Search Results (515)

We review some recent findings on thermal properties of the magnetic dual chiral density wave (MDCDW) condensate in the Nambu–Jona-Lasinio (NJL) model of dense quark matter, as well as a convenient method for investigating this phase with a high-order Ginzburg–Landau (GL) expansion. We show how a recently discovered formula for the GL coefficients can be used to compute key physical properties of the condensate, such as its ground state order parameters and critical temperature in the mean-field approximation and its stability against thermal phonon fluctuations. We find that magnetic fields of order ${10}^{18}$ G significantly increase the condensate magnitude and critical temperature, eventually making the condensate favored up to temperatures a few times 10 MeV over the entire range of densities in the model. At much smaller fields, the condensate is still preferred and thermally stable over a range of densities relevant to cold neutron stars. We emphasize how the topological features of MDCDW are encoded in certain terms of the GL expansion, which can be used to show that the preceding effects have a topological origin. Finally, we present a new result on the convergence properties of the GL expansion, proving that it converges when ${|m|}^2+{|b|}^2<\sqrt{{\mu }^2+{\left(\pi T\right)}^2},$ where m and b are order parameters proportional to the condensate magnitude and spatial modulation, respectively. This condition holds over a large region of parameter space, including the region of interest for neutron star applications. Full article

Spectral line shapes are used to diagnose parameters of white dwarfs and neutron stars in particular. In magnetized plasmas, the motion of the radiating atom in the plasma needs to be considered in the collision process as the electronic structure of the atom depends on its center-of-mass translational momentum. More broadly, collision models do not explicitly or fully account for the motion of the nucleus, accounting for deflection through conservation of momentum. Traditionally, the correlation between electronic and nuclear motion has been captured through mass-polarization terms involving momenta scalar products between different electrons. We reformulate the collision problem accounting for the motion of the nucleus, taking advantage of unitary transformations. In this new formulation, Coulomb interactions between the atom and projectile/plasma particle become displaced Coulomb interactions, and exchange interactions include corrections of order $1/M_A$. We demonstrate the resulting impact on the elastic scattering T-matrices of the $1s$ state of hydrogen, where the lowest-energy electrons increase the real part by 20–30% while leaving the imaginary part practically unaltered. Lastly, we present a formulation so that the atomic motion can be explicitly included in the collision problem for magnetic-field applications. Full article

The density dependence of nuclear symmetry energy remains one of the key uncertainties in contemporary nuclear physics, with significant implications for the structure of exotic nuclei, the dynamics of heavy-ion collisions, and the properties of astrophysical objects such as neutron stars and core-collapse supernovae. However, extracting robust constraints requires observables that are minimally affected by final-state interactions and are reliably predicted by transport models. This review synthesizes recent theoretical and experimental advancements in constraining the symmetry energy by leveraging isospin diffusion in heavy-ion reactions within the Fermi energy domain. Recent results from the INDRA-FAZIA collaboration, including isospin transport ratio data and Boltzmann–Uehling–Uhlenbeck (BUU) transport model calculations, are highlighted. Confidence regions for the symmetry energy are extracted from isospin transport ratios and isospin diffusion currents by utilizing state-of-the-art nuclear functionals, including both ab initio and phenomenological approaches, with a particular focus on the density regions probed by these experiments. The resulting constraints will aid future Bayesian studies of the nuclear equation of state and contribute to a more unified understanding of dense matter in both terrestrial experiments and astrophysical environments. Full article

Be/X-ray binaries constitute the largest and most diverse subgroup of neutron star high-mass X-ray binaries. These systems feature a rapidly rotating Be star surrounded by a circumstellar decretion disk that serves as the primary reservoir of accreted matter onto a strongly magnetized neutron star. While a few Be/X-ray binaries remain persistently active, the majority manifest as hard X-ray transient sources, becoming detectable only during X-ray outbursts. This review synthesizes current understanding of their rich phenomenology across optical and X-ray wavelengths, focusing on variability ocurring over timescales that span from seconds to years. Full article

The optical counterpart of the gravitational wave event GW170817, known as kilonova, has provided strong evidence that binary neutron star mergers are favourable sites to host the r-process nucleosynthesis. Kilonova is a quasi-thermal electromagnetic emission powered by the radioactive decay of heavy neutron-rich nuclei produced by the r-process. Considering the variety of elements contributing to kilonova ejecta, essential information about its composition can be achieved through spectral characterisation, radiative transfer simulations, and opacities. The latter represents one of the most challenging aspects of the modelling, as it relies on accurate atomic structure calculations of energy levels and transitions. Since light r-process elements are major opacity contributors in early (<2 days) scenario, this work focuses on atomic calculations for Zr I–IV. Energy levels and bound-bound transitions are determined using the GRASP2018 code, assuming two different datasets for each ionisation stage: one including, and one excluding core-core and core-valence correlations. Results demonstrate that the inclusion of f shell and core correlations impacts on both energy levels and transitions. A systematic assessment of the accuracy is performed through detailed comparisons with the NIST ASD and literature references. Finally, these Zr data are integrated on the open access MARTINI platform. Full article

