Modified Urca Process Research Articles

Emergence of Microphysical Bulk Viscosity in Binary Neutron Star Postmerger Dynamics

In nuclear matter in isolated neutron stars, the flavor content (e.g., proton fraction) is subject to weak interactions, establishing flavor (β-)equilibrium. However, there can be deviations from this equilibrium during the merger of two neutron stars. We study the resulting out-of-equilibrium dynamics during the collision by incorporating direct and modified Urca processes (in the neutrino-transparent regime) into general-relativistic hydrodynamics simulations with a simplified neutrino transport scheme. We demonstrate how weak-interaction-driven bulk viscosity in postmerger simulations can emerge and assess the bulk viscous dynamics of the resulting flow. We further place limits on the impact of the postmerger gravitational-wave strain. Our results show that weak-interaction-driven bulk viscosity can potentially lead to a phase shift of the postmerger gravitational-wave spectrum, although the effect is currently on the same level as the numerical errors of our simulation.

Evolution of Urca Pairs in the Crusts of Highly Magnetized Neutron Stars

We report on the effects of strong magnetic fields on neutrino emission in the modified Urca process. We show that the effect of Landau levels on the various Urca pairs affects the neutrino emission spectrum and leads to an angular asymmetry in the neutrino emission. For low magnetic fields, the Landau levels have almost no effect on the cooling. However, as the field strength increases, the electron chemical potential increases resulting in a lower density at which Urca pairs can exist. For intermediate field strength, there is an interesting interference between the Landau level distribution and the Fermi distribution. For high enough field strength, the entire electron energy spectrum is eventually confined to a single Landau level producing dramatic spikes in the emission spectrum.

Number spectra of nonequilibrium neutrinos from neutron stars: Impact of realistic nuclear interactions and free hyperons

The number spectrum of nonequilibrium neutrinos resulting from neutron star matter deviation from beta equilibrium is calculated. For this purpose, the chemical potential gap ($\ensuremath{\delta}\ensuremath{\mu}$) in a typical density change and in the high degeneracy regime (${k}_{B}T\ensuremath{\le}0.1\phantom{\rule{0.28em}{0ex}}\text{MeV}$) is estimated. This is done under the application of realistic descriptions of nuclear matter and the assumption of the presence of free hyperons (${\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}, {\mathrm{\ensuremath{\Lambda}}}^{0}$) in the neutron star core. Realistic nuclear interactions are taken into account by a microscopic many-body approach---the lowest order constrained variational (LOCV) method---for asymmetric nuclear matter. We find that addition of a three-body force interaction leads to a dramatic change in the trend of the symmetry energy at high densities and consequently the change in the sign of $\ensuremath{\delta}\ensuremath{\mu}$. This, as a result, changes the dominant flavor of the nonequilibrium neutrinos. By accounting for the hyperons' presence, we see that apart from the sign change of $\ensuremath{\delta}\ensuremath{\mu}$ its value can change noticeably. In hypernuclear matter, the effect of particle fractions is also very important. We have also investigated the effect of deviation from beta equilibrium on the dynamic part of the number spectra of nonequilibrium neutrinos produced in the neutron branch of the modified Urca process. It is shown that this part of the number spectra is not affected significantly by the nonequilibrium process and can be replaced by its equilibrium value.

Proto-neutron star evolution with improved charged-current neutrino–nucleon interactions

ABSTRACT We perform simulations of the Kelvin–Helmholtz cooling phase of proto-neutron stars with a new numerical code in spherical symmetry and using the quasi-static approximation. We use for the first time the full set of charged-current neutrino–nucleon reactions, including neutron decay and modified Urca processes, together with the energy-dependent numerical representation for the inclusion of nuclear correlations with random phase approximation. Moreover, convective motions are taken into account within the mixing length theory. As we vary the assumptions for computing neutrino–nucleon reaction rates, we show that the dominant effect on the cooling time-scale, neutrino signal, and composition of the neutrino-driven wind comes from the inclusion of convective motion. Computation of nuclear correlations within the random phase approximation, as compared to mean field approach, has a relatively small impact.

