Heating in Magnetar Crusts from Electron Captures

The persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. The compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of light elements that may have been accreted onto the surface from the fallback of supernova debris, from a disk or from the interstellar medium. This scenario bears some resemblance to deep crustal heating in accreting neutron stars, although the matter composition and the thermodynamic conditions are very different. The maximum possible amount of heat that can be released by each reaction and their locations are determined analytically taking into account the Landau–Rabi quantization of electron motion. Numerical results are also presented using experimental, as well as theoretical nuclear data. Whereas the heat deposited is mainly determined by atomic masses, the locations of the sources are found to be very sensitive to the magnetic field strength, thus providing a new way of probing the internal magnetic field of magnetars. Most sources are found to be concentrated at densities 1010–1011 g cm−3 with heat power W∞∼1035–1036 erg/s, as found empirically by comparing cooling simulations with observed thermal luminosity. The change of magnetic field required to trigger the reactions is shown to be consistent with the age of known magnetars. This suggests that electron captures and pycnonuclear fusion reactions may be a viable heating mechanism in magnetars. The present results provide consistent microscopic inputs for neutron star cooling simulations, based on the same model as that underlying the Brussels-Montreal unified equations of state.

A Multifluid Perspective on Multimessenger Modeling

This brief review introduces the notion of a relativistic multifluid system—a multi-component system with identifiable relative flows—and outlines a set of models for scenarios relevant for different astronomical observation channels. The specific problems used to illustrate the key principles include superfluid hydrodynamics (with relevance for radio and x-ray pulsar timing and gravitational-wave searches), heat flow (connecting to the problem of neutron star cooling and associated x-ray observations) and the coupling between matter and electromagnetism (linking to explosive phenomena like gamma-ray bursts and more subtle issues like the long-term evolution of a neutron star's magnetic field). We also comment on the coupling between matter and radiation, for which the multifluid approach would seem less appropriate. The main motivation of the survey is to illustrate less familiar aspects that come into play in multifluid problems, establish the relevant “language” and provide a platform for more detailed work on these issues.

Developing an end-to-end simulation framework of supernova neutrino detection

Massive stars can explode as supernovae at the end of their life cycle, releasing neutrinos whose total energy reaches 1053erg. Moreover, neutrinos play key roles in supernovae, heating and reviving the shock wave as well as cooling the resulting proto-neutron star. Therefore, neutrino detectors are waiting to observe the next galactic supernova and several theoretical simulations of supernova neutrinos are underway. While these simulation concentrate mainly on only the first one second after the supernova bounce, the only observation of a supernova with neutrinos, SN 1987A, revealed that neutrino emission lasts for more than 10 seconds. For this reason, long-time simulation and analysis tools are needed to compare theories with the next observation. Our study is to develop an integrated supernova analysis framework to prepare an analysis pipeline for treating galactic supernovae observations in the near future. This framework deals with the core-collapse, bounce and proto-neutron star cooling processes, as well as with neutrino detection on earth in a consistent manner. We have developed a new long-time supernova simulation in one dimension that explodes successfully and computes the neutrino emission for up to 20 seconds. Using this model we estimate the resulting neutrino signal in the Super-Kamiokande detector to be about 1,800 events for an explosion at 10 kpc and discuss its implications in this paper. We compare this result with the SN 1987A observation to test its reliability.

Neutron Star Cooling Within the Equation of State With Induced Surface Tension

We study the thermal evolution of neutron stars described within the equation of state with induced surface tension (IST) that reproduces properties of normal nuclear matter, fulfills the proton flow constraint, provides a high-quality description of hadron multiplicities created during the nuclear-nuclear collision experiments, and it is equally compatible with the constraints from astrophysical observations and the GW170817 event. The model features strong direct Urca processes for the stars above 1.91M⊙. The IST equation of state shows very good agreement with the available cooling data, even without introducing nuclear pairing. We also analysed the effect of the singlet proton/neutron and triplet neutron pairing on the cooling of neutron stars of different mass. We show that the description of the compact object in the center of the Cassiopeia A does not necessarily require an inclusion of neutron superfluidity and/or proton superconductivity. Our results indicate that data of Cassiopeia A can be adequately well reproduced by a 1.66M⊙ star with an atmosphere of light elements. Moreover, the IST EoS reproduces each of the observational datasets for the surface temperature of Cassiopeia A either by a rapidly cooling ∼1.955M⊙ star with paired and unpaired matter or by a 1.91M⊙ star with the inclusion of neutron and proton pairings in the singlet channel.

Nuclear Pairing Gaps and Neutron Star Cooling

We study the cooling of isolated neutron stars with particular regard to the importance of nuclear pairing gaps. A microscopic nuclear equation of state derived in the Brueckner-Hartree-Fock approach is used together with compatible neutron and proton pairing gaps. We then study the effect of modifying the gaps on the final deduced neutron star mass distributions. We find that a consistent description of all current cooling data can be achieved and a reasonable neutron star mass distribution can be predicted employing the (slightly reduced by about 40%) proton 1S0 Bardeen-Cooper-Schrieffer (BCS) gaps and no neutron 3P2 pairing.

Neutron star equations of state and their applications

This paper reviews the properties of neutron stars based on the recent multi-messenger observations including electromagnetic waves from the low-mass X-ray binaries and gravitational waves from the merger of neutron star binaries. Based on these observations, we investigate theoretical models for dense nuclear matter and discuss their implications to the neutron star observations such as mass, radius, cooling, and tidal deformability. We also discuss the uncertainties in the neutron star cooling, neutron star properties with Bayesian approaches, and an expansion scheme applied to the nuclear energy density functional theory.

Thermal luminosities of cooling neutron stars

ABSTRACT Ages and thermal luminosities of neutron stars, inferred from observations, can be interpreted with the aid of the neutron star cooling theory to gain information on the properties of superdense matter in neutron-star interiors. We present a survey of estimated ages, surface temperatures, and thermal luminosities of middle-aged neutron stars with relatively weak or moderately strong magnetic fields, which can be useful for these purposes. The catalogue includes results selected from the literature, supplemented with new results of spectral analysis of a few cooling neutron stars. The data are compared with the theory. We show that overall agreement of theoretical cooling curves with observations improves substantially for models where neutron superfluidity in stellar core is weak.

Cooling of hybrid neutron stars with microscopic equations of state

ABSTRACT We model the cooling of hybrid neutron stars combining a microscopic nuclear equation of state in the Brueckner–Hartree–Fock approach with different quark models. We then analyse the neutron star cooling curves predicted by the different models and single out the preferred ones. We find that the possibility of neutron p-wave pairing can be excluded in our scenario.