arXiv subjects

Nicolas Chamel

Publications and source records attributed to Nicolas Chamel.

At least 19 recordsLinked to original sources

arXiv2026

On the impact of the carbon fusion rate over the properties of superbursts -- Numerical simulations of superbursts with MESA

Context: Superbursts are very energetic explosions in the crust of neutron stars in Low-Mass X-ray Binaries (LMXBs). These are triggered by unstable carbon burning at $T\leq 10^{9}$ K. In recent years, there has been a re-examination of the carbon fusion rate, finding that at these temperatures it might be either smaller or higher with respect to the standard rate from Caughlan \& Fowler (1988). Aims: We explore the consequences changing the carbon fusion rate has over the physics of superbursts. Methods: For simulating superbursts, we employ the public code \mesa\ v.24.08.1, as well as four versions of the carbon fusion reaction rate. Results: A significant enhancement of the reaction rate at $T\leq 10^9$~K would reduce the recurrence and decay times of the superburst, as well as the column depth at ignition. The opposite behavior is observed when the carbon fusion rate is reduced. The maximum temperature reached during the explosion is also sensitive to a change in the carbon fusion rate, leading to either an enhancement or a reduction in the synthesis of $\alpha$-nuclides. These changes are comparable to the effect of reducing the amount of base heating at the bottom of the envelope. Conclusions: The modelling of X-ray superburst is sensitive to the adopted carbon fusion rate at $T\leq 10^{9}$ K. Given its role for determining the ignition conditions, this rate needs to be better experimentally constrained to reduce the uncertainty in nuclear physics when modeling the superburst light curve and the nucleosynthesis of the explosion. At the same time, multizone simulations of superburst also requires improvement in the description of the initially accreted material and stellar conditions prior to the explosion.

astro-ph.HE↗

arXiv2026

Internal constitution of the outer crust of non-accreted neutron stars and magnetars

Context. Determining the internal constitution of the outer crust of magnetars is important for interpreting several of their astrophysical manifestations. In particular, the crustal composition is a key input for simulations of r-process nucleosynthesis in giant flare ejecta. However, traditional methods are computationally expensive, limiting their use in large-scale studies. Although faster iterative approaches exist, they are restricted to unmagnetized matter and strongly quantizing magnetic fields, leaving the intermediate field strengths characteristic of observed magnetars without an efficient treatment. Aims. We developed the program magcrust to extend these existing iterative approaches, enabling the rapid computation of the outer-crust composition of cold, non-accreted magnetars over the full range of the magnetic-field strengths inferred for these objects. Methods. Transitions between adjacent crustal layers are computed by solving approximate equilibrium conditions at the interface. Nuclear abundances and layer depths are estimated from approximate solutions of Einstein's equations of general relativity. Results. The performance and accuracy of the program were assessed against detailed numerical calculations. Relative deviations from exact transition properties remain within a few percent, and crustal compositions are well reproduced across 17 nuclear mass tables and 1300 magnetic-field strengths from 1E13 to 1E16 G. Computation times are reduced by factors of 1E3-1E7 compared to traditional approaches. Conclusions. This program provides a robust and efficient tool for determining the stratification of magnetars' outer crust over the full range of astrophysically relevant magnetic-field strengths. Its computational speed makes it well suited to systematic calculations, including sensitivity analyses, uncertainty quantification, and ensemble studies.

astro-ph.HE↗

arXiv2026

Superfluid fraction and effective ion mass in the crystalline crust of a neutron star: role of interband response

