arXiv subjects

Hiroki Nagakura

Publications and source records attributed to Hiroki Nagakura.

At least 55 records · Page 3Linked to original sources

arXiv2023

Simple method for determining asymptotic states of fast neutrino-flavor conversion

Neutrino-neutrino forward scatterings potentially induce collective neutrino oscillation in dense neutrino gases in astrophysical sites such as core-collapse supernovae (CCSN) and binary neutron star mergers (BNSM). In this paper, we present a detailed study of fast neutrino-flavor conversion (FFC), paying special attention to asymptotic states, by linear stability analysis and local simulations with a periodic boundary condition. We find that asymptotic states can be characterized by two key properties of FFC: (1) the conservation of lepton number for each flavor of neutrinos and (2) the disappearance of ELN(electron neutrino-lepton number)-XLN(heavy-leptonic one) angular crossings in the spatial- or time-averaged distributions. The system which initially has the positive (negative) ELN-XLN density reaches a flavor equipartition in the negative (positive) ELN-XLN angular directions, and the other part compensates it to preserve the conservation laws. These properties of FFCs offer an approximate scheme determining the survival probability of neutrinos in asymptotic states without solving quantum kinetic equations. We also demonstrate that the total amount of flavor conversion can vary with species-dependent neutrino distributions for identical ELN-XLN ones. Our results suggest that even shallow or narrow ELN angular crossings have the ability to drive large flavor conversion, exhibiting the need for including the effects of FFCs in the modeling of CCSN and BNSM.

astro-ph.HE↗

arXiv2023

Connecting small-scale to large-scale structures of fast neutrino-flavor conversion

We present a systematic study of fast neutrino-flavor conversion (FFC) with both small-scale and large-scale numerical simulations in spherical symmetry. We find that FFCs can, in general, reach a quasi-steady state, and these features in the non-linear phase are not characterized by the growth rate of FFC instability but rather angular structures of electron neutrino lepton number (ELN) and heavy one (XLN). Our result suggests that neutrinos can almost reach a flavor equipartition even in cases with low growth rate of instability (e.g., shallow ELN crossing) and narrow angular regions (in momentum space) where flavor conversions occur vigorously. This exhibits that ELN and XLN angular distributions can not provide a sufficient information to determine total amount of flavor conversion in neutrinos and antineutrinos of all flavors. Based on the results of our numerical simulations, we provide a new approximate scheme of FFC that is designed so that one can easily incorporate effects of FFCs in existing classical neutrino transport codes for the study of core-collapse supernova (CCSN) and binary neutron star merger (BNSM). The scheme has an ability to capture key features of quasi-steady state of FFCs without solving quantum kinetic neutrino transport, which will serve to facilitate access to FFCs for CCSN and BNSM theorists.

astro-ph.HE↗

arXiv2022

Protoneutron Star Convection Simulated with a New General Relativistic Boltzmann Neutrino Radiation-Hydrodynamics Code

We investigate the protoneutron star (PNS) convection using our newly developed general relativistic Boltzmann neutrino radiation-hydrodynamics code. This is a pilot study for more comprehensive investigations later. As such, we take a snapshot of a PNS at 2.3 seconds after bounce from a 1D PNS cooling calculation and run our simulation for $\sim160\,\mathrm{ms}$ in 2D under axisymmetry. The original PNS cooling calculation neglected convection entirely and the initial condition is linearly unstable to convection. We find in our 2D simulation that convection is instigated there indeed and expands inward after being full-fledged. The convection is then settled to a quasi-steady state in $\sim100\,\mathrm{ms}$, being sustained by the negative $Y_e$ gradient, which is in turn maintained by neutrino emissions. It enhances the luminosities and mean energies of all species of neutrinos compared to 1D. Taking advantage of the Boltzmann solver, we analyze the possible occurrence of the neutrino fast flavor conversion (FFC). We found that FFC is likely to occur in the regions, where $Y_e$ is lower, and that the growth rate can be as high as $\sim 10^{-1}\,{\mathrm{cm}^{-1}}$.

