~M_\odot$ and a maximum mass of around $1.38~M_\odot$. If the mass of CTCV J2056--3014 is close to the lower mass limit, its equatorial radius would be on the order of $10^4$~km due to rapid rotation. Such a radius is significantly larger than that of a nonrotating WD of average mass ($0.6\, M_\odot$), which is on the order of $7\times 10^3$~km. The effects on the minimum mass of J2056-3014 due to changes in the temperature and composition of the stellar matter were found to be negligibly small.

astro-ph.SR↗

arXiv2021

Hybrid stars with color superconducting cores in an extended FCM model

We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell description, which assumes a sharp hadron-quark phase transition. The equation of state of hadronic matter is computed for a density-dependent relativistic lagrangian treated in the mean-field approximation, with parameters given by the SW4L nuclear model. This model described the interactions among baryons in terms of $σ, ω, ρ, σ^*$, and $ϕ$ mesons. Quark matter is assumed to be in either the CFL or the 2SC+s color superconducting phase. The possibility of sequential (hadron-quark, quark-quark) transitions in ultra-dense matter is investigated. Observed data related to massive pulsars, gravitational-wave events, and NICER are used to constrain the parameters of the extended FCM model. The successful equations of state are used to explore the mass-radius relationship, radii, and tidal deformabilities of hybrid stars. A special focus lies on investigating consequences that slow or fast conversions of quark-hadron matter have on the stability and the mass-radius relationship of hybrid stars. We find that if slow conversion should occur, a new branch of stable massive stars would exist whose members have radii that are up to 1.5~km smaller than those of conventional neutron stars of the same mass. Such objects could be possible candidates for the stellar high-mass object of the GW190425 binary system.

nucl-th↗

arXiv2021

Effects of anisotropy on strongly magnetized neutron and strange quark stars in general relativity

We investigate the properties of anisotropic, spherically symmetric compact stars, especially neutron stars and strange quark stars, made of strongly magnetized matter. The neutron stars are described by SLy equation of state, the strange quark stars by an equation of state based on the MIT Bag model. The stellar models are based on an a priori assumed density dependence of the magnetic field and thus anisotropy. Our study shows that not only the presence of a strong magnetic field and anisotropy, but also the orientation of the magnetic field itself, have an important influence on the physical properties of stars. Two possible magnetic field orientations are considered, a radial orientation, where the local magnetic fields point in the radial direction, and a transverse orientation, where the local magnetic fields are perpendicular to the radial direction. Interestingly, we find that for a transverse orientation of the magnetic field, the stars become more massive with increasing anisotropy and magnetic field strength and increase in size, since the repulsive, effective anisotropic force increases in this case. In the case of a radially orientated magnetic field, however, the masses and radii of the stars decrease with increasing magnetic field strength, because of the decreasing effective anisotropic force. Importantly, we also show that in order to achieve hydrostatic equilibrium configurations of magnetized matter, it is essential to account for both the local anisotropy effects as well as the anisotropy effects caused by a strong magnetic field. Otherwise, hydrostatic equilibrium is not achieved for magnetized stellar models.

astro-ph.HE↗

arXiv2021

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 $g^{2}/μ^{2}$ in the range of 4.58 GeV$^{-2}\leq g^{2}/μ^{2}\leq$ 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.

astro-ph.HE↗

arXiv2020

Rapidly rotating $Δ$-resonance-admixed hypernuclear compact stars

We use a set of hadronic equations of state derived from covariant density functional theory to study the impact of their high-density behavior on the properties of rapidly rotating $Δ$-resonance-admixed hyperonic compact stars. In particular, we explore systematically the effects of variations of the bulk energy isoscalar skewness, $Q_{\mathrm{sat}}$, and the symmetry energy slope, $L_{\mathrm{sym}}$, on the masses of rapidly rotating compact stars. With models for equation of state satisfying all the modern astrophysical constraints, excessively large gravitational masses of around $2.5 \, M_{\odot}$ are only obtained under three conditions: (a) strongly attractive $Δ$-resonance potential in nuclear matter, (b) maximally fast (Keplerian) rotation, and (c) parameter ranges $Q_{\mathrm{sat}}\gtrsim500$ MeV and $L_{\mathrm{sym}}\lesssim50$ MeV. These values of $Q_{\mathrm{sat}}$ and $L_{\mathrm{sym}}$ have a rather small overlap with a large sample (total of about 260) parametrizations of covariant nucleonic density functionals. The extreme nature of requirements (a)-(c) reinforces the theoretical expectation that the secondary object involved in the GW190814 event is likely to be a low-mass black hole rather than a supramassive neutron star.