Neutron superfluidity in the inner crust of a neutron star is further investigated, focusing on the role of the interband response in the superfluid fraction and the effective mass of crustal ions induced by their motion through the superfluid. Calculations are performed within the linear response theory of the self-consistent time-dependent Hartree-Fock-Bogoliubov equations with Skyrme nuclear energy density functionals in the Bardeen-Cooper-Schrieffer approximation. The absence of interband response in previous analyses is clarified. The neutron superfluid density is formally shown to be consistent with the entrainment matrix derived earlier in homogeneous neutron-proton superfluid mixture, thus providing a unified description of entrainment effects in the inner crust and outer core of a neutron star within the same microscopic framework. The relative importance of the intraband and interband responses in different regions of the crust is numerically assessed from three-dimensional band-structure calculations, taking into account quantum zero-point motion of ions about their equilibrium position. The neutron superfluid fraction is found to be enhanced by the interband response, resulting in effective ion masses that remain close to the mass of quantum mechanically bound nucleons for realistic neutron pairing gaps. Results are compared to predictions from classical hydrodynamics with different prescriptions for the permeability of ions to superfluidity.

astro-ph.HE↗

arXiv2026

Superfluid fraction in the crystalline crust of a neutron star: role of quantum zero-point motion of ions

The suppression of the neutron superfluid fraction in the inner crust of a cold neutron star is mitigated by the quantum zero-point motion of ions about their equilibrium position. In turn, the crustal dynamics is altered by the presence of the neutron superfluid. These effects are studied self-consistently to assess the validity of the usual assumption of a perfect rigid lattice. To this end, fully three-dimensional band-structure calculations of the superfluid fraction are carried out in the weak-coupling approximation, considering body- and face-centered cubic lattices. In both cases, the superfluid fraction is still found to be strongly suppressed in the intermediate region of the inner crust. In turn, the effective mass of the ions is dramatically increased, thus further damping the ion fluctuations. These results are of relevance for the rotational and thermal evolutions of neutron stars.

astro-ph.HE↗

arXiv2026

Tidal deformations of general-relativistic multifluid compact stars

Over the past decade, gravitational-wave astronomy has opened a new window onto the extreme states of matter inside compact stars. At some point during the inspiral of a binary system, each star starts to experience adiabatic tides, characterized by tidal deformabilities. The dominant tidal deformability, first measured with the GW170817 event, has already constrained the dense-matter equation of state. With the advent of third-generation detectors, tidal deformabilities are expected to be inferred with much higher precision, potentially revealing subleading tidal contributions. This motivates the development of more accurate compact-star models that incorporate richer microphysics. With this in mind, we move beyond the commonly adopted perfect-fluid approximation and model compact stars through a multifluid framework. In this work, we present the fully general-relativistic description of adiabatic tidal deformations of compact stars composed of an arbitrary number of interacting fluids, using Carter's multifluid variational formalism. A distinctive feature of this approach is the presence of nondissipative mutual entrainment between fluid species. We derive the hydrostatic equilibrium equations for multifluid configurations, along with the perturbed equations governing stationary gravitoelectric and gravitomagnetic tidal responses of arbitrary order. We then investigate how entrainment modifies the corresponding tidal deformabilities. Using an analytical representation of the multifluid equation of state, we show that entrainment leaves adiabatic tidal responses unchanged and therefore produces no measurable effect on the gravitational-wave signal emitted during the inspiral long before the excitation of internal mode resonances. We subsequently discuss two specific applications: superfluid neutron stars and dark matter admixed compact stars.

gr-qc↗

arXiv2025

Stellar properties indicating the presence of hyperons in neutron stars

We describe distinctive stellar features indicating the presence of hyperons in neutron stars as compared to purely nucleonic systems. A strongly negative curvature of the mass-radius relation $R(M)$ is characteristic of hyperons, which can be determined from measurements of neutron stars with three different masses. Similarly, a reduced second derivative of the tidal deformability as function of mass \lambda(M) points to hyperonic degrees of freedom in NS matter. The slopes of such curves R(M) and \lambda(M) can distinguish a hyperonic equation of state from purely nucleonic models if they appear increased (decreased for \lambda(M)) relative to the maximum mass of neutron stars.

astro-ph.HE↗

arXiv2025

Gapless neutron superfluidity in the crust of the accreting neutron stars KS 1731-260 and MXB 1659-29