Fast radio bursts (FRBs) are millisecond-duration radio transients of extragalactic origin, while ultra-high-energy cosmic rays (UHECRs; $E\gtrsim {10}^{18}$ eV) remain among the most important unresolved problems in astroparticle physics. This review examines the viability of FRBs and their central engines as sources of UHECRs within a comprehensive multi-messenger framework. We summarize the observational constraints on UHECR source populations imposed by the energy spectrum, nuclear composition, anisotropy measurements, diffuse $\gamma$-ray background, and high-energy neutrino observations, which, together, favor source classes capable of accelerating heavy nuclei with hard injection spectra, modest cosmological evolution, and sufficiently high source densities. We then review the current landscape of FRB progenitor and engine models, including magnetars, supramassive neutron stars, compact-object mergers, and accretion-powered systems, emphasizing their energetics, environments, and particle-acceleration capabilities through relativistic shocks, magnetic reconnection, magnetar wind nebulae, and direct electromagnetic acceleration by ultra-relativistic FRB pulses. We discuss how these scenarios are constrained by neutrino and $\gamma$-ray observations from IceCube, KM3NeT, and Fermi-LAT, as well as by large-scale UHECR anisotropy measurements from the Pierre Auger Observatory and Telescope Array. Finally, we examine the observational tests that will become possible in the coming decade through large samples of localized FRBs, composition-resolved UHECR measurements, next-generation neutrino observatories, and wide-field $\gamma$-ray facilities. We emphasize that FRB dispersion and rotation measures provide unique probes of the baryonic and magnetic environments relevant for UHECR acceleration and propagation, enabling a new form of multi-messenger tomography of cosmic-ray source environments and allowing the FRB–UHECR connection to become a quantitatively testable astrophysical framework. Full article

Robust age measurements for isolated neutron stars (NSs) are not easily available. That is why the characteristic age ${\tau }_{\mathrm{ch}}=P/2\dot{P}$ is often used as a proxy. Here, P is the spin period of the NS and $\dot{P}$ is the time derivative of P. Additional assumptions related to the initial properties and spin-down evolution are made to derive ${\tau }_{\mathrm{ch}}$. As a result, it is expected that ${\tau }_{\mathrm{ch}}$ is an upper limit for the real age ${\tau }_{\mathrm{real}}$. Recently, Chrimes et al. presented measurements of kinematic ages ${\tau }_{\mathrm{kin}}$ for several magnetars. Surprisingly, for the majority of these sources, ${\tau }_{\mathrm{kin}}>{\tau }_{\mathrm{ch}}$. We present a simple model that includes a realistic approximation for magnetic field decay in magnetars and a simple phenomenological description of field re-emergence following fallback after the birth of an NS. We demonstrate that this simple model can explain the observed relation ${\tau }_{\mathrm{kin}}>{\tau }_{\mathrm{ch}}$ for a realistic set of parameters. Full article

It is well known that axions or axion-like particles can mediate spin-dependent interactions. If such interactions exist, they may violate the equivalence principle of general relativity, causing a polarized fermion with nonzero mass to experience different gravitational effects for different spin orientations. In this work, we derive the spin-dependent gravitational interaction generated by a spherically symmetric celestial body and apply the formalism to the Earth. We compare existing experimental searches for spin-dependent gravitational effects with the constraints implied by axion-mediated interactions. We further note that a rotating polarized neutron star may generate a time-dependent spin–mass interaction field acting on an unpolarized probe mass, which could be detected with high-frequency, high-Q torsion oscillators. Full article

Neutron capture reactions provide essential nuclear physics input for modeling the synthesis of heavy elements in stars. The growing precision of stellar spectroscopy and isotopic measurements in presolar SiC grains now demands cross sections with improved accuracy over the full energy range, and access to unstable nuclei relevant to slow (s-) process branchings and the intermediate (i-) process. This article reviews recent progress in direct neutron capture measurements, focusing on time-of-flight (TOF) experiments at CERN n_TOF and complementary activation techniques. Substantial advances have been achieved for stable s-only and bottleneck isotopes, significantly improving constraints on s-process models. In parallel, the combination of high instantaneous neutron fluxes and advanced detector systems has facilitated first-time neutron capture measurements on several radioactive branching-point nuclei. Feasibility studies, however, reveal current limitations related to sample availability, background conditions, and restricted energy coverage. In this context, the complementarity between TOF and activation emerges as a central strategy. Future developments, including high-flux facilities and novel inverse kinematics experiments in ion storage rings, are expected to extend the boundaries of neutron capture measurements, overcoming current limitations and helping unlock new frontiers in our understanding of stellar nucleosynthesis. Full article