astro-ph.HE↗

arXiv2022

Nuclei in Core-Collapse Supernovae Engine

Herein, we review the nuclear equations of state (EOSs) %for core-collapse supernova simulations and the constituent nuclei of core-collapse supernovae (CCSNe) and their roles in CCSN simulations. Various nuclei such as deuterons, iron, and extremely neutron-rich nuclei compose in the central engines of CCSNe. The center of a collapsing core is dominated by neutron-rich heavy nuclei prior to the occurrence of core bounce. Their weak interactions significantly affect the neutrino emission and the size of the produced proto-neutron star. After a core bounce, heavy nuclei are dissolved to protons, neutrons, and light nuclei between the expanding shock wave and the newly formed neutron star (NS). Some of the key components in determining the shock-wave dynamics and supernova explosion of outer envelopes are neutrino interactions of nucleons and light nuclei such as deuterons. An EOS provides the relations between thermodynamical properties and the nuclear composition, and is needed to simulate this explosion. Further investigations on uniform and non-uniform nuclear matter are needed to improve the understanding of the mechanism of CCSNe and the properties of supernova nuclei. The knowledge of the EOS for uniform nuclear matter is being continually improved by a combination of microscopic calculations, terrestrial experiments, and NS observations. With reference to various nuclear experiments and current theories, the finite temperature effects on heavy nuclei, formation of light nuclei in dilute nuclear matter, and transition to uniform nuclear matter should be improved in the model of the EOS for non-uniform nuclear matter.

nucl-th↗

arXiv2022

Explosive nucleosynthesis with fast neutrino-flavor conversion in core-collapse supernovae

Fast neutrino-flavor conversion (FFC) is a possible game-changing ingredient in core-collapse supernova (CCSN) theory. In this paper, we examine the impact of FFC on explosive nucleosynthesis by including the effects of FFC in conjunction with asymmetric neutrino emission into nucleosynthetic computations in a parametric way. We find that the ejecta compositions are not appreciably affected by FFC for elements lighter than Co, while the compositions are influenced by FFC for the heavier elements. We also find that the role of FFC varies depending on the asymmetric degree of neutrino emission ($m_{\rm asy}$) and the degree of neutrino-flavor mixing. The impact of FFC is not monotonic to $m_{\rm asy}$; The change in the ejecta composition increases for higher $m_{\rm asy}$ up to $\sim 10\%$ compared with that without FFC, whereas FFC has little effect on the nucleosynthesis in very large asymmetric neutrino emission ($\gtrsim 30 \%$). Our results suggest that FFC facilitates the production of neutron-rich ejecta in most cases, although it makes the ejecta more proton-rich if anti-neutrino conversion is more vigorous than that of neutrino. The key ingredient accounting for this trend is neutrino absorption, whose effects on nucleosynthesis can be quantified by simple diagnostics.

astro-ph.HE↗

arXiv2022

Effects of nuclear matter and composition in core-collapse supernovae and long-term proto-neutron star cooling

We study the influence of hot and dense matter in core-collapse supernovae by adopting up-to-date nuclear equation of state (EOS) based on the microscopic nuclear many-body frameworks. We explore effects of EOS based on the Dirac Brueckner Hartree-Fock theory through comparisons with those based on the variational method. We also examine effects of the differences in the composition of nuclei and nucleons by using the same EOS by the variational method but employing two different treatments in computations of nuclear abundances. We perform numerical simulations of core-collapse supernovae adopting the three EOSs. We also perform numerical simulations of the long-term evolution over 70 s of the proto-neutron star cooling. We show that impacts by different modeling of composition are remarkable as in those by different treatments of uniform matter in the gravitational collapse, bounce, and shock propagation. The cooling of proto-neutron star and the resulting neutrino emission are also affected by the compositional difference even if the same treatment in computing uniform matter of EOS.

