Prospects of detecting baryon and quark superfluidity from cooling neutron stars

Baryon and quark superfluidity in the cooling of neutron stars are investigated. Future observations will allow us to constrain combinations of the neutron or Lambda-hyperon pairing gaps and the star's mass. However, in a hybrid star with a mixed phase of hadrons and quarks, quark gaps larger than a few tenths of an MeV render quark matter virtually invisible for cooling. If the quark gap is smaller, quark superfluidity could be important, but its effects will be nearly impossible to distinguish from those of other baryonic constituents.     

Neutron superfluidity and unusual nuclear shapes in neutron stars crusts

We calculate in the frame-work of a semiclassical model the neutron superfluidity in the crust of neutron stars, accounting for the presence of unusual nuclear shape. We have found a marked spatial anisotropy in the calculated pairing energy gap.     

Thermal properties and detectability of neutron stars - I cooling and heating of neutron stars

In this report, we first review earlier and recent developments in some of thermodynamic problems of neutron stars, especially those involving cooling mechanisms and theoretical predictions of surface temperatures of neutron stars. Emphasis is placed particularly on: the effect of equations of state and hence that of nuclear and strong interactions; the effect of better treatment of various neutrino cooling mechanisms, especially those involving pion condensates; and implication of these better and more detailed theoretical estimates on the prospect of directly observing thermal radiation from the surface of neutron stars. In connection with the last problem, we briefly review recent developments on the observational side — the HEAO-B and other programs already existing or expected to be planned for near future, which are directly related to the above problem. In connection with the possibilities of observing older neutron stars we briefly summarise various heating mechanisms.From these studies, we see that exciting possibilities exist through the HEAO-B and some other programs which may be realised in the 1980's, that we may observe radiation directly from neutron star surfaces if they are ? (3?5) × 10⁵°K. If such radiation is detected, the observed surface temperatures and further spectral studies may give invaluable insight into various important problems, such as magnetic properties of dense matter, equations of state, pion condensates, and other fundamental problems in nuclear, particle and high energy physics. If the surface temperatures of younger members of these stars (? 10⁴ years) are observationally found to be less than ≈ (5?10) × 10⁵°K (depending on the individual objects), we note that at the moment only pion coolings are consistent with observations, and the outcome may be equally far reaching. Among various observed neutron stars (pulsars) and neutron star candidates (e.g. supernova remnants), the Vela pulsar may prove to be the most rewarding one. If regular pulsar-like periodicities are discovered in radiations from any of supernova remnants, we can assume the presence of neutron stars in these objects. In that case, some supernova remnants, such as SN 1006, may also turn out to be promising. If we defect surface radiations from older pulsars (? 10⁵ years), that may support some of heating theories. At the end, we point out that there may be many point sources of very soft weak thermal X-rays across the sky (as old neutron stars accrete interstellar matter) and some of the closest ones may be detectable through the HEAO-B and similar devices.     

Thermal conduction across neutron star crusts and cooling of young neutron stars

We calculate thermal conduction times in the crust and core of a neutron star and find that for certain neutron star models the surface remains thermally isolated from the core at initial times. The surface temperature of a few hundred year old neutron star in these models can be insensitive to the presence of a pion-condensed core and might well be within the limits of observability of the HEAO satellite which can help to distinguish between different neutron star models.     

Effect of superfluidity on the structure of the inner crust of neutron stars

A self-consistent, completely quantum calculation of the structure of the inner crust of neutron stars is carried out in the Wigner-Seitz approximation with a realistic phenomenological nuclear energy functional, where pair correlations of neutrons and protons are included in the explicit form. It has been shown that the superfluidity of neutrons and protons affects the structure of the ground state of the crust.     

Rapid cooling of the neutron star in Cassiopeia A triggered by neutron superfluidity in dense matter

We propose that the observed cooling of the neutron star in Cassiopeia A is due to enhanced neutrino emission from the recent onset of the breaking and formation of neutron Cooper pairs in the (3)P(2) channel. We find that the critical temperature for this superfluid transition is ?0.5×10(9) K. The observed rapidity of the cooling implies that protons were already in a superconducting state with a larger critical temperature. This is the first direct evidence that superfluidity and superconductivity occur at supranuclear densities within neutron stars. Our prediction that this cooling will continue for several decades at the present rate can be tested by continuous monitoring of this neutron star.     

3PF2 neutron superfluidity in neutron stars and three-body force effect

We investigate the ^3PF2 neutron superfluidity in H-stable neutron star matter and neutron stars by using the BCS theory and the Brueckner-Hartree-Fock approach. We adopt the Argonne V18 potential supplemented with a microscopic three-body force as the realistic nucleon-nucleon interaction. We have concentrated on studying the threebody force effect on the ^3PF2 neutron pairing gap. It is found that the three-body force effect is to enhance remarkably the ^3PF2 neutron superfluidity in neutron star matter and neutron stars.     