The interpretation of the thermal evolution of the transiently accreting neutron stars MXB 1659-29 and KS 1731-260 after an outburst is challenging, both within the traditional deep-crustal heating paradigm and the thermodynamically consistent approach of Gusakov and Chugunov that accounts for neutron diffusion throughout the crust. All these studies assume that the neutron superfluid in the crust is at rest. However, we have recently shown that a finite superflow could exist and could lead to a new gapless superfluid phase if quantized vortices are pinned. We have revisited the cooling of MXB 1659-29 and KS 1731-260 and we have found that gapless superfluidity could naturally explain their late time cooling. We pursue here our investigation by performing new simulations of the thermal relaxation of the crust of MXB 1659-29 and KS 1731-260 within a Markov Chain Monte Carlo method accounting for neutron diffusion and allowing for gapless superfluidity. We have varied the global structure of the neutron star, the composition of the heat-blanketing envelope, and the mass accretion rate. In all cases, observations are best fitted by models with gapless superfluidity. Finally, we make predictions that could be tested by future observations.

astro-ph.HE↗

arXiv2025

Neutron Star Inner Crust at Finite Temperatures: A Comparison Between Compressible Liquid Drop and Extended Thomas-Fermi Approaches

We investigate the effects of temperature on the properties of the inner crust of a non-accreting neutron star. To this aim, we employ two different treatments: the compressible liquid drop model (CLDM) and the temperature-dependent extended Thomas-Fermi (TETF) method. Our systematic comparison shows an agreement between the two methods on their predictions for the crust thermodynamic properties. We find that the CLDM description can also reproduce reasonably well the TETF composition especially if the surface energy is optimized on the ETF calculation. However, the neglect of neutron skin in CLDM leads to an overestimation of the proton radii.

nucl-th↗

arXiv2025

Filling fractions for the formation of nuclear pasta in neutron stars: semiclassical vs liquid-drop predictions

Historically, a sequence of nuclear pasta shapes was predicted to appear in the deepest region of the inner crust of a neutron star within the compressible liquid-drop picture, when the filling fraction $u$ exceeds some threshold values. However, later calculations showed that these values depend on the details of the liquid-drop model. Here we investigate the existence of pasta in neutron stars within the semiclassical extended Thomas-Fermi approach using various generalized Skyrme functionals. The filling fractions for the different transitions are found to be quasi-universal, unlike the pasta density ranges governed by the symmetry energy at relevant densities. In particular, pasta emerge at $u_\mathrm{sp}\approx0.13-0.15$. By applying a simplified stability criterion within the liquid-drop framework, we show that these values of $u_\mathrm{sp}$ can be explained by the nuclear curvature correction. In this way, the abundance of pasta can be easily estimated. This criterion can also be used to optimize the search of pasta within the more realistic extended Thomas-Fermi approach.

astro-ph.HE↗

arXiv2025

Roadmap for warm dense matter physics

This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the warm dense matter physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations \& experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments and contemporary challenges that are faced by theoretical methods, and experimental techniques needed to create and diagnose warm dense matter. A large part of this roadmap is dedicated to specific warm dense matter systems and applications in astrophysics, inertial confinement fusion and novel material synthesis.

physics.plasm-ph↗

arXiv2025

Pressure and chemical potentials in the inner crust of a cold neutron star within Hartree-Fock and extended Thomas-Fermi methods

Self-consistent mean-field methods with Skyrme-type effective interactions and semiclassical approximations, such as the Thomas-Fermi approach and its extensions are particularly well-suited for describing in a thermodynamically consistent way the various phases of the dense matter present in the interior of neutron stars. These methods have been applied to predict the composition of the different regions, including the inner crust constituted by nuclear clusters coexisting with free neutrons and electrons. Because of the computational cost, the energy is typically calculated for a few selected average baryon number densities, and the results are interpolated to obtain the pressure numerically. However, this may introduce systematic errors in the calculations of the global structure of a neutron star and its dynamical evolution. In this paper, we show how the full equation of state can be consistently calculated within the same framework by deriving exact formulas for the chemical potentials and for the pressure that can be easily implemented in existing computer codes. These formulas are applicable to both catalyzed and accreted crusts. We discuss in each case the suitable conditions to impose to determine the composition. Numerical examples are also presented and discussed. Results from refined calculations of the BSk24 equation of state for the inner crust of nonaccreted neutron stars and the corresponding adiabatic index are provided.