(1) Background: For the study of highly magnetized neutron stars observed as magnetars and to quantify the effect of this intense magnetic field on the star’s structure and shape, which can be particularly relevant for the study of the emission of continuous gravitational waves, both numerical and perturbative approaches have been developed. (2) Methods: We compare these two approaches in General Relativity with the limitation to the case where the magnetic field has a purely poloidal structure. The perturbative one assumes that the deformation induced by the magnetic field is small and that this field arises only from dipole currents. The fully numerical one is based on the lorene library. (3) Results: We used both approaches to compute the magnetic-field distribution and the deformation of the star, varying the value of the magnetic field at the pole, the compactness of the star and its equation of state. (4) Conclusions: Whereas the perturbative approach breaks down for very high polar magnetic-field values (typically above a few times ${10}^{16}$ G), it achieves very good results for observed values, even in magnetars. On the contrary, the numerical code exhibits resolution problems for relatively low magnetic-field values (typically ${10}^{10}$ G), which translates into imprecise computation of the star’s deformation and mass quadrupole moment. Full article

We study the effect of trapped neutrinos on the properties of the deconfinement phase transition from hot $\beta$-equilibrated, electrically neutral hadronic matter to quark matter. To describe the thermodynamic properties of hot hadronic matter, an extended relativistic mean field (RMF) theory is used, which also incorporates the isovector–Lorentz-scalar $\delta$-meson effective field. The three-flavor quark phase is described within the framework of the local Nambu–Jona-Lasinio (NJL) model. It was assumed that the surface tension at the quark-hadron interface is so strong that the phase transition occurs according to Maxwell’s construction. The thermodynamic properties of the quark and hadronic phases were calculated for both neutrino-trapped and neutrino-transparent regimes at various temperatures ranging from 0 to 100 MeV and baryon number densities from 0 to 1.8 ${\mathrm{fm}}^{-3}$. The impact of trapped neutrinos on the thermodynamic properties of the coexistence state has been investigated. It has been demonstrated that the baryon chemical potential in the coexistence state decreases as temperature increases. The critical endpoint parameters in the $T-n_B$ plane of the phase diagram were obtained for the case of trapped neutrinos (74 MeV; 0.269 ${\mathrm{fm}}^{-3}$) and for the case of the absence of neutrinos (75.6 MeV; 0.255 ${\mathrm{fm}}^{-3}$). Full article

We discuss existing theoretical expectations and propose new mechanisms of electromagnetic radiation before, during, and after the merger of relativistic objects involving neutron stars. Furthermore, theoretical expectations for electromagnetic phenomena during a potential neutron star–black hole merger are discussed, using the reported transients AT2025bbo and FRB 20250206A as illustrative motivation. Full article

This review traces how our understanding of low- and intermediate-mass stars (hereafter LMS and IMS, respectively) evolved in time, in parallel with our knowledge of slow neutron-capture phenomena (the s-process). I shall focus in particular on the main component of this nucleosynthesis phenomenon, occurring in the above-mentioned stars close to the end of their lifetimes. They start ascending the Asymptotic Giant Branch (AGB), where both H- and He-shells exist, burning alternatively during the phases most relevant to our discussion: the so-called thermal pulses (hence, the name of TP-AGB stages for the final evolutionary period of these stars). I shall outline how such final stages were discovered to be a crucial source for neutron captures. Finally, I will briefly discuss what observational constraints and nuclear measurements have taught us about the status of our theoretical models in this field of nuclear and stellar physics. Full article

Binary neutron star mergers and proto-neutron stars provide unique environments where dense matter is hot, lepton-rich, and potentially undergoes a transition from hadronic to deconfined quark matter. We investigate the thermodynamics and stellar properties of hybrid matter under such conditions. The hadronic phase is described within a covariant density functional framework, while the quark phase is modeled using a Nambu–Jona-Lasinio (NJL) model that includes repulsive vector interactions, the axial $U_A\left(1\right)$-breaking ’t Hooft determinant interaction, and two-flavor color-superconducting (2SC) pairing. The phase transition between hadronic and quark matter is constructed using a mixed-phase prescription that enforces baryon and lepton number conservation, allowing us to follow thermodynamic trajectories at fixed entropy per baryon and a fixed lepton fraction. We analyze the phase structure of dense matter at a finite temperature and study the composition of the hadronic, mixed, and quark phases in both neutrino-trapped and neutrino-free regimes. Our results show that neutrino trapping significantly modifies the particle composition and shifts the onset of deconfinement to higher densities. The mixed phase exhibits a density-dependent pressure due to the presence of multiple conserved charges. Using the resulting equations of state, we compute static stellar configurations and examine the influence of the temperature and lepton content on the mass–radius relation in hybrid stars. Hot, neutrino-rich configurations are found to have larger radii and slightly higher maximum masses than their cold counterparts. As the star cools and deleptonizes, its radius contracts at an approximately constant baryonic mass, potentially triggering changes in the internal phase structure. These results highlight the roles of color superconductivity, lepton trapping, and thermal effects in shaping the structure and evolution of hybrid stars in transient astrophysical environments. Full article

Matter at ultra-high densities finds a physical realization inside neutron stars. It is generally acknowledged that huge magnetic fields are present in these stellar objects, and if not for the presence of the magnetic fields, neutron stars would be much more “silent” and practically invisible. However, a series of questions still remain concerning the role of magnetic fields in the neutron star structure and internal dynamics. We present an overview of these topics, pointing out the importance of a new set of observations (glitches and related events, and precession) and old questions that must be accommodated by a more complete theory of neutron star physics. Full article