Neutrino and Antineutrino pair-Emission in Strong Magnetic Field in Relativistic Quantum Approach

We study the v\bar v\-pair emission from electrons and protons in a relativistic quantum approach. In this work we calculate the luminosity of the v\bar v\-pairs emitted from neutron-star-matter with a strong magnetic field, and find that this luminosity is much larger than that in the modified Urca process. The v\bar v\-pair emission processes in strong magnetic fields significantly contribute to the cooling of the magnetars.

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.

Fast neutrino cooling of nuclear pasta in neutron stars: Molecular dynamics simulations

The direct URCA process of rapid neutrino emission can occur in nonuniform nuclear pasta phases that are expected in the inner crust of neutron stars. Here, the periodic potential for a nucleon in nuclear pasta allows momentum conservation to be satisfied for direct URCA reactions. We improve on earlier work by modeling a rich variety of pasta phases (gnocchi, waffle, lasagna, and anti-spaghetti) with large-scale molecular dynamics simulations. We find that the neutrino luminosity due to direct URCA reactions in nuclear pasta can be 3 to 4 orders of magnitude larger than that from the modified URCA process in the NS core. Thus neutrino radiation from pasta could dominate radiation from the core and this could significantly impact the cooling of neutron stars

Formula omitted]-Pair synchrotron emission in neutron-star matter based on a relativistic quantum approach

We study the νν¯-pair synchrotron emission from electrons and protons in a relativistic quantum approach. This process occurs only in the presence of a strong magnetic field, and it is considered to be one of effective processes for neutron star cooling. In this work we calculate the luminosity of the νν¯-pairs emitted from neutron-star-matter with a magnetic field of about 1015 G. We find that the energy loss is much larger than that of the modified Urca process. The νν¯-pair emission processes in strong magnetic fields is expected to contribute significantly to the cooling of the magnetars.

Cooling of neutron stars in “nuclear medium cooling scenario” with stiff equation of state including hyperons

We demonstrate that the existing neutron-star cooling data can be appropriately described within “the nuclear medium cooling scenario” including hyperons under the assumption that different compact-star sources have different masses. We use a stiff equation of state of the relativistic mean-field model MKVORHϕ with hadron effective couplings and masses dependent on the scalar field. It fulfills a large number of experimental constraints on the equation of state of the nuclear matter including the 2M⊙ lower bound for the maximum compact-star mass and the constraint for the pressure from the heavy-ion particle flow. We select appropriate S01 proton and Λ hyperon pairing gap profiles from those exploited in the literature and allow for a variation of the effective pion gap controlling the efficiency of the medium modified Urca process. The P23 neutron pairing gap is assumed to be negligibly small in our scenario. The possibility of the pion, kaon and charged ρ-meson condensations is for simplicity suppressed. The resulting cooling curves prove to be sensitive to the value and the density dependence of the pp pairing gap and rather insensitive to the values of the S01 neutron pairing gaps.

Bulk viscosity in a neutron star mantle

We study the bulk viscosity in two (anti-spaghetti and Swiss cheese) phases of non- spherical nuclei in the mantle of an oscillating neutron star near the boundary with the stellar core. The bulk viscosity is produced by non-equilibrium Urca neutrino emis- sion processes. In the mantle, the direct Urca process may be open (Gusakov et al., 2004) if neutrons and protons move in a periodic potential created by a lattice of non- spherical nuclei (which allows the nucleons to have large quasi-momenta and satisfy direct Urca momentum-conservation). This bulk viscosity can dominate over that due to the modified Urca process in the outer stellar core and over the shear viscosity. The bulk viscosity depends strongly on temperature, oscillation frequency and nucleon superfluidity. The enhanced bulk viscosity in the mantle can control propagation and damping of neutron star oscillations.