astro-ph.HE↗

arXiv2022

Effects of energy-dependent scatterings on fast neutrino flavor conversions

Neutrino self-interactions in a dense neutrino gas can induce collective neutrino flavor conversions. Fast neutrino flavor conversions (FFCs), one of the collective neutrino conversion modes, potentially change the dynamics and observables in core-collapse supernovae and binary neutron star mergers. In cases without neutrino-matter interactions (or collisions), FFCs are essentially energy-independent, and therefore the single energy treatment has been used in previous studies. However, neutrino-matter collisions in general depend on neutrino energy, suggesting that energy-dependent features may emerge in FFCs with collisions. In this paper, we perform dynamical simulations of FFCs with iso-energetic scatterings (emulating nucleon scatterings) under multi-energy treatment. We find that cancellation between in- and out-scatterings happens in high energy region, which effectively reduces the number of collisions and then affects the FFC dynamics. In fact, the lifetime of FFCs is extended compared to the single-energy case, leading to large flavor conversions. Our result suggests that the multi-energy treatment is mandatory to gauge the sensitivity of FFCs to collisions. We also provide a useful quantity to measure the importance of multi-energy effects of collisions on FFCs.

astro-ph.HE↗

arXiv2022

Time-dependent and quasi-steady features of fast neutrino-flavor conversion

Despite the theoretical indication that fast neutrino-flavor conversion (FFC) ubiquitously occurs in core-collapse supernova and binary neutron star merger, the lack of global simulations has been the greatest obstacle to study their astrophysical consequences. In this {\it Letter}, we present large-scale ($50 {\rm km}$) simulations of FFC in spherical symmetry by using a novel approach. We effectively rescale the oscillation scale of FFC by reducing the number of injected neutrinos in the simulation box, and then extrapolate back to the case of the target density of neutrinos with a convergence study. We find that FFC in all models achieves quasi-steady state in the non-linear regime, and its saturation property of FFC is universal. We also find that temporal- and spatial variations of FFC are smeared out at large radii due to phase cancellation through neutrino self-interactions. Finally, we provide a new diagnostic quantity, ELN-XLN angular crossing, to assess the non-linear saturation of FFC.

astro-ph.HE↗

arXiv2022

General-Relativistic Quantum-Kinetics Neutrino Transport

We developed a new general-relativistic quantum-kinetics neutrino transport code, GRQKNT, for numerical studies of quantum kinetics of non-equilibrium neutrinos in six-dimensional phase space. This code is intended for use in both local and global simulations of neutrino transport in core-collapse supernova and binary neutron star merger. It has been widely recognized that global simulations of collective neutrino oscillations, in particular fast neutrino-flavor conversions (FFC), require unfeasible computational resources due to large disparity of scales between flavor conversion and astrophysical phenomena. We propose a novel approach to tackle the issue. This paper is devoted to describe the philosophy, design, and numerical implementation of GRQKNT with a number of tests ensuring correct implementation of each module. The code is based on a discrete-ordinate Sn method, finite-difference realization of mean-field quantum kinetic equation. The transport equation is solved based on a conservative formalism, and we use a fifth-order weighted essentially non-oscillatory scheme with fourth-order TVD Runge-Kutta time-integration. The transport module is designed to work with arbitrary spacetimes and currently three different stationary spacetimes (flat spacetime, Schwarzschild black hole, and Kerr black hole) are implemented. The collision term including neutrino emission, absorption, and momentum-exchanged scatterings are also implemented into our code. The oscillation Hamiltonian consists of vacuum, matter, and self-interactions. Both two- and three neutrino-flavor scenarios can be applied. Fluid-velocity dependences in transport-, collision-, and oscillation modules, are also treated self-consistently by using two-energy-grid technique, that has been already established in our another code with full Boltzmann neutrino transport.

