Neutrino Emission Processes Research Articles

Astroparticles from X-Ray Binary Coronae

The recent observation of high-energy neutrinos from the Galactic plane implies an abundant population of hadronic cosmic-ray sources in the Milky Way. We explore the role of the coronae of accreting stellar-mass black holes as such astroparticle emitters. We show that the particle acceleration and interaction timescales in the coronal region are tied to the compactness of the X-ray source. Thus, neutrino emission processes may similarly happen in the cores of active galactic nuclei and black hole X-ray binaries (XRBs), despite their drastically different masses and physical sizes. We apply the model to the well-measured XRB Cygnus X-1 and find that the cascaded gamma rays accompanying the neutrino emission naturally explain the GeV emission that only presents during the source’s hard state, while the state-averaged gamma-ray emission explains the LHAASO observation above 20 TeV. We show that XRB coronae could contribute significantly to the Galactic cosmic-ray and Galactic plane neutrino fluxes. Our model predicts variable high-energy neutrino emission from bright Galactic XRBs that may be observed by IceCube and future neutrino observatories.

Baryon–Baryon interaction effects on neutrino emission from direct URCA processes of baryons and modified URCA processes of nucleons in neutron star matter

In the framework of the relativistic mean field theory, the neutrino emission from the baryonic direct URCA processes and nucleonic modified URCA processes in neutron star (NS) matter is investigated. We discuss particularly the influence of the extension of the SU(6) spin-flavor symmetry to the SU(3) flavor symmetry on the sizes of the neutrino energy losses, baryonic density and mass ranges of the above neutrino emission processes. In the nucleonic direct URCA processes, the neutrino energy losses in NS matter consisted of npe[Formula: see text] are observably higher than these in NS matter involving hyperons. For NS matter involving hyperons, the neutrino energy losses with the SU(3) flavor symmetry are slightly lower than those with the SU(6) spin-flavor symmetry. In the hyperonic direct URCA processes, the neutrino energy losses for the reactions of [Formula: see text] and [Formula: see text] with the SU(3) flavor symmetry are obviously smaller than these with the SU(6) spin-flavor symmetry. However, the neutrino energy loss of the reaction of [Formula: see text] with the SU(3) flavor symmetry is observably higher than that with the SU(6) spin-flavor symmetry. Moreover, the threshold densities of processes for [Formula: see text] with the SU(3) flavor and SU(6) spin-flavor symmetries are 0.868[Formula: see text]fm[Formula: see text] and 0.806[Formula: see text]fm[Formula: see text] (corresponding to 2.118[Formula: see text][Formula: see text] and 1.838[Formula: see text][Formula: see text]), respectively, which mean that it only happens in the interior of the massive NSs. For the neutron and proton branch modified URCA processes, the neutrino emission rates with the SU(3) flavor symmetry are also less than those with the SU(6) spin-flavor symmetry.

Impact of a Magnetic Field on Neutrino–Matter Interactions in Core-collapse Supernovae

Abstract We explore the impact of a magnetic field on neutrino–matter interactions in core-collapse supernovae. We first derive the modified source terms for neutrino–nucleon scattering and neutrino absorption and emission processes in the moment formalism. Then, we perform full relativistic, three-dimensional, magnetorotational core-collapse supernova simulations of a star with spectral neutrino transport. Our simulations self-consistently treat the parity-violation effects of weak interaction in the presence of an external magnetic field. The result shows significant global asymmetry, mostly confined in the proto-neutron star, clearly reflecting the magnetic field structure. The asymmetric property arises from two factors: the angle between the neutrino flux and magnetic field, and the term that is parallel to the magnetic field and is also proportional to the deviation of the distribution function of neutrinos from thermal equilibrium. The typical correction value amounts to ∼1% relative to the total neutrino–matter interaction rate for the magnetic field strength of G. Although these asymmetric properties do not immediately affect the explosion dynamics, our results imply that they would be significant once the neutrinos diffuse out of the proto-neutron- star core carrying those asymmetries away. We also show that, during our simulation time of ∼370 ms after bounce, our results indicate that the correction value due to the modified inelastic scattering process dominates over that of the modified neutrino absorption and emission process.

