Effective contact pairing forces from realistic calculations in infinite homogeneous nuclear matter

Non-empirical effective contact pairing forces to be used in self-consistent mean-field calculations are presented. These pairing forces, constructed so as to reproduce exactly any given microscopic pairing gaps in infinite homogeneous nuclear matter for any isospin asymmetry, are given in analytical form. As a by-product, this work provides an analytical solution of the BCS gap equations which could be applied to describe various many-body systems.Comment: 6 pages, 5 figures ; accepted for publication in Physical Review

Neutron star crust beyond the Wigner-Seitz approximation

For more than three decades, the inner crust of neutron stars, formed of a solid lattice of nuclear clusters coexisting with a gas of electrons and neutrons, has been traditionally studied in the Wigner-Seitz approximation. The validity of this approximation is discussed in the general framework of the band theory of solids, which has been recently applied to the nuclear context. Using this novel approach, it is shown that the unbound neutrons move in the crust as if their mass was increased.Comment: 8 pages, 2 figures. Proceedings of the International Symposium on Exotic States of Nuclear Matter, Catania (Italy), June 11-15, 200

Constraint on the internal structure of a neutron star from Vela pulsar glitches

Pulsars are spinning extremely rapidly with periods as short as about $1.4$ milliseconds and delays of a few milliseconds per year at most, thus providing the most accurate clocks in the Universe. Nevertheless, sudden spin ups have been detected in some pulsars like the emblematic Vela pulsar. These abrupt changes in the pulsar's rotation period have long been thought to be the manifestation of a neutron superfluid permeating the inner crust of neutron stars. However, the neutron superfluid has been recently found to be so strongly coupled to the crust that it does not carry enough angular momentum to explain the Vela data. We explore the extent to which pulsar-timing observations can be reconciled with the standard glitch theory considering the lack of knowledge of the dense-matter equation of state.Comment: Proceedings of the conference "The Modern Physics of Compact Stars 2015" held in Erevan, Armenia, from 30 September 2015 to 3 October 2015. To appear in Proceedings of Scienc

Self-interaction errors in nuclear energy density functionals

When applied to a single nucleon, nuclear energy density functionals may yield a non-vanishing internal energy thus implying that the nucleon is interacting with itself. It is shown how to avoid this unphysical feature for semi-local phenomenological functionals containing all possible bilinear combinations of local densities and currents up to second order in the derivatives. The method outlined in this Rapid Communication could be easily extended to functionals containing higher order terms, and could serve as a guide for constraining the time-odd part of the functional

Superfluidity and entrainment in neutron-star crusts

Despite the absence of viscous drag, the neutron superfluid permeating the inner crust of a neutron star can still be strongly coupled to nuclei due to non-dissipative entrainment effects. Neutron superfluidity and entrainment have been systematically studied in all regions of the inner crust of a cold non-accreting neutron star in the framework of the band theory of solids. It is shown that in the intermediate layers of the inner crust a large fraction of "free" neutrons are actually entrained by the crust. The results suggest that a revision of the interpretation of many observable astrophysical phenomena might be necessary.Comment: 4 pages, to appear in the proceedings of the ERPM conference, Zielona Gora, Poland, April 201

Pairing: from atomic nuclei to neutron-star crusts

Nuclear pairing is studied both in atomic nuclei and in neutron-star crusts in the unified framework of the energy-density functional theory using generalized Skyrme functionals complemented with a local pairing functional obtained from many-body calculations in homogeneous nuclear matter using realistic forces.Comment: 16 pages, 3 figures. Contribution for the book "50 years of nuclear BCS", edited by R.A. Broglia and V. Zelevinsk

Superfluid dynamics in neutron star crusts

A simple description of superfluid hydrodynamics in the inner crust of a neutron star is given. Particular attention is paid to the effect of the lattice of nuclei on the properties of the superfluid neutrons, and the effects of entrainment, the fact that some fraction of the neutrons are locked to the motion of the protons in nuclei

Global numerical simulations of the rise of vortex-mediated pulsar glitches in full general relativity

In this paper, we study in detail the role of general relativity on the global dynamics of giant pulsar glitches as exemplified by Vela. For this purpose, we carry out numerical simulations of the spin up triggered by the sudden unpinning of superfluid vortices. In particular, we compute the exchange of angular momentum between the core neutron superfluid and the rest of the star within a two-fluid model including both (non-dissipative) entrainment effects and (dissipative) mutual friction forces. Our simulations are based on a quasi-stationary approach using realistic equations of state (EoSs). We show that the evolution of the angular velocities of both fluids can be accurately described by an exponential law. The associated characteristic rise time $\tau_{\text{r}}$, which can be precisely computed from stationary configurations only, has a form similar to that obtained in the Newtonian limit. However, general relativity changes the structure of the star and leads to additional couplings between the fluids due to frame-dragging effects. As a consequence, general relativity can have a large impact on the actual value of $\tau_{\text{r}}$: the errors incurred by using Newtonian gravity are thus found to be as large as $\sim 40 \%$ for the models considered. Values of the rise time are calculated for Vela and compared with current observational limits. Finally, we study the amount of gravitational waves emitted during a glitch. Simple expressions are obtained for the corresponding characteristic amplitudes and frequencies. The detectability of glitches through gravitational wave observatories is briefly discussed.Comment: 19 pages, 12 figures, minor changes to match version to be published in MNRA

Neutron drip transition in accreting and nonaccreting neutron star crusts

The neutron-drip transition in the dense matter constituting the interior of neutron stars generally refers to the appearance of unbound neutrons as the matter density reaches some threshold density $\rho_\textrm{drip}$. This transition has been mainly studied under the cold catalyzed matter hypothesis. However, this assumption is unrealistic for accreting neutron stars. After examining the physical processes that are thought to be allowed in both accreting and nonaccreting neutron stars, suitable conditions for the onset of neutron drip are derived and general analytical expressions for the neutron drip density and pressure are obtained. Moreover, we show that the neutron-drip transition occurs at lower density and pressure than those predicted within the mean-nucleus approximation. This transition is studied numerically for various initial composition of the ashes from X-ray bursts and superbursts using microscopic nuclear mass models.Comment: 24 pages, accepted for publication in Physical Review