astro-ph.HE↗

arXiv2022

Self-consistency in models of neutrino scattering and fast flavor conversion

Several recent numerical studies have examined the effects of neutrino neutral-current scattering on fast flavor conversion (FFC). Those studies are in apparent conflict, with some finding enhancement of flavor conversion and others finding suppression. The ones that report enhancement all use homogeneous models, and we discuss in detail the self-consistency issues that they face as a result. We reproduce scattering-enhanced FFC in our own homogeneous calculations, showing that it occurs for both neutral- and charged-current scattering and that it is due to isotropization of the angular distributions over time. Because this is the very feature of the calculations that is not self-consistent, we conclude that the enhancement effect is of unclear astrophysical relevance and may not occur in natural environments.

hep-ph↗

arXiv2022

Principal-Axis Analysis of the Eddington Tensor for the Early Post-Bounce Phase of Rotational Core-Collapse Supernovae

Using full Boltzmann neutrino transport, we performed two-dimensional (2D) core-collapse supernova simulations in axisymmetry for two progenitor models with 11.2M and 15.0M both rotational and non-rotational. We employed the results obtained in the early post-bounce phase (t < 20 ms) to assess performance under rapid rotation of some closure relations commonly employed in the truncated moment method. We first made a comparison in 1D under spherical symmetry, though, of the Eddington factor p defined in the fluid rest frame (FR). We confirmed that the maximum entropy closure for the Fermionic distribution (MEFD) performs better than others near the proto-neutron star surface, where p < 1/3 occurs, but does not work well even in 1D when the phase space occupancy satisfies e < 0.5 together with p < 1/3, the condition known to be not represented by MEFD. For the 2D models with the rapid rotation, we employed the principal-axis analysis of the Eddington tensor. We paid particular attention to the direction of the longest principal axis. We observed in FR that it is aligned neither with the radial direction nor with the neutrino flux in 2D, particularly so in convective and/or rapidly rotating regions, the fact not accommodated in the moment method. We repeated the same analysis in the laboratory frame (LB) and found again that the direction of the longest principal axis is not well reproduced by MEFD because the interpolation between the optically thick and thin limits is not very accurate in this frame.

astro-ph.HE↗

arXiv2022

Efficient estimation method for time evolution of proto-neutron star mass and radius from supernova neutrino signal

In this paper we present a novel method to estimate the time evolution of proto-neutron star (PNS) structure from the neutrino signal in core-collapse supernovae (CCSN). Employing recent results of multi-dimensional CCSN simulations, we delve into a relation between total emitted neutrino energy (TONE) and PNS mass/radius, and we find that they are strongly correlated with each other. We fit the relation by simple polynomial functions connecting TONE to PNS mass and radius as a function of time. By combining another fitting function representing the correlation between TONE and cumulative number of event at each neutrino observatory, PNS mass and radius can be retrieved from purely observed neutrino data. We demonstrate retrievals of PNS mass and radius from mock data of neutrino signal, and we assess the capability of our proposed method. While underlining the limitations of the method, we also discuss the importance of the joint analysis with gravitational wave signal. This would reduce uncertainties of parameter estimations in our method, and may narrow down the possible neutrino oscillation model. The proposed method is a very easy and inexpensive computation, which will be useful in real data analysis of CCSN neutrino signal.

astro-ph.HE↗

arXiv2021

Prospects of fast flavor neutrino conversion in rotating core-collapse supernovae