Long-term temperature evolution of neutron stars undergoing episodic accretion outbursts

Context. Transient neutron star low-mass X-ray binaries undergo episodes of accretion, alternated with quiescent periods. During an accretion outburst, the neutron star heats up due to exothermic accretion-induced processes taking place in the crust. Besides the long-known deep crustal heating of nuclear origin, a likely non-nuclear source of heat, dubbed “shallow heating”, is present at lower densities. Most of the accretion-induced heat slowly diffuses into the core on a timescale of years. Over many outburst cycles, a state of equilibrium is reached when the core temperature is high enough that the heating and cooling (photon and neutrino emission) processes are in balance.Aims. We investigate how stellar characteristics and outburst properties affect the long-term temperature evolution of a transiently accreting neutron star. For the first time the effects of crustal properties are considered, particularly that of shallow heating.Methods. Using our codeNSCool, we tracked the thermal evolution of a neutron star undergoing outbursts over a period of 105yr. The outburst sequence is based on the regular outbursts observed from the neutron star transient Aql X-1. For each model we calculated the timescale over which equilibrium was reached and we present these timescales along with the temperature and luminosity parameters of the equilibrium state.Results. We performed several simulations with scaled outburst accretion rates, to vary the amount of heating over the outburst cycles. The results of these models show that the equilibrium core temperature follows a logarithmic decay function with the equilibrium timescale. Secondly, we find that shallow heating significantly contributes to the equilibrium state. Increasing its strength raises the equilibrium core temperature. We find that if deep crustal heating is replaced by shallow heating alone, the core would still heat up, reaching only a 2% lower equilibrium core temperature. Deep crustal heating may therefore not be vital to the heating of the core. Additionally, shallow heating can increase the quiescent luminosity to values higher than previously expected. The thermal conductivity in the envelope and crust, including the potentially low-conductivity pasta layer at the bottom of the crust, is unable to significantly alter the long-term internal temperature evolution. Stellar compactness and nucleon pairing in the core change the specific heat and the total neutrino emission rate as a function of temperature, with the consequences for the properties of the equilibrium state depending on the exact details of the assumed pairing models. The presence of direct Urca emission leads to the lowest equilibrium core temperature and the shortest equilibrium timescale.

Time evolution of rotating and magnetized white dwarf stars

We investigate the evolution of isolated, zero and finite temperature, massive, uniformly rotating and highly magnetized white dwarf stars under angular momentum loss driven by magnetic dipole braking. We consider the structure and thermal evolution of white dwarf isothermal cores taking also into account the nuclear burning and neutrino emission processes. We estimate the white dwarf lifetime before it reaches the condition either for a type Ia supernova explosion or for the gravitational collapse to a neutron star. We study white dwarfs with surface magnetic fields from $10^6$ to $10^{9}$~G and masses from $1.39$ to $1.46~M_\odot$ and analyze the behavior of the white dwarf parameters such as moment of inertia, angular momentum, central temperature and magnetic field intensity as a function of lifetime. The magnetic field is involved only to slow down white dwarfs, without affecting their equation of state and structure. In addition, we compute the characteristic time of nuclear reactions and dynamical time scale. The astrophysical consequences of the results are discussed.

Search for Multimessenger Sources of Gravitational Waves and High-energy Neutrinos with Advanced LIGO during Its First Observing Run, ANTARES, and IceCube

Abstract Astrophysical sources of gravitational waves, such as binary neutron star and black hole mergers or core-collapse supernovae, can drive relativistic outflows, giving rise to non-thermal high-energy emission. High-energy neutrinos are signatures of such outflows. The detection of gravitational waves and high-energy neutrinos from common sources could help establish the connection between the dynamics of the progenitor and the properties of the outflow. We searched for associated emission of gravitational waves and high-energy neutrinos from astrophysical transients with minimal assumptions using data from Advanced LIGO from its first observing run O1, and data from the Antares and IceCube neutrino observatories from the same time period. We focused on candidate events whose astrophysical origins could not be determined from a single messenger. We found no significant coincident candidate, which we used to constrain the rate density of astrophysical sources dependent on their gravitational-wave and neutrino emission processes.