In-medium enhancement of the modified Urca neutrino reaction rates

We calculate modified Urca neutrino emission rates in the dense nuclear matter in neutron star cores. We find that these rates are strongly enhanced in the beta-stable matter in regions of the core close to the direct Urca process threshold. This enhancement can be tracked to the use of the in-medium nucleon spectrum in the virtual nucleon propagator. We describe the in-medium nucleon scattering in the non-relativistic Bruckner–Hartree–Fock framework taking into account two-body as well as the effective three-body forces, although the proposed enhancement does not rely on a particular way of the nucleon interaction treatment. Finally we suggest a simple approximate expression for the emissivity of the neutron branch of the modified Urca process that can be used in the neutron stars cooling simulations with any nucleon equation of state of dense matter.

Role of nuclear correlations and kinematic effects on neutrino emission from the modified Urca processes

The neutrino emissivity from the proton branch of the modified Urca process is calculated. Angular integrations in momentum space are performed numerically without recourse to widely used, but unjustified in this specific case, approximations. To deal with nuclear correlation effects, the independent-pair approximation for nucleon quasiparticles is assumed. The realistic nuclear correlation effects are extracted from a modified extended version of the lowest-order constrained variational method in asymmetric nuclear matter in $\ensuremath{\beta}$ equilibrium with electrons and muons that allows for three-body nucleon interactions and fits all semiempirical saturation parameters of nuclear matter quite well. Two-body nucleon interaction is modeled by a realistic Argonne AV18 potential and the three-body potential used is a phenomenological Urbana UIX, generalizing parametrization of its component strengths in a manner suitable for application in asymmetric nuclear matter. Limiting to the two-body forces only, we find that, at fixed temperature, neutrino emissivity is a (weakly) decreasing function of density. This is also the case for the neutron branch of the modified Urca process analyzed by us in a recent paper. Inclusion of the modified three-body force effects changes this behavior. Quantitatively the value of the emissivity at each density increases and qualitatively the overall effect of the modified three-body force addition is such that dependence on density becomes weak in a density range from ${n}_{0}$ to $3.5{n}_{0}$. An updated version of neutron branch results obtained using our modified three-body force model is also presented. This is done using exact treatment of the angular integrations of nuclear transition rate in momentum space.

Model-dependence of neutrino emissivities and neutrino luminosities of neutron stars from the direct Urca processes and the modified Urca processes

The neutrino emissivities in β-stable neutron star matter from the direct Urca (DU) processes and the modified Urca (MU) processes have been investigated by adopting 26 Skyrme interactions. Several physical quantities related to the MU processes and the DU processes have been calculated and discussed. The model-dependence of the neutrino emissivities from the DU processes is found to stem mainly from the model-dependence of the effective mass, while the neutrino emissivities from the MU processes are determined by the competition between the effects of the symmetry energy and the effective mass. Besides, we have investigated the total neutrino luminosities of neutron stars, with the masses of 1.2,1.4,1.6 and 1.8M⊙, from the DU processes and the MU processes. The neutrino luminosity of a neutron star is found to be primarily determined by whether the electron DU process is allowed or not. As long as the electron DU process can occur, the total luminosity turns out to be 5 to 8 orders of magnitude larger as compared with the case that the DU process is forbidden, which indicates that the strongest model-dependence of the neutrino luminosity comes from that of the symmetry energy and the equation of state (EOS) of neutron star matter. In the case that the DU processes are allowed, the discrepancy of the calculated neutrino luminosity using various Skyrme interactions remains noticeable, which is essentially attributed to the model-dependence of the symmetry energy, the EOS of NS matter and the effective masses.