astro-ph.HE↗

arXiv2020

Non-Newtonian gravity in strange quark stars and constraints from the observations of PSR J0740+6620 and GW170817

We investigate the effects of non-Newtonian gravity on the properties of strange quark stars (QSs) and constrain the parameters of the standard MIT bag model used to describe strange quark matter (SQM) by employing the mass of PSR J0740+6620 and the tidal deformability of GW170817. We find that, for the standard MIT bag model, these mass and tidal deformability observations would rule out the existence of QSs if non-Newtonian gravity effects are ignored. For a strange quark mass of $m_{s}=95$ MeV, we find that QSs can exist for values of the non-Newtonian gravity parameter $g^{2}/μ^{2}$ in the range of 1.37 GeV$^{-2}\leq g^{2}/μ^{2}\leq$ 7.28 GeV$^{-2}$ and limits on the bag constant and the strong interaction coupling constant of the SQM model given by 141.3 MeV$\leq B^{1/4}\leq$ 150.9 MeV and $α_{S}\leq 0.56$. For a strange quark mass of $m_{s}=150$ MeV, QSs can exist for 1.88 GeV$^{-2}\leq g^{2}/μ^{2}\leq$ 6.27 GeV$^{-2}$ and limits on the parameters of the SQM model given by 139.7 MeV$\leq B^{1/4}\leq$ 147.3 MeV and $α_{S}\leq 0.49$.

astro-ph.HE↗

arXiv2020

Delta baryons and diquark formation in the cores of neutron stars

We investigate the hadron-quark phase transition in cold neutron stars in light of (i) the observed limits on the maximum-mass of heavy pulsars, (ii) constraints on the tidal properties inferred from the gravitational waves emitted in binary neutron-star mergers, and (iii) mass and radius constraints derived from the observation of hot spots on neutron star observed with NICER. Special attention is directed to the possible presence of $Δ(1232)$ baryons in neutron star matter. Our results indicate that this particle could make up a large fraction of the baryons in neutron stars and thus have a significant effect on the properties of such objects, particularly on their radii. This is partially caused by the low density appearance of $Δ$s for a wide range of theoretically defensible sets of meson-hyperon, SU(3) ESC08 model, and meson-$Δ$ coupling constants. The transition of hadronic matter to quark matter, treated in the 2SC+s condensation phase, is found to occur only in neutron stars very close to the mass peak. Nevertheless, quark matter may still constitute an appreciable fraction of the stars' total matter if the phase transition is treated as Maxwell-like (sharp), in which case the neutron stars located beyond the gravitational mass peak would remain stable against gravitational collapse. In this case, the instability against gravitational collapse is shifted to a new (terminal) mass different from the maximum-mass of the stellar sequence, giving rise to stable compact objects with the same gravitational masses as those of the neutron stars on the traditional branch, but whose radii are smaller by up to 1 km. All models for the equation of state of our study fall comfortably within the bound established very recently by Annala {\it et al.} (Nature Physics, 2020)

astro-ph.HE↗

arXiv2020

Confronting GW190814 with hyperonization in dense matter and hypernuclear compact stars

We examine the possibility that the light companion in the highly asymmetric binary compact object coalescence event GW190814 is a hypernuclear star. We use density functional theory with functionals that have been tuned to the properties of $Λ$ hypernuclei as well as astrophysical constraints placed by the masses of the most massive millisecond pulsars, the mass-radius range inferred from the NICER experiment, and the binary neutron star merger event GW170817. We compute general-relativistic static and maximally rotating Keplerian configurations of purely nucleonic and hypernuclear stars. We find that while nucleonic stars are broadly consistent with a neutron star being involved in GW190814, this would imply no new degrees of freedom in the dense matter up to 6.5 times the nuclear saturation density. Allowing for hyperonization of dense matter, we find that the maximal masses of hypernuclear stars, even for maximal rapidly rotating configurations, are inconsistent with a stellar nature interpretation of the light companion in GW190814, implying that this event involved two black holes rather than a neutron star and a black hole.

astro-ph.HE↗

arXiv2019

Accretion-induced collapse to third family compact stars as trigger for eccentric orbits of millisecond pulsars in binaries