Spectra and rates of bremsstrahlung neutrino emission in stars

We calculate the energy-differential rate for neutrino emission from electron-nucleus bremsstrahlung in stellar interiors taking into account the effects of electron screening and ionic correlations. We compare the energy-differential and the net rates, as well as the average $\bar{\nu}_e$ and $\bar{\nu}_x$ $(x = {\mu}, {\tau})$ energies, for this process with those for $e^{\pm}$ pair annihilation, plasmon decay, and photo-neutrino emission over a wide range of temperature and density. We also compare our updated energy loss rates for the above thermal neutrino emission processes with the fitting formulas widely used in stellar evolution models and determine the temperature and density domain in which each process dominates. We discuss the implications of our results for detection of $\bar{\nu}_e$ from massive stars during their pre-supernova evolution and find that pair annihilation makes the predominant contribution to the signal from the thermal emission processes.

IMPACT OF NEUTRINO FLAVOR OSCILLATIONS ON THE NEUTRINO-DRIVEN WIND NUCLEOSYNTHESIS OF AN ELECTRON-CAPTURE SUPERNOVA

Neutrino oscillations, especially to light sterile states, can affect nucleosynthesis yields because of their possible feedback effect on the electron fraction (Ye). For the first time, we perform nucleosynthesis calculations for neutrino-driven wind trajectories from the neutrino-cooling phase of an 8.8 electron-capture supernova (SN), whose hydrodynamic evolution was computed in spherical symmetry with sophisticated neutrino transport and whose Ye evolution was post-processed by including neutrino oscillations between both active and active–sterile flavors. We also take into account the α-effect as well as weak magnetism and recoil corrections in the neutrino absorption and emission processes. We observe effects on the Ye evolution that depend in a subtle way on the relative radial positions of the sterile Mikheyev–Smirnov–Wolfenstein resonances, on collective flavor transformations, and on the formation of α particles. For the adopted SN progenitor, we find that neutrino oscillations, also to a sterile state with eV mass, do not significantly affect the element formation and in particular cannot make the post-explosion wind outflow neutron-rich enough to activate a strong r-process. Our conclusions become even more robust when, in order to mimic equation-of-state-dependent corrections due to nucleon potential effects in the dense-medium neutrino opacities, six cases with reduced Ye in the wind are considered. In these cases, despite the conversion of active neutrinos to sterile neutrinos, Ye increases or is not significantly lowered compared to the values obtained without oscillations and active flavor transformations. This is a consequence of a complicated interplay between sterile-neutrino production, neutrino–neutrino interactions, and α-effect.

The neutron star in Cassiopeia A: equation of state, superfluidity, and Joule heating

The thermomagnetic evolution of the young neutron star in Cassiopea A is studied by considering fast neutrino emission processes. In particular, we consider neutron star models obtained from the equation of state computed in the framework of the Brueckner-Bethe-Goldstone many-body theory and variational methods, and models obtained with the Akmal-Pandharipande-Ravenhall equation of state. It is shown that it is possible to explain a fast cooling regime as the one observed in the neutron star in Cassiopea A if the Joule heating produced by dissipation of the small-scale magnetic field in the crust is taken into account. We thus argue that it is difficult to put severe constraints on the superfluid gap if the Joule heating is considered.

MODELING MAGNETAR OUTBURSTS: FLUX ENHANCEMENTS AND THE CONNECTION WITH SHORT BURSTS AND GLITCHES

The availability of a large amount of observational data recently collected from magnetar outbursts is now calling for a complete theoretical study of outburst characteristics. In this letter (the first of a series dedicated to model magnetar outbursts), we tackle the long-standing open issue of whether or not short bursts and glitches are always connected to long-term radiative outbursts. We show that the recent detection of short bursts and glitches seemingly unconnected to outbursts is only misleading our understanding of these events. We show that, in the framework of the starquake model, neutrino emission processes in the magnetar crust limit the temperature, and therefore the luminosity. This natural limit to the maximum luminosity makes outbursts associated with bright persistent magnetars barely detectable. These events are simply seen as a small luminosity increase over the already bright quiescent state, followed by a fast return to quiescence. In particular, this is the case for 1RXS J1708-4009, 1E 1841-045, SGR 1806-20, and other bright persistent magnetars. On the other hand, a similar event (with the same energetics) in a fainter source will drive a more extreme luminosity variation and longer cooling time, as for sources such as XTE J1810-197, 1E 1547-5408 and SGR 1627-41. We conclude that the non-detection of large radiative outbursts in connection with glitches and bursts from bright persistent magnetars is not surprising per se, nor it needs of any revision on the glitches and burst mechanisms as explained by current theoretical models.

