arXiv subjects

Ryuichiro Akaho

Publications and source records attributed to Ryuichiro Akaho.

At least 19 recordsLinked to original sources

arXiv2026

Numerical Studies of Accretion Flows onto a Neutron Star Engulfed in a Massive Star

Massive stars commonly form binaries that can evolve into compact systems via common envelope evolution (CEE), a critical but poorly understood phase -- especially when the companion is a neutron star. Understanding the drag force exerted on a neutron star during CEE is a key to the quantitative evaluation of orbital decay, merger timescale, and compactness of the resultant binary. In this paper, we conduct general-relativistic hydrodynamical simulations under a novel strategy of multi-layer domain-decomposition to treat the vast disparity of $10^4$--$10^7$ between the neutron star radius and the accretion radius. Our 10-model survey spans diverse physical conditions that the neutron star encounters in the envelope of a massive star. We find that nested bow shocks with alternating orientations commonly form. This configuration is qualitatively different from those in the conventional picture and results in an enhancement of the drag force by one to two orders of magnitude from what the Bondi--Hoyle--Lyttleton formula predicts. Moreover, the direction of the net force can reverse depending on the envelope conditions, contrary to the standard picture in which the drag always decelerates the companion. These results will serve as a basis for improvements of the drag force prescription in CEE modeling, and have implications for binary evolution theory.

astro-ph.HE↗

arXiv2026

Momentum Space Resolution Dependence in Boltzmann Neutrino Radiation Hydrodynamics Simulations of Core-collapse Supernovae

Finite momentum space resolution is a primary source of uncertainty in solving the Boltzmann equation with the discrete-ordinates method. In this paper, the momentum space resolution dependence of Boltzmann neutrino transport is studied by performing a series of two-dimensional core-collapse supernova (CCSN) simulations. The effects on the explosion dynamics are discussed by individually varying the resolutions of the zenith and azimuth angles in momentum space, and of the energy. It is found that a coarse zenith angle resolution artificially facilitates the explosion, even turning non-exploding models into exploding ones. This is because the coarse zenith angle grid cannot capture the forward-peaked distribution, thereby underestimating the flux factor and making neutrinos stay in the gain region for a longer time. The dependence on the azimuth angle resolution is found to have little effect for the present non-rotating models, which is understandable given the sphericity of the CCSN core. In contrast to the zenith angle resolution, a coarse energy resolution is found to artificially suppress the explosion. This is because the neutrino heating rate at intermediate energies ($\sim30$--$50\,\mathrm{MeV}$) is underestimated in the low-resolution case. Finally, the resolution dependence is studied for a one-dimensional black hole (BH) forming model. Unlike the explosion dynamics, BH formation does not exhibit a strong angular dependence. On the other hand, a coarse energy resolution is found to delay the BH formation time. This is because the momentum feedback is overestimated at low resolution, which allows a more massive neutron star to be supported.

astro-ph.HE↗

arXiv2026

Comprehensive neutrino light curves and spectra: from pre-supernova evolution to early supernova phase

We present the first systematic study of neutrino emissions from massive stars, continuously tracking the late evolutionary stages through the early core-collapse supernova phase. Using progenitor and supernova models, we analyze the neutrino luminosities and spectra for progenitors with initial masses of 10--40~$M_\odot$. Our systematic analysis reveals that the compactness parameter ($ξ_{2.5}$) and carbon-oxygen core mass ($M_{\text{CO}}$) exhibit strong correlations with neutrino emission. In the pre-supernova phase, the time-integrated number of neutrinos correlates with $ξ_{2.5}$ when integrated over the final day and with $M_{\text{CO}}$ for longer durations. For the early supernova phase ($<200$ ms post-bounce), the neutrino properties are relatively insensitive to the specific stellar evolution code used, allowing for a reliable extraction of physical correlations. We confirm that the neutrino emission features, including the electron neutrino burst properties and accretion-powered luminosity of other species, reflect the progenitor's compactness. An evaluation of the observational feasibility for a nearby progenitor using a False Alarm Rate approach suggests that these correlations can persist even under practical detection conditions. Such a joint analysis of both phases provides complementary constraints on the internal structure. All calculated time-series data will be made publicly available.

astro-ph.HE↗

arXiv2026

Bifurcated Impact of Neutrino Fast Flavor Conversion on Core-collapse Supernovae Informed by Multi-angle Neutrino Radiation Hydrodynamics