nucl-th↗

arXiv2024

Superfluid fraction in the crystalline crust of a neutron star: role of BCS pairing

The breaking of translational symmetry in the inner crust of a neutron star leads to the depletion of the neutron superfluid reservoir similarly to cold atomic condensates in optical lattices and in supersolids. This effect is studied in the general framework of the self-consistent time-dependent Hartree-Fock-Bogoliubov (HFB) theory, treating the crust as a perfect crystal. The superfluid fraction is derived in the Bardeen-Cooper-Schrieffer approximation for superfluid velocities much smaller than Landau's critical velocity within the linear-response theory. The different assumptions made in previous studies are clarified. Fully three-dimensional band-structure calculations of superfluid neutrons in a body-centered cubic lattice are carried out. Although the formation of Cooper pairs is essential for the occurrence of superfluidity, the superfluid fraction is found to be insensitive to the pairing gap, as in uniform neutron matter. In the intermediate region of the inner crust at the average baryon number density 0.03 fm$^{-3}$, only 8\% of the free neutrons are found to participate to the superflow. Such very low superfluid fraction challenges the classical interpretation of pulsar frequency glitches and calls for more systematic calculations within the full HFB approach.

astro-ph.HE↗

arXiv2024

Skyrme-Hartree-Fock-Bogoliubov mass models on a 3D mesh: IV. Improved description of the isospin dependence of pairing

Providing reliable data on the properties of atomic nuclei and infinite nuclear matter to astrophysical applications remains extremely challenging, especially when treating both properties coherently within the same framework. Methods based on energy density functionals (EDFs) enable manageable calculations of nuclear structure throughout the entire nuclear chart and of the properties of infinite nuclear matter across a wide range of densities and asymmetries. To address these challenges, we present BSkG4, the latest Brussels-Skyrme-on-a-Grid model. It is based on an EDF of the extended Skyrme type with terms that are both momentum and density-dependent, and refines the treatment of $^1S_0$ nucleon pairing gaps in asymmetric nuclear matter as inspired by more advanced many-body calculations. The newest model maintains the accuracy of earlier BSkGs for known atomic masses, radii and fission barriers with rms deviations of 0.633 MeV w.r.t. 2457 atomic masses, 0.0246 fm w.r.t. 810 charge radii, and 0.36 MeV w.r.t 45 primary fission barriers of actinides. It also improves some specific pairing-related properties, such as the $^1S_0$ pairing gaps in asymmetric nuclear matter, neutron separation energies, $Q_\beta$ values, and moments of inertia of finite nuclei. This improvement is particularly relevant for describing the $r$-process nucleosynthesis as well as various astrophysical phenomena related to the rotational evolution of neutron stars, their oscillations, and their cooling.

nucl-th↗

arXiv2024

Role of neutron pairing with density-gradient dependence in the semi-microscopic treatment of the inner crust of neutron stars

Using the fourth-order extended Thomas-Fermi method with Strutinsky-integral shell and pairing corrections, we calculate the inner crust of neutron stars with the BSk31 functional, whose pairing has two terms: i) a term that is fitted to the results of microscopic calculations on homogeneous nuclear matter (accounting for both medium polarization and self-energy effects) that are more realistic than those of our earlier functionals; ii) an empirical term that is dependent on the density gradient, which permits an excellent fit to nuclear masses. Both proton and neutron pairing are taken into account, the former in the BCS theory and the latter in the local density approximation. We found that the equilibrium value of the proton number $Z$ remains 40 over the entire density range considered, whether or not neutron pairing is included. The new equation of state and the composition are very similar to those of our previously preferred functional, BSk24. However, the predicted neutron pairing fields are quite different. In particular, clusters are found to be impermeable to the neutron superfluid. The implications for the neutron superfluid dynamics are briefly discussed. Since the new pairing is more realistic, the functional BSk31 is better suited for investigating neutron superfluidity in neutron-star crusts.