A numerical rotating neutron star solver is used to study the temporal evolution of accreting neutron stars using a multi-polytrope model for the nuclear equation of state named ACB5. The solver is based on a quadrupole expansion of the metric, but confirms the results of previous works, revealing the possibility of an abrupt transition of a neutron star from a purely hadronic branch to a third-family branch of stable hybrid stars, passing through an unstable intermediate branch. The accretion is described through a sequence of stationary rotating {stellar} configurations which lose angular momentum through magnetic dipole emission while, at the same time, gaining angular momentum through mass accretion. The model has several free parameters which are inferred from observations. The mass accretion scenario is studied in dependence on the effectiveness of angular momentum transfer which determines at which spin frequency the neutron star will become unstable against gravitational collapse to the corresponding hybrid star on the stable third-family branch. It is conceivable that the neutrino burst which accompanies the deconfinement transition may trigger a pulsar kick which results in the eccentric orbit. A consequence of the present model is the prediction of a correlation between the spin frequency of the millisecond pulsar in the eccentric orbit and its mass at birth.

astro-ph.HE↗

arXiv2019

Phase transitions in neutron stars and their links to gravitational waves

The recent direct observation of gravitational wave event $GW170817$ and its $GRB170817A$ signal has opened up a new window to study neutron stars and heralds a new era of Astronomy referred to as the Multimessenger Astronomy. Both gravitational and electromagnetic waves from a single astrophysical source have been detected for the first time. This combined detection offers an unprecedented opportunity to place constraints on the neutron star matter equation of state. The existence of a possible hadron-quark phase transition in the central regions of neutron stars is associated with the appearance of g-modes, which are extremely important as they could signal the presence of a pure quark matter core in the centers of neutron stars. Observations of g-modes with frequencies between 1 kHz and 1.5 kHz could be interpreted as evidence of a sharp hadron-quark phase transition in the cores of neutron stars. In this article, we shall review the description of the dense matter composing neutron stars, the determination of the equation of state of such matter, and the constraints imposed by astrophysical observations of these fascinating compact objects.

astro-ph.HE↗

arXiv2019

Astrophysical aspects of general relativistic mass twin stars

In this chapter we will introduce an effective equation of state (EoS) model based on polytropes that serves to study the so called "mass twins" scenario, where two compact stars have approximately the same mass but (significant for observation) quite different radii. Stellar mass twin configurations are obtained if a strong first-order phase transition occurs in the interior of a compact star. In the mass-radius diagram of compact stars, this will lead to a third branch of gravitationally stable stars with features that are very distinctive from those of white dwarfs and neutron stars. We discuss rotating hybrid star sequences in the slow rotation approximation and in full general relativity and draw conclusions for an upper limit on the maximum mass of nonrotating compact stars that has recently be deduced from the observation of the merger event GW170817.

astro-ph.HE↗

arXiv2019

Cooling of hypernuclear compact stars: Hartree-Fock models and high-density pairing

The thermal evolution of hypernuclear compact stars is studied for stellar models constructed on the basis of covariant density functional theory in Hartree and Hartree-Fock approximation. Parametrizations of both types are consistent with the astrophysical mass constraints on compact stars and available hypernuclear data. We discuss the differences of these density functionals and highlight the effects they have on the composition and on the cooling of hypernuclear stars. It is shown that hypernuclear stars computed with density functional models that have a low symmetry energy slope, $L$, are fairly consistent with the cooling data of observed compact stars. The class of stellar models based on larger $L$ values gives rise to the direct Urca process at low densities, which leads to significantly faster cooling. We conjecture high-density pairing for protons and $Λ$'s in the $P$-wave channel and provide simple scaling arguments to obtain these gaps. As a consequence the most massive stellar models with masses $1.8 \le M/M_{\odot} \le2$ experience slower cooling by hyperonic dUrca processes which involve $Λ$'s and protons.

nucl-th↗

arXiv2018

Hyperonic Neutron Star Matter in Light of GW170817

Since 2013 the mass of pulsar PSR J0348+0432 ($M = 2.01\,M_{\odot}$) has provided a tight constraint on neutron star equation of state. However, a number of different analyses of the recently detected BNS merger (GW170817) point to a maximum neutron star mass around $2.16\,M_{\odot}$. In addition, a recent study determined the mass of the millisecond pulsar PSR J2215+5135 to be $2.27^{+0.17}_{-0.15}\,M_{\odot}$. In this work we investigate the presence of hyperons in neutron star matter in light of these new mass measurements using equations of state calculated in the relativistic mean-field approximation. Particular attention is paid to the use of the available empirical data from the study of hypernuclei and that of the SU(3) symmetry relations in fixing the meson-hyperon coupling constants. We find that hyperonic equations of state with reasonable choices for the meson-hyperon coupling constants can satisfy these new mass constraints, with hyperons potentially accounting for more than 10% of the baryons in the core of a neutron star.

nucl-th↗