Neutrino emissivity of3P2-3F2superfluid cores in neutron stars

The influence of the admixture of the $^{3}F_{2}$ state onto collective spin oscillations and neutrino emission processes in the triplet superfluid neutron liquid is studied in the BCS approximation. The eigen mode of spin oscillations with $\omega\simeq\sqrt{58/35}\Delta$ is predicted to exist in the triplet superfluid neutron condensate besides the already known mode $\omega\simeq\Delta/\sqrt{5}$. Excitation of the high-frequency spin oscillations in the condensate occurs through the tensor interactions between quasiparticles. Neutrino energy losses through neutral weak currents are found to consist of three separate contributions caused by a recombination of broken Cooper pairs and by weak decays of the collective modes of spin oscillations. Neutrino decays of the low-frequency spin waves can play an important role in the cooling scenario of neutron stars. Weak decays of the high-frequency oscillations that occur only if the tensor forces are taken into account in the pairing interactions does not modify substantially the total energy losses. Simple expressions are suggested for the total neutrino emissivity.

The Impact of Magnetic Field on the Thermal Evolution of Neutron Stars

The impact of strong magnetic fields B>10e13 G on the thermal evolution of neutron stars is investigated, including crustal heating by magnetic field decay. For this purpose, we perform 2D cooling simulations with anisotropic thermal conductivity considering all relevant neutrino emission processes for realistic neutron stars. The standard cooling models of neutron stars are called into question by showing that the magnetic field has relevant (and in many cases dominant) effects on the thermal evolution. The presence of the magnetic field significantly affects the thermal surface distribution and the cooling history of these objects during both, the early neutrino cooling era and the late photon cooling era. The minimal cooling scenario is thus more complex than generally assumed. A consistent magneto-thermal evolution of magnetized neutron stars is needed to explain the observations.

Magnetohydrodynamics of Neutrino-Cooled Accretion Tori around a Rotating Black Hole in General Relativity

We present our first numerical results of axisymmetric magnetohydrodynamic simula- tions for neutrino-cooled accretion tori around rotating black holes in general relativity. We consider tori of mass ∼ 0.1-0.4Maround a black hole of mass M =4 Mand spin a =0 - 0.9M ; such systems are candidates for the central engines of gamma-ray bursts (GRBs) formed after the collapse of massive rotating stellar cores and the merger of a black hole and a neutron star. In this paper, we consider the short-term evolution of a torus for a duration of ≈ 60 ms, focusing on short-hard GRBs. Simulations were performed with a plausible microphysical equation of state that takes into account neutronization, the nuclear statistical equilibrium of a gas of free nucleons and α-particles, black body radiation, and a relativistic Fermi gas (neutrinos, electrons, and positrons). Neutrino-emission processes, such as e ± capture onto free nucleons, e ± pair annihilation, plasmon decay, and nucleon- nucleon bremsstrahlung are taken into account as cooling processes. Magnetic braking and the magnetorotational instability in the accretion tori play a role in angular momentum redistribution, which causes turbulent motion, resultant shock heating, and mass accretion onto the black hole. The mass accretion rate is found to be ˙ M∗ ∼ 1-10M� /s, and the shock heating increases the temperature to ∼ 10 11 K. This results in a maximum neutrino emission rate of Lν = several ×10 53 ergs/s and a conversion efficiency Lν / ˙ M∗c 2 on the order of a few percent for tori with mass Mt ≈ 0.1-0.4Mand for moderately high black hole spins. These results are similar to previous results in which the phenomenological α-viscosity prescription with the α-parameter of αv =0 .01-0.1 is used. It is also found that the neutrino luminosity can be enhanced by the black hole spin, in particular for large spins, i.e., a & 0.75M ;i f the accretion flow is optically thin with respect to neutrinos, the conversion efficiency may be & 10% for a & 0.9M. Angular momentum transport, and the resulting shock heating caused by magnetic stress induce time-varying neutrino luminosity, which is a favorable property for explaining the variability of the luminosity curve of GRBs.