Contribution of the massive photon decay channel to neutrino cooling of neutron stars

We consider massive photon decay reactions via intermediate states of electron-electron-holes and proton-proton-holes into neutrino-antineutrino pairs in the course of neutron star cooling. These reactions may become operative in hot neutron stars in the region of proton pairing where the photon due to the Higgs-Meissner effect acquires an effective mass m _γ that is small compared to the corresponding plasma frequency. The contribution of these reactions to neutrino emissivity is calculated; it varies with the temperature and the photon mass as T ^3/2 m _γ ^7/2 exp(−m _γ /T) for T<m ^γ . Estimates show that these processes appear as extra efficient cooling channels of neutron stars at temperatures T≅10⁹–10¹⁰ K. Zh. éksp. Teor. Fiz. 114, 385–397 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Abnormal neutron stars

Quasi-classical field theory is used to predict abnormal behaviour of neutron matter in a model with neutrons, scalar mesons and vector mesons. We predict two stable families of neutron stars, a conventional one and an unconventional one with surface density in excess of 10¹⁵g cm^?3, roughly.     

Effect of magnetic field decay on the chemical heating of cooling neutron stars

The effect of magnetic field decay on the chemical heating and thermal evolution of neutron stars is discussed in this paper. Our main goal is to study how the chemical heating mechanism and thermal evolution are changed by the field decay and how the magnetic field decay is modified by the thermal evolution. We compare stars cooling with chemical heating with one without chemical heating and find that the decay of the magnetic field is delayed significantly by the chemical heating. We find that the effect of chemical heating has been suppressed through the decaying magnetic field by the spin-down of the stars at a later stage. Compared with typical chemical heating, we find the decay of the magnetic field can even cause the surface temperature to turn down at an older age. When we discuss the cooling of neutron stars, we should consider the coupling effect of the magnetic field and the rotational evolution of neutron stars on the heating mechanisms.     

Constraining hadronic superfluidity with neutron star precession

I show that the standard picture of the neutron star core containing coexisting neutron and proton superfluids, with the proton component forming a type II superconductor threaded by flux tubes, is inconsistent with observations of long-period (approximately 1 yr) precession in isolated pulsars. I conclude that either the two superfluids coexist nowhere in the stellar core, or the core is a type I superconductor rather than type II. Either possibility would have interesting implications for neutron star cooling and theories of spin jumps (glitches).     

X-rays from neutron stars

The basic theoretical ideas in the models of regularly pulsating X-ray sources are discussed, and put in relation to the observations. The topics covered include physics of the magnetosphere of an accreting neutron star, hydrodynamics of the accretion column, physical processes close to the surface of the neutron star such as proton-electron collisions, photon-electron interactions.     

Hyperons in neutron stars

Generalized beta equilibrium involving nucleons, hyperons and isobars is examined for neutron star matter. The hyperons produce a considerable softening of the equation of state. It is shown that the observed masses of neutron stars can be used to settle a recent controversy concerning the nuclear compressibility. Compressibilities less than 200 MeV are incompatible with observed masses.     

The Zoo of neutron stars

In these lecture notes, I briefly discuss the present day situation and new discoveries in astrophysics of neutron stars focusing on isolated objects. The latter include soft gamma repeaters, anomalous x-ray pulsars, central compact objects in supernova remnants, the Magnificent Seven, and rotating radio transients. In the last part of the paper, I describe available tests of cooling curves of neutron stars and discuss different additional constraints that can help to confront theoretical calculations of cooling with observational data. The text was submitted by the author in English.     

Superfluidity in neutron stars

Arguments are presented to show that the BCS theory of superfluidity in its original form may not be applicable to neutron star matter over a wide range of density.     

Structure and cooling of compact stars

We study the structure and evolution of neutron stars (NSs), the interiors of which are modeled using microscopic approaches and constrained by the condition that the equation of state (EoS) of matter extrapolated to high densities should not contradict known observational data from compact stars and experimental data from heavy-ion collisions (HIC). We use modern cooling simulations to extract distributions of NS masses required to reproduce those of the yet sparse data in the Temperature-Age (TA) plane. By comparing the results with the mass distribution for young, nearby NSs used in population synthesis, we can sharpen the NS cooling constraints. The text was submitted by the author in English.     

Internal structure of neutron stars

The equation of state and the structure and composition of neutron star matter are investigated in the density region 3.1 × 10¹¹−2 × 10¹⁵g/cm³. Below the density 3.1 × 10¹¹ g/cm³ the matter is a solid consisting of neutron-rich nuclei in a degenerate electron gas. At 3.1 × 10¹¹g/cm³ neutrons start to drip out of the nuclei; as the density increases, the lattice spacing continuously decreases while the geometrical size of the nuclei only slightly increases, until at about 15 × 10¹³g/cm³ the nuclei disappear by coalescing into a homogeneous liquid in an almost continuous phase transition. The maximum proton number per nucleus is 40, which is obtained between the densities 1−2.5 × 10¹³g/cm³; after that the proton number decreases until at the solid-to-liquid phase boundary it is about 20. In the liquid-core region, muons appear at the density 20.5 × 10¹³g/cm³.