There is mounting evidence that neutrinos undergo fast flavor conversion (FFC) in core-collapse supernova (CCSN). In this letter, we investigate the roles of stellar rotation on the occurrence of FFC by carrying out axisymmetric CCSN simulations with full Boltzmann neutrino transport. Our result suggests that electron neutrino lepton number (ELN) angular crossings, which are the necessary and sufficient condition to trigger FFC, preferably occur in the equatorial region for rotating CCSNe. By scrutinizing the neutrino--matter interaction and neutrino radiation field, we find some pieces of evidence that the stellar rotation facilitates the occurrence of FFC. The low-electron-fraction region in the post-shock layer expands by centrifugal force, enhancing the disparity of neutrino absorption between electron-type neutrinos ($ν_{\rm e}$) and their anti-particles ($\barν_{\rm e}$). This has a significant impact on the angular distribution of neutrinos in momentum space, in which $ν_{\rm e}$ tends to be more isotropic than $\barν_{\rm e}$; consequently, ELN crossings emerge. The ELN crossing found in this study is clearly associated with rotation, which motivates further investigation on how the subsequent FFC influences explosion dynamics, nucleosynthesis, and neutrino signals in rotating CCSNe.

astro-ph.HE↗

arXiv2021

New method for detecting fast neutrino flavor conversions in core-collapse supernova models with two-moment neutrino transport

Fast-pairwise neutrino oscillations potentially affect many aspects of core-collapse supernova (CCSN): the explosion mechanism, neutrino signals, and nucleosynthesis in the ejecta. This particular mode of collective neutrino oscillations has a deep connection to the angular structure of neutrinos in momentum space; for instance, the appearance of electron neutrinos lepton number (ELN) angular crossings in momentum space is a good indicator of occurrences of the flavor conversions. However, many multi-dimensional (multi-D) CCSN simulations are carried out with approximate neutrino transport (such as two-moment methods), which limits the access to the angular distributions of neutrinos, i.e., inhibits ELN-crossing searches. In this paper, we develop a new method of ELN-crossing search in these CCSN simulations. The required data is the zero-th and first angular moments of neutrinos and matter profile, all of which are available in CCSN models with two-moment method. One of the novelties of our new method is to use a ray-tracing neutrino transport to determine ELNs in the direction of the stellar center. It is designed to compensate for shortcomings of the crossing searches only with the two angular moments. We assess the capability of the method by carrying out a detailed comparison to results of full Boltzmann neutrino transport in 1D and 2D CCSN models. We find that the ray-tracing neutrino transport improves the accuracy of crossing searches; indeed, the appearance/disappearance of the crossings is accurately detected even in the region of forward-peaked angular distributions. The new method is computationally cheap and has a benefit of efficient parallelization; hence, it will be useful for ELN-crossing searches in any CCSN models employed two-moment neutrino transport.

astro-ph.HE↗

arXiv2021

Neutrino transport with Monte Carlo method: II. Quantum Kinetic Equations

Neutrinos have an unique quantum feature as flavor conversions. Recent studies suggested that collective neutrino oscillations play important roles in high-energy astrophysical phenomena. Quantum kinetic equation (QKE) is capable of describing the neutrino flavor conversion, transport and matter collision self-consistently. However, we have experienced many technical difficulties in their numerical implementation. In this paper, we present a new QKE solver based on Monte Carlo (MC) approach. This is an upgraded version of our classical MC neutrino transport solver; in essence, a flavor degree of freedom including mixing state is added into each MC particle. This extension requires updating numerical treatments of collision terms, in particular for scattering processes. We deal with the technical problem by generating a new MC particle at each scattering event. To reduce statistical noise inherent in MC methods, we develop the effective mean free path method. This suppresses a sudden change of flavor state due to collisions without increasing the number of MC particles. We present a suite of code tests to validate these new modules with comparing to the results reported in previous studies. Our QKE-MC solver is developed with fundamentally different philosophy and design from other deterministic- and mesh methods, suggesting that it will be complementary to others, and potentially provide new insights into physical processes of neutrino dynamics.

astro-ph.HE↗

arXiv2021

Where, when and why: occurrence of fast-pairwise collective neutrino oscillation in three-dimensional core-collapse supernova models