Neutrino crown of a protoneutron star and analysis of its convective instability

A numerical solution to the problem of the structure of the neutrino crown of a protoneutron star that is formed upon an iron-star-core collapse, which is peculiar to all massive stars at the end of their thermonuclear evolution, is given. The structure of a neutrino crown, which is semitransparent to neutrino radiation from a spherical layer between the neutrinosphere and the front of the accretion shock wave, is determined by a set of nonlinear ordinary differential equations of spherically symmetric neutrino hydrodynamics with allowance for a complete set of beta processes in a Boltzmann free-nucleon gas and an ultrarelativistic Fermi-Dirac electron-positron gas that form neutrino-crown matter. The problem of consistently taking into account nonequilibrium neutrino-absorption and neutrino-emission processes and the problem of formulating boundary conditions for a neutrino crown were the main problems in constructing the numerical solution in question, which was obtained by means of a dedicated algorithm. The problem at hand features a number of parameters: the protoneutron-star mass, M0; the rate of accretion of the outer layers of the collapsing star being considered, ⊙M0; the effective temperature of the neutrinosphere and the effective neutrino chemical potential there, Tveff and ψveff, respectively; and, finally, the total neutrino emissivity of the neutrinosphere, \(L_{v\tilde v} \). Two of these parameters, M0 and \(L_{v\tilde v} \), are varied within broad intervals in accordance with the hydrodynamic theory of a collapse. On one hand, the numerical solutions constructed in the present study give an idea of the physical conditions in the immediate vicinity of a protoneutron star in the course of its continuing gravitational collapse; on the other hand, they make it possible to obtain exhaustive information about its convective instability, which is the most important property of a so-called soundless collapse—that is, a collapse not accompanied by an explosion of a supernova scale. The increment of the development of a convective instability is obtained at a linear stage, this giving sufficient grounds to introduce the hypothesis that the instability in question plays a key role in the origin of observed gamma-ray bursts. More precisely, these bursts may result from the development of the instability at the subsequent nonlinear stage, which has yet to be studied theoretically—in particular, on the basis of non-one-dimensional numerical models of neutrino hydrodynamics.

The neutron star soft X-ray transient 1H 1905+000 in quiescence

In this paper, we report on our analysis of a ∼25 ks. Chandra X-ray observation of the neutron star soft X-ray transient (SXT) 1H 1905+000 in quiescence. Furthermore, we discuss our findings of the analysis of optical photometric observations which we obtained using the Magellan telescope and photometric and spectroscopic observations which we obtained using the Very Large Telescope (VLT) at Paranal. The X-ray counterpart of 1H 1905+000 was not detected in our Chandra data, with a 95 per cent confidence limit to the source count rate of 1.2 × 10−4 counts s−1. For different spectral models this yields an upper limit on the luminosity of 1.8 × 1031 erg s−1 (for an upper limit on the distance of 10 kpc). This luminosity limit makes 1H 1905+000 the faintest neutron star SXT in quiescence observed to date. The neutron star luminosity is so low that it is similar to the lowest luminosities derived for black hole (BH) SXTs in quiescence. This low luminosity for a neutron star SXT challenges the hypothesis presented in the literature that BH SXTs in quiescence have lower luminosities than neutron star SXTs as a result of the presence of a BH event horizon. Furthermore, the limit on the neutron star luminosity obtained less than 20 yr after the outburst has ceased, constrains the thermal conductivity of the neutron star crust. Finally, the neutron star core must be so cold that unless the time-averaged mass accretion rate is lower than 2 × 10−12 M⊙ yr−1, core cooling has to proceed via enhanced neutrino emission processes. The time-averaged mass accretion rate can be derived from binary evolution models if the orbital period of the system is known. Our optical observations show that the optical counterpart discovered when the source was in outburst has faded. Near the outburst optical position we find two stars with a separation of 0.7 arcsec and I= 19.3 ± 0.1 and 21.3 ± 0.1. VLT optical spectroscopy revealed that the spectrum of the brighter of the two sources is a G5–7V star. However, the outburst astrometric position of the optical counterpart does not coincide with the position of the G5–7V star nor with that of the fainter star. We derive a limit on the absolute I-band magnitude of the quiescent counterpart of MI > 7.8 assuming the source is at 10 kpc. This is in line with 1H 1905+000 being an ultracompact X-ray binary, as has been proposed based on the low-outburst V-band absolute magnitude.