Gap-bridging enhancement of modified Urca processes in nuclear matter

In nuclear matter at neutron-star densities and temperatures, Cooper pairing leads to the formation of a gap in the nucleon excitation spectra resulting in exponentially strong Boltzmann suppression of many transport coefficients. Previous calculations have shown evidence that density oscillations of sufficiently large amplitude can overcome this suppression for flavor-changing $\ensuremath{\beta}$ processes, via the mechanism of ``gap bridging.'' We address the simplifications made in that initial work, and show that gap bridging can counteract Boltzmann suppression of neutrino emissivity for the realistic case of modified Urca processes in matter with $^{3}P_{2}$ neutron pairing.

Emissivity and Mean-free Paths of Neutrinos in Neutron Star Matter via Modified Urca Processes

Emissivity and Mean-free Paths of Neutrinos in Neutron Star Matter via Modified Urca Processes

Nuclear correlations and neutrino emissivity from the neutron branch of the modified Urca process

The neutrino emissivity from the neutron branch of the modified Urca process is calculated. The nuclear correlation effects are taken into account by employing the correlation functions extracted from the lowest-order constrained variational (LOCV) method applied to asymmetric nuclear matter. Two-body nucleon interaction is modeled by a realistic Argonne AV18 potential. In order to get consistency with semiempirical saturation parameters of nuclear matter and the existence of $2{M}_{\ensuremath{\bigodot}}$ pulsars, we add a phenomenological Urbana UIX three-body potential to the nucleon Hamiltonian and apply a newly formulated version of the LOCV method that allows for three-body nucleon interactions. We find that at fixed temperature neutrino emissivity is a (weakly) decreasing function of density, due to quenching of the contribution from tensor correlations with increasing density. This is in variance with all previous works. We also find that three-body forces allow for the opening of the direct Urca process at nucleon density $0.3\phantom{\rule{0.28em}{0ex}}{\mathrm{fm}}^{\ensuremath{-}3}$.

Influence of the stiffness of the equation of state and in-medium effects on the cooling of compact stars

Measurements demonstrate the existence of compact stars with masses in a broad range from 1.2 to 2 $M_\odot$. The most massive of these objects might be hybrid stars. To fulfill the constraint $M_{\rm max}>2~M_{\odot}$ with a reserve we exploit the stiff DD2 hadronic equation of state (EoS) without and with excluded volume (DD2vex) correction, which produce maximum neutron star masses of $M_{\rm max} = 2.43~M_{\odot}$ and $2.70~M_{\odot}$, respectively. We show that the stiffness of the EoS does not preclude an explanation of the whole set of cooling data within "nuclear medium cooling" scenario for compact stars by a variation of the star masses. We select appropriate proton gap profiles from those exploited in the literature and allow for a variation of the effective pion gap controlling the efficiency of the medium modified Urca process. However, we suppress the possibility of pion condensation. In general, the stiffer the EoS the steeper a decrease with density of the effective pion gap is required. Results are compared with previously obtained ones for the HDD EoS for which $M_{\rm max} =2.06~M_{\odot}$. The cooling of the compact star in the supernova remnant Cassiopeia A (Cas A) is explained mainly by an efficient medium modified Urca process. With the DD2vex EoS and using an effective pion gap steeper decreasing with the density and/or a proton gap shifted to smaller densities we are also able to reproduce both a strong decline compatible with ACIS-S data and HRC-S instrument data. The mass of Cas A is estimated as $1.6 - 1.9~M_{\odot}$, above the value $1.5~M_{\odot}$, which we have evaluated with the softer HDD equation of state. Different mass choices for the hottest object XMMU J173203.3-344518 are discussed. We make general remarks also on hybrid star cooling and on its dependence on the stiffness of the hadronic EoS.