Fast-pairwise collective neutrino oscillation represents a key uncertainty in the theory of core-collapse supernova (CCSN). Despite the potentially significant impact on CCSN dynamics, it is usually neglected in numerical models of CCSN because of the formidable technical difficulties of self-consistently incorporating this physics. In this paper, we investigate the prospects for the occurrence of fast flavor conversion by diagnosing electron neutrino lepton number (ELN) crossing in more than a dozen state-of-the-art three-dimensional CCSN models. ELN crossings is a necessary condition for triggering flavor conversion. Although only zeroth and first angular moments are available from the simulations, our new method enables us to look into the angular distributions of neutrinos in momentum space and provide accurate insight into ELN crossings. Our analysis suggests that fast flavor conversion generally occurs in the post-shock region of CCSNe, and that explosive models provide more favorable conditions for the flavor conversion than failed CCSNe. We also find that there are both common and progenitor-dependent characteristics. Classifying ELN crossings into two types, we analyze the generation mechanism of each case by scrutinizing the neutrino radiation field and matter interactions. We find key ingredients of CCSN dynamics driving the ELN crossings: proto-neutron star (PNS) convection, asymmetric neutrino emission, neutrino absorptions and scatterings. This study suggests that we need to accommodate fast flavor conversions in realistic CCSN models.

astro-ph.HE↗

arXiv2021

Supernova neutrino signals based on long-term axisymmetric simulations

We study theoretical neutrino signals from core-collapse supernova (CCSN) computed using axisymmetric CCSN simulations that cover the post-bounce phase up to $\sim 4$~s. We provide basic quantities of the neutrino signals such as event rates, energy spectra, and cumulative number of events at some terrestrial neutrino detectors, and then discuss some new features in the late phase that emerge in our models. Contrary to popular belief, neutrino emissions in the late phase are not always steady, but rather have temporal fluctuations, the vigor of which hinges on the CCSN model and neutrino flavor. We find that such temporal variations are not primarily driven by proto-neutron star (PNS) convection, but by fallback accretion in exploding models. We assess the detectability of these temporal variations, and find that IceCube is the most promising detector with which to resolve them. We also update fitting formulae first proposed in our previous paper for which the total neutrino energy (TONE) emitted at the CCSN source is estimated from the cumulative number of events in each detector. This will be a powerful technique with which to analyze real observations, particularly for low-statistics data.

astro-ph.HE↗

arXiv2021

Constructing angular distributions of neutrinos in core collapse supernova from zero-th and first moments calibrated by full Boltzmann neutrino transport

Two-moment neutrino transport methods have been widely used for developing theoretical models of core-collapse supernova (CCSN), since they substantially reduce the computational burden inherent in the multi-dimensional neutrino-radiation hydrodynamical simulations. The approximation, however, comes at a price; the detailed structure of angular distribution of neutrinos is sacrificed, that is the main drawback of this approach. In this paper, we develop a novel method by which to construct angular distributions of neutrinos from the zero-th and first angular moments. In our method, the angular distribution is expressed with two quadratic functions of the neutrino angle in a piecewise fashion. We determine the best parameters in the fitting function by comparing to the neutrino data in a spherically symmetric CCSN model with full Boltzmann neutrino transport. We demonstrate the capability of our method by using our recent 2D CCSN model. We find that the essential features of the angular distributions can be well reconstructed, whereas the angular distributions of incoming neutrinos tend to have large errors that increase with flux factor ($κ$). This issue originates from the insensitiveness of incoming neutrinos to $κ$, that is an intrinsic limitation in moment methods. Based on the results of the demonstration, we assess the reliability of ELN-crossing searches with two-moment neutrino transport. This analysis is complementary to our another paper that scrutinizes the limitation of crossing searches with a few moments. We find that the systematic errors of angular distributions for incoming neutrinos lead to misjudgements of the crossing at $κ\gtrsim 0.5$. This casts doubt on the results of ELN-crossing searches based on two-moment methods in some previous studies.

astro-ph.HE↗