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Realistic energies for vortex pinning in intermediate-density neutron star matter
No full textRealistic values for the pinning energies of vortices in the neutron superfluid expected in the inner crust of neutron stars are crucial for the theory of pulsar glitches. To this end, we supplement our consistent semi-classical model for the vortex-nucleus interaction with general properties of intermediate-d. fermion systems with large neg. scattering lengths, such as neutron matter at the densities corresponding to the inner crust. We also implement the redn. of pairing expected from the polarization of the strongly correlated neutron medium, although allowing for the present large theor. uncertainties on the amt. of redn. Finally, we better evaluate the kinetic contributions to pinning accounting also for the quantum structure of the vortex core, which sustains divergenceless flow. When compared to existing results, we find weaker values for the pinning energies per site (EP < 3.5 MeV); moreover, significant nuclear pinning occurs only in a restricted d. range (about 2 * 1013 .ltorsim. r .ltorsim. 5 * 1013 g/cm3 or 0.07 r0 .ltorsim. r .ltorsim. 0.2 r0, with r0 the nuclear satn. d.). The rest of the crust presents either interstitial pinning (r < 0.07 r0) or collective super-weak pinning (r &rt; 0.2 r0), both negligible at the macroscopic scale relevant to vortex unpinning and glitches
Fully consistent semi-classical treatment of vortex-nucleus interaction in rotating neutron stars
No full textWe present the first realistic and fully consistent model to study the vortex-nucleus interaction in the inner crust of a rotating neutron star, where a gas of unbound superfluid neutrons threaded by vortex lines coexists with a lattice of neutron-rich nuclei. Within the framework of the local density approximation, the model determines unambiguously the structure and radius of the vortex core along the crust, and takes into account all energy contributions to evaluate the vortex-nucleus configuration with lowest energy. The results show that, quite independent from the pairing interaction used, pinning of vortices on nuclei occurs only in the deepest layers of the crust and even there the average pinning forces are quite weak. If confirmed by complete quantum calculations, such a limited region of weak nuclear pinning may be relevant to the explanation of pulsar glitches
Axially symmetric equations for differential pulsar rotation with superfluid entrainment
Full text linkIn this article we present an analytical two-component model for pulsar rotational dynamics. Under the assumption of axial symmetry, implemented by a paraxial array of straight vortices that thread the entire neutron superfluid, we are able to project exactly the 3D hydrodynamical problem to a 1D cylindrical one. In the presence of density dependent entrainment the superfluid rotation is non-columnar: we circumvent this by using an auxiliary dynamical variable directly related to the areal density of vortices. The main
result is a system of differential equations that take consistently into account the stratified spherical structure of the star, the dynamical effects of non-uniform entrainment, the differential rotation of the superfluid component and its coupling to the normal crust. These equations represent a mathematical framework in which to test quantitatively the macroscopic consequences of the presence of a stable vortex array, a working hypothesis widely used in glitch models. Even without solving the equations explicitly, we
are able to draw some general quantitative conclusions; in particular, we show that the reservoir of angular momentum (corresponding to recent values of the pinning forces) is enough to reproduce the largest glitch observed in the Vela pulsar, provided its mass is not too large
Pinning and Binding Energies for Vortices in Neutron Stars : Comments on Recent Results
Full text linkAngular momentum transfer in Vela-like pulsar glitches
Full text linkThe angular momentum transfer associated with Vela-like glitches has never been calculated directly within a realistic scenario for the storage and release of superfluid vorticity; therefore, the explanation of giant glitches in terms of vortices has not yet been tested against observations. We present the first physically reasonable model, both at the microscopic and macroscopic level (spherical geometry, n = 1 polytropic density profile, density-dependent pinning forces compatible with vortex rigidity), to determine where in the star the vorticity is pinned, how much of it is pinned, and for how long. For standard neutron star parameters (M = 1.4 M ☉, Rs = 10 km, Hz s–1), we find that maximum pinning forces of order fm ≈ 1015 dyn cm–1 can accumulate ΔL gl ≈ 1040 erg s of superfluid angular momentum, and release it to the crust at intervals Δt gl ≈ 3 years. This estimate of ΔL gl is one order of magnitude smaller than that implied indirectly by current models for post-glitch recovery, where the core and inner-crust vortices are taken as physically disconnected; yet, it successfully yields the magnitudes observed in recent Vela glitches for both jump parameters, ΔΩgl and , provided one assumes that only a small fraction (<10%) of the total star vorticity is coupled to the crust on the short timescale of a glitch. This is reasonable in our approach, where no layer of normal matter exists between the core and the inner-crust, as indicated by existing microscopic calculation. The new scenario presented here is nonetheless compatible with current post-glitch models