Dynamical Evolution of Neutrino‐cooled Accretion Disks: Detailed Microphysics, Lepton‐driven Convection, and Global Energetics

We present a detailed, two dimensional numerical study of the microphysical conditions and dynamical evolution of accretion disks around black holes when neutrino emission is the main source of cooling. Such structures are likely to form after the gravitational collapse of massive rotating stellar cores, or the coalescence of two compact objects in a binary (e.g., the Hulse--Taylor system). The physical composition is determined self consistently by considering two regimes: neutrino--opaque and neutrino--transparent, with a detailed equation of state which takes into account neutronization, nuclear statistical equilibrium of a gas of free nucleons and alpha particles, blackbody radiation and a relativistic Fermi gas of arbitrary degeneracy. Various neutrino emission processes are considered, with electron/positron capture onto free nucleons providing the dominant contribution to the cooling rate. We find that important temporal and spatial scales, related to the optically thin/optically thick transition are present in the disk, and manifest themselves clearly in the energy output in neutrinos. This transition produces an inversion of the lepton gradient in the innermost regions of the flow which drives convective motions, and affects the density and disk scale height radial profiles. The electron fraction remains low in the region close to the black hole, and if preserved in an outflow, could give rise to heavy element nucleosynthesis. Our specific initial conditions arise from the binary merger context, and so we explore the implications of our results for the production of gamma ray bursts.

Neutron star cooling

The impact of nuclear physics theories on cooling of isolated neutron stars is analyzed. Physical properties of neutron star matter important for cooling are reviewed such as composition, the equation of state, superfluidity of various baryon species, neutrino emission mechanisms. Theoretical results are compared with observations of thermal radiation from neutron stars. Current constraints on theoretical models of dense matter, derived from such a comparison, are formulated.

Neutron Star Cooling

▪ Abstract Observation of cooling neutron stars can potentially provide information about the states of matter at supernuclear densities. We review physical properties important for cooling such as neutrino emission processes and superfluidity in the stellar interior, surface envelopes of light elements owing to accretion of matter, and strong surface magnetic fields. The neutrino processes include the modified Urca process and the direct Urca process for nucleons and exotic states of matter, such as a pion condensate, kaon condensate, or quark matter. The dependence of theoretical cooling curves on physical input and observations of thermal radiation from isolated neutron stars are described. The comparison of observation and theory leads to a unified interpretation in terms of three characteristic types of neutron stars: high-mass stars, which cool primarily by some version of the direct Urca process; low-mass stars, which cool via slower processes; and medium-mass stars, which have an intermediate behavior. The related problem of thermal states of transiently accreting neutron stars with deep crustal burning of accreted matter is discussed in connection with observations of soft X-ray transients. Observations imply that some stars cool more rapidly than can be explained on the basis of nonsuperfluid neutron star models cooling via the modified Urca process, whereas other star cool less rapidly. We describe possible theoretical models that are consistent with observations.

Baryon Superfluidity and Neutrino Emissivity of Neutron Stars

For neutron stars with hyperon-mixed core, neutrino emissivity is studied under the equation of state, obtained by introducing repulsive three-body force universal for all baryons so as to assure the maximum mass compatible with the observation. By paying attention to the density-dependence of critical temperatures of baryon superfluids, which reflect the nature of baryon-baryon interaction and control neutron star cooling, we show what neutrino emission processes are efficient in the regions with and without hyperon mixing and remark its implications related to neutron star cooling.

Baryon superfluidity and neutrino emissivity of neutron stars

For neutron stars with hyperon-mixed cores, neutrino emissivity is studied using the properties of neutron star matter determined under the equation of state, which is obtained by introducing a repulsive three-body force universal for all the baryons so as to assure the maximum mass of neutron stars compatible with observations. The case without a meson condensate is treated. We choose the inputs provided by nuclear physics, with a reliable allowance. Paying attention to the density dependence of the critical temperatures of the baryon superfluids, which reflect the nature of the baryon-baryon interaction and control neutron star cooling, we show what neutrino emission processes are efficient in regions both with and without hyperon mixing. By comparing the calculated emissivities with respect to densities, we can conclude that at densities lower than about 4 times the nuclear density, the Cooper-pair process arising from the neutron 3 P2 superfluid dominates, while at higher densities the hyperon direct Urca process dominates. For the hyperon direct Urca process to be a candidate responsible for rapid cooling compatible with observations, a moderately large energy gap of the Λ-particle 1 S0 superfliud is required to suppress its large emissivity. The implications of these results are discussed in the relation to thermal evolution of neutron stars.