nucl-th↗

arXiv2024

Time-dependent nuclear energy-density functional theory toolkit for neutron star crust: Dynamics of a nucleus in a neutron superfluid

We present a new numerical tool designed to probe the dense layers of neutron star crusts. It is based on the time-dependent Hartree-Fock-Bogoliubov theory with generalized Skyrme nuclear energy-density functionals of the Brussels-Montreal family. We use it to study the time evolution of a nucleus accelerating through superfluid neutron medium in the inner crust of a neutron star. We extract an effective mass in the low velocity limit. We observe a threshold velocity and specify mechanisms of dissipation: phonon emission, Cooper pairs breaking, and vortex rings creation. These microscopic effects are of key importance for understanding various neutron star phenomena. Moreover, the mechanisms we describe are general and apply also to other fermionic superfluids interacting with obstacles like liquid helium or ultracold gases.

nucl-th↗

arXiv2024

Gapless superfluidity in neutron stars: Thermal properties

The interior of mature neutron stars is expected to contain superfluid neutrons and superconducting protons. The influence of temperature and currents on superfluid properties is studied within the self-consistent time-dependent nuclear energy-density functional theory. We find that this theory predicts the existence of a regime in which nucleons are superfluid (the order parameter remains finite) even though the energy spectrum of quasiparticle excitations exhibits no gap. We show that the disappearance of the gap leads to a specific heat that is not exponentially suppressed at low temperatures as in the BCS regime but can be comparable to that in the normal phase. Introducing some dimensionless effective superfluid velocity, we show that the behavior of the specific heat is essentially universal and we derive general approximate analytical formulas for applications to neutron-star cooling simulations.

nucl-th↗

arXiv2024

Gapless neutron superfluidity can explain the late time cooling of transiently accreting neutron stars

The current interpretation of the observed late time cooling of transiently accreting neutron stars in low-mass X-ray binaries during quiescence requires the suppression of neutron superfluidity in their crust at variance with recent ab initio many-body calculations of dense matter. Focusing on the two emblematic sources KS~1731$-$260 and MXB~1659$-$29, we show that their thermal evolution can be naturally explained by considering the existence of a neutron superflow driven by the pinning of quantized vortices. Under such circumstances, we find that the neutron superfluid can be in a gapless state in which the specific heat is dramatically increased compared to that in the classical BCS state assumed so far, thus delaying the thermal relaxation of the crust. We have performed neutron-star cooling simulations taking into account gapless superfluidity and we have obtained excellent fits to the data thus reconciling astrophysical observations with microscopic theories. The imprint of gapless superfluidity on other observable phenomena is briefly discussed.

astro-ph.HE↗

arXiv2023

Torsional oscillations of magnetized neutron stars: Impacts of Landau-Rabi quantization of electron motion

Torsional oscillations of magnetized neutron stars have been well studied since they may be relevant to the physical interpretation of some of the observed quasiperiodic oscillations in the magnetar giant flares. In the crustal region of a magnetar, the strong magnetic field can alter the equation of state and composition due to the Landau-Rabi quantization of electron motion. In this paper, we study this effect on the crust-confined, torsional oscillation modes of neutron stars with mixed poloidal-toroidal magnetic fields in general relativity under the Cowling approximation. Furthermore, the inner and outer crusts are treated consistently based on the nuclear-energy density functional theory. Depending on the magnetic-field configurations, we find that the Landau-Rabi quantization of electrons can change the frequencies of the fundamental torsional oscillation mode of $1.4 M_\odot$ neutron star models with a normal fluid core by about 10% when the magnetic field strength at the pole reaches the order of $10^{16}$ G. The shift can even approach 20% at a field strength of $10^{15}$ G for neutron stars with a simple model of superconducting core where the magnetic field is assumed to be expelled completely.

astro-ph.HE↗