arXiv subjects

Len Brandes

Publications and source records attributed to Len Brandes.

6 recordsLinked to original sources

arXiv2024

Implications of latest NICER data for the neutron star equation of state

As an update to our previously performed Bayesian inference analyses of the neutron star matter equation-of-state and related quantities, the additional impact of the recently published NICER data of PSR J0437-4751 is examined. Including the mass and radius distributions of this pulsar in our data base results in modest shifts from previously inferred median posterior values of radii $R$ and central densities $n_c$ for representative $1.4\,M_\odot$ and $2.1\,M_\odot$ neutron stars: radii are reduced by about $0.2-0.3$ km to values of $R_{1.4} = 12.1\pm 0.5$ km and $R_{2.1} = 11.9^{+0.5}_{-0.6}$ km (at the 68\% level), and central densities increase slightly to values of $n_c(1.4\,M_\odot)/n_0 = 2.8\pm 0.3$ and $n_c(2.1\,M_\odot)/n_0 = 3.8_{-0.7}^{+0.6}$ (in units of equilibrium nuclear matter density, $n_0 = 0.16$ fm$^{-3}$), i.e., they still fall below five times nuclear saturation density at the 68\% level. As a further significant result, the evidence established by analyzing Bayes factors for a negative trace anomaly measure, $\Delta = 1/3-P/\varepsilon < 0$, inside heavy neutron stars is raised to strong.

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arXiv2024

Neural Simulation-Based Inference of the Neutron Star Equation of State directly from Telescope Spectra

Neutron stars provide a unique opportunity to study strongly interacting matter under extreme density conditions. The intricacies of matter inside neutron stars and their equation of state are not directly visible, but determine bulk properties, such as mass and radius, which affect the star's thermal X-ray emissions. However, the telescope spectra of these emissions are also affected by the stellar distance, hydrogen column, and effective surface temperature, which are not always well-constrained. Uncertainties on these nuisance parameters must be accounted for when making a robust estimation of the equation of state. In this study, we develop a novel methodology that, for the first time, can infer the full posterior distribution of both the equation of state and nuisance parameters directly from telescope observations. This method relies on the use of neural likelihood estimation, in which normalizing flows use samples of simulated telescope data to learn the likelihood of the neutron star spectra as a function of these parameters, coupled with Hamiltonian Monte Carlo methods to efficiently sample from the corresponding posterior distribution. Our approach surpasses the accuracy of previous methods, improves the interpretability of the results by providing access to the full posterior distribution, and naturally scales to a growing number of neutron star observations expected in the coming years.

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arXiv2023

Constraints on Phase Transitions in Neutron Star Matter

Recent inference results of the sound velocity in the cores of neutron stars are summarized. Implications for the equation of state and the phase structure of highly compressed baryonic matter are discussed. In view of the strong constraints imposed by the heaviest known pulsars, the equation of state must be very stiff in order to ensure the stability of these extreme objects. This required stiffness limits the possible appearance of phase transitions in neutron star cores. For example, a Bayes factor analysis quantifies strong evidence for squared sound velocities $c_s^2 > 0.1$ in the cores of 2.1 solar-mass and lighter neutron stars. Only weak first-order phase transitions with a small phase coexistence density range $\Delta\rho/\rho < 0.2$ (at the 68\% level) in a Maxwell construction still turn out to be possible within neutron stars. The central baryon densities in even the heaviest neutron stars do not exceed five times the density of normal nuclear matter. In view of these data-based constraints, much discussed issues such as the quest for a phase transition towards restored chiral symmetry, and the active degrees of freedom in cold and dense baryonic matter, are reexamined.

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arXiv2023

Evidence against a strong first-order phase transition in neutron star cores: impact of new data

With the aim of exploring the evidence for or against phase transitions in cold and dense baryonic matter, the inference of the sound speed and equation-of-state for dense matter in neutron stars is extended in view of recent new observational data. The impact of the heavy (2.35 $M_\odot$) black widow pulsar PSR J0952-0607 and of the unusually light supernova remnant HESS J1731-347 is inspected. In addition a detailed re-analysis is performed of the low-density constraint based on chiral effective field theory and of the perturbative QCD constraint at asymptotically high densities, in order to clarify the influence of these constraints on the inference procedure. The trace anomaly measure, $\Delta = 1/3 - P/\varepsilon$, is also computed and discussed. A systematic Bayes factor assessment quantifies the evidence (or non-evidence) of low averaged sound speeds $(c_s^2 \leq 0.1)$, a prerequisite for a phase transition, within the range of densities realized in the core of neutron stars. One of the consequences of including PSR J0952-0607 in the data base is a further stiffening of the equation-of-state, resulting for a 2.1 solar-mass neutron star in a reduced central density of less than five times the equilibrium density of normal nuclear matter at the 68\% level. The evidence against small sound speeds in neutron star cores is further strengthened. Within the inferred 68\% posterior credible bands, only a weak first-order phase transition with a coexistence density interval $\Delta n/n \lesssim 0.2$ would be compatible with the observed data.

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arXiv2022

Inference of the sound speed and related properties of neutron stars

Information on the phase structure of strongly interacting matter at high baryon densities can be gained from observations of neutron stars and their detailed analysis. In the present work Bayesian inference methods are used to set constraints on the speed of sound in the interior of neutron stars, based on recent multi-messenger data in combination with limiting conditions from nuclear physics at low densities. Two general parametric representations are introduced for the sound speed $c_s$ in order to examine the independence with respect to choices for the parametrisation of Priors. Credible regions for neutron star properties are analysed, in particular with reference to the quest for possible phase transitions in cold dense matter. The evaluation of Bayes factors implies extreme evidence for a violation of the conformal bound, $c_s^2 \leq 1/3$, inside neutron stars. Given the presently existing data base, it can be concluded that the occurrence of a first-order phase transition in the core of even a two-solar-mass neutron star is unlikely, while a continuous crossover cannot be ruled out. At the same time it is pointed out that the discovery of a superheavy neutron star with a mass $ M \sim 2.3 - 2.4\, M_\odot$ would strengthen evidence for a phase change in the deep interior of the star.

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arXiv2021

Fluctuations and phases in baryonic matter

The phase structure of baryonic matter is investigated with focus on the role of fluctuations beyond the mean-field approximation. The prototype test case studied is the chiral nucleon-meson model, with added comments on the chiral quark-meson model. Applications to the liquid-gas phase transition in nuclear matter and extensions to dense matter are performed. The role of vacuum fluctuations and thermal excitations is systematically explored. It is pointed out that such fluctuations tend to stabilise the hadronic phase characterised by spontaneously broken chiral symmetry, shifting the chiral restoration transition to very high densities. This stabilisation effect is shown to be further enhanced by additional dynamical fluctuations treated with functional renormalisation group methods.

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