Investigating superconductivity in neutron star interiors with glitch models
Full text linkThe high-density interior of a neutron star is expected to contain superconducting protons and superfluid neutrons. Theoretical estimates suggest that the protons will form a type II superconductor in which the stellar magnetic field is carried by flux tubes. The strong interaction between the flux tubes and the neutron rotational vortices could lead to strong "pinning," i.e., vortex motion could be impeded. This has important implications especially for pulsar glitch models as it would lead to a large part of the vorticity of the star being decoupled from the "normal" component to which the electromagnetic emission is locked. In this Letter, we explore the consequences of strong pinning in the core on the "snowplow" model for pulsar glitches, making use of realistic equations of state and relativistic background models for the neutron star. We find that, in general, a large fraction of the pinned vorticity in the core is not compatible with observations of giant glitches in the Vela pulsar. Thus, the conclusion is that either most of the core is in a type I superconducting state or the interaction between vortices and flux tubes is weaker than previously assumed
Superfluidity in the inner crust of neutron stars
No full textWe present a mean-field quantum calculation of the specific heat in the inner crust of neutron stars, taking into account the inhomogeneous character of the system, in which a lattice of neutron-rich nuclei coexists with a gas of unbound neutrons
Constraints on pulsar masses from the maximum observed glitch
No full textNeutron stars are unique cosmic laboratories in which fundamental physics can be probed in extreme conditions not accessible to terrestrial experiments. In particular, the precise timing of rotating magnetized neutron stars (pulsars) reveals sudden jumps in rotational frequency in these otherwise steadily spinning-down objects. These 'glitches' are thought to be due to the presence of a superfluid component in the star, and offer a unique glimpse into the interior physics of neutron stars. In this paper we propose an innovative method to constrain the mass of glitching pulsars, using observations of the maximum glitch observed in a star, together with state-of-the-art microphysical models of the pinning interaction between superfluid vortices and ions in the crust. We study the properties of a physically consistent angular momentum reservoir of pinned vorticity, and we find a general inverse relation between the size of the maximum glitch and the pulsar mass. We are then able to estimate the mass of all the observed glitchers that have displayed at least two large events. Our procedure will allow current and future observations of glitching pulsars to constrain not only the physics of glitch models but also the superfluid properties of dense hadronic matter in neutron star interiors
Mesoscopic pinning forces in neutron star crusts
Full text linkThe crust of a neutron star is thought to be comprised of a lattice of nuclei immersed in a sea of free electrons and neutrons. As the neutrons are superfluid their angular momentum is carried by an array of quantized vortices. These vortices can pin to the nuclear lattice and prevent the neutron superfluid from spinning down, allowing it to store angular momentum which can then be released catastrophically, giving rise to a pulsar glitch. A crucial ingredient for this model is the maximum pinning force that the lattice can exert on the vortices, as this allows us to estimate the angular momentum that can be exchanged during a glitch. In this paper we perform, for the first time, a detailed and quantitative calculation of the pinning force per unit length acting on a vortex immersed in the crust and resulting from the mesoscopic vortex-lattice interaction. We consider realistic vortex tensions, allow for displacement of the nuclei and average over all possible orientation of the crystal with respect to the vortex. We find that, as expected, the mesoscopic pinning force becomes weaker for longer vortices and is generally much smaller than previous estimates, based on vortices aligned with the crystal. Nevertheless the forces we obtain still have maximum values of order fpin ≈ 1015 dyn/cm, which would still allow for enough angular momentum to be stored in the crust to explain large Vela glitches, if part of the star is decoupled during the event
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