Growth of Accreting Supermassive Black Hole Seeds and Neutrino Radiation

In the framework of microscopic theory of black hole (MTBH), which explores the most important processes of rearrangement of vacuum state and spontaneous breaking of gravitation gauge symmetry at huge energies, we have undertaken a large series of numerical simulations with the goal to trace an evolution of the mass assembly history of 377 plausible accreting supermassive black hole seeds in active galactic nuclei (AGNs) to the present time and examine the observable signatures today. Given the redshifts, masses, and luminosities of these black holes at present time collected from the literature, we compute the initial redshifts and masses of the corresponding seed black holes. For the present masses MBH/M⊙≃1.1×106 to 1.3×1010 of 377 black holes, the computed intermediate seed masses are ranging from MBHSeed/M⊙≃26.4 to 2.9×105. We also compute the fluxes of ultrahigh energy (UHE) neutrinos produced via simple or modified URCA processes in superdense protomatter nuclei. The AGNs are favored as promising pure UHE neutrino sources, because the computed neutrino fluxes are highly beamed along the plane of accretion disk, peaked at high energies, and collimated in smaller opening angle (θ≪1).

Rotochemical heating of millisecond and classical pulsars with anisotropic and density-dependent superfluid gap models

When a rotating neutron star loses angular momentum, the progressive reduction of the centrifugal force makes it contract. This perturbs each fluid element, raising the local pressure and originating deviations from beta equilibrium, inducing reactions that release heat (rotochemical heating). This effect has previously been studied by Fern\'andez & Reisenegger (2005) for non-superfluid neutron stars and by Petrovich & Reisenegger (2010) for superfluid millisecond pulsars. Both studies found that pulsars reach a quasi-steady state in which the compression driving the matter out of beta equilibrium is balanced by the reactions trying to restore the equilibrium. We extend previous studies by considering the effect of density-dependence and anisotropy of the superfluid energy gaps, for the case in which the dominant reactions are the modified Urca processes, the protons are non-superconducting, and the neutron superfluidity is parametrized by models proposed in the literature. By comparing our predictions with the surface temperature of the millisecond pulsar PSR J0437-4715 and upper limits for twenty-one classical pulsars, we find the millisecond pulsar can be only explained by the models with the effectively largest energy gaps (type B models), the classical pulsars require with the gap models that vanish for some angle (type C) and two different envelope compositions. Thus, no single model for neutron superfluidity can simultaneously account for the thermal emission of all available observations of non-accreting neutron stars, possibly due to our neglect of proton superconductivity.

Ultra-high energy neutrino fluxes from supermassive AGN black holes

We compute the ultra-high energy (UHE) neutrino fluxes from plausible accreting supermassive black holes closely linking to the 377 active galactic nuclei (AGNs). They have well-determined black hole masses collected from the literature. The neutrinos are produced via simple or modified URCA processes, even after the neutrino trapping, in superdense proto-matter medium. The resulting fluxes are ranging from: (1) (quark reactions)—\(J^{q}_{\nu\varepsilon}/(\varepsilon_{d}\ \mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1})\simeq8.29\times 10^{-16}\) to 3.18×10−4, with the average \(\overline{J}^{q}_{\nu\varepsilon}\simeq5.53\times 10^{-10}\varepsilon_{d}\ \mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1}\), where ed∼10−12 is the opening parameter; (2) (pionic reactions)—\(J^{\pi}_{\nu\varepsilon} \simeq0.112J^{q}_{\nu\varepsilon}\), with the average \(J^{\pi}_{\nu\varepsilon} \simeq3.66\times 10^{-11}\varepsilon_{d}\ \mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1}\); and (3) (modified URCA processes)—\(J^{URCA}_{\nu\varepsilon}\simeq7.39\times10^{-11} J^{q}_{\nu\varepsilon}\), with the average \(\overline{J}^{URCA}_{\nu\varepsilon} \simeq2.41\times10^{-20} \varepsilon_{d}\ \mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1}\). We conclude that the AGNs are favored as promising pure neutrino sources, because the computed neutrino fluxes are highly beamed along the plane of accretion disk, peaked at high energies and collimated in smaller opening angle θ∼ed.