In this {\it Letter}, we present a compelling and robust argument for the roles of neutrino fast flavor conversion (FFC) in the explosion mechanism of core-collapse supernova (CCSN), combining the {\it multi-angle} FFC subgrid model rooted in quantum kinetic theory with the multi-dimensional four-species Boltzmann neutrino radiation hydrodynamics. Employing various progenitor masses and the nuclear equations of states, we find that the effect of FFC on CCSN explosion is bifurcated depending on the progenitors. For the lowest-mass progenitor, FFC facilitates the shock revival and enhances the explosion energy, whereas for higher-mass progenitors its impact is inhibitory. We identify the mass accretion rate as the key determinant governing this bifurcation. When the mass accretion rate is low (high), the contribution of FFC to neutrino heating becomes positive (negative), because the heating efficiency enhancement via FFC-driven spectral hardening of electron-type neutrinos dominates over (is outweighed by) the concurrent reduction in neutrino luminosity. Our results further highlight the limitations of approximate neutrino transport, and demonstrate that a multi-angle treatment is essential for accurately capturing FFC effects; otherwise, FFCs are missed and even generated spuriously.

astro-ph.HE↗

arXiv2025

Neutrino Constraints on Black Hole Formation in M31

We investigate neutrino signals associated with black hole formation resulting from the gravitational collapse of massive stars, motivated by the candidate failed supernova M31-2014-DS1 in the Andromeda Galaxy (M31). By compiling numerical simulation results for stellar collapse, we predict the expected neutrino emission and compare these predictions with observational limits from Super-Kamiokande (SK). The simulations reveal a characteristic precursor signal consisting of a short, intense burst whose average neutrino energy rises rapidly and then ceases abruptly once the black hole forms. We examine several nuclear equations of state, specifically the Lattimer \& Swesty, Shen, Togashi, and SFHo models, to evaluate how the emission depends on neutron-star properties and nuclear-physics uncertainties. Comparison of the predicted event counts with SK's non-detection of neutrinos coincident with M31-2014-DS1 already rules out part of the model space and highlights the sensitivity of current neutrino detectors to both progenitor mass and the EOS. These findings demonstrate the capability of neutrino astronomy to probe core collapse and black hole formation in failed supernova scenarios.

astro-ph.HE↗

arXiv2025

A Dual-Resolution Prescription in the $S_N$ Method for Boltzmann Neutrino Transport I: Proof of Principle and the Resolution of Collision Term

We propose a dual-resolution prescription meant for the Boltzmann neutrino transport, in which the advection and collision terms are calculated with different angular resolutions in momentum space. The purpose is to address the issue of the low resolution that afflicts the $S_N$ method in the multi-dimensional neutrino transport simulations for core-collapse supernovae. We handle with a high resolution the advection term alone, assuming that the collision term does not require such high resolutions. To confirm this surmise as well as our new conversion scheme, from low- to high-angular resolutions and vice versa, we run a couple of experimental one-zone (in space) simulations. Neutrino scatterings on nucleons are considered with small recoils fully taken into account whereas the advection term is replaced by the angle- and energy-dependent source terms that are designed to mimic the results of a Boltzmann simulation, inducing the anisotropy in momentum space. For the conversion from a low-resolution distribution function to a high-resolution one, we employ a polynomial interpolations in the zenith and azimuth directions separately with the number conservation and continuity (and periodicity only in the azimuth direction). We find that this dual-resolution scheme works well and that the current angular resolution employed in the canonical supernova simulations with our Boltzmann solver or a bit better in the $ϕ_ν$ direction will be sufficient for the collision terms if they are coupled with the advection terms calculated with a high-angular resolution via this prescription.

astro-ph.HE↗

arXiv2025

Asymmetric emissions of neutrinos in the cooling of rotating proto-neutron stars

We evaluate global asymmetry in the luminosities of neutrinos emitted from rapidly-rotating proto-neutron stars (PNS's). We build axisymmetric models of PNS's in mechanical equilibrium with rotation by adding prescribed angular momentum distributions by hand to non-rotational PNS models, which are extracted from a one-dimensional (spherically symmetric) PNS cooling calculation at different times: $t=2, 6, 10, 20, 30$s after a supernova explosion. We then conduct two-dimensional (spatially axisymmetric) neutrino transport calculations on top of them with the matter profiles (and the spacetime geometry) fixed. We find for the rapidly-rotating models with $T/|W|\sim 5\times 10^{-2}$ that the neutrino luminosity changes by $\sim 3 \% $ depending on the observer position. We give detailed analyses of the neutrino-hemispheres as well as the neutrino luminosities that are defined observer-wise. We also calculate the low-frequency ($\lesssim 1{\rm Hz}$) gravitational waves produced by the neutrinos radiated asymmetrically. We find that those gravitational waves, if emitted from the Galactic center, can be detected by planned detectors such as B-DECIGO, DECIGO and AILA. Finally, we look for crossings in the energy-integrated angular distributions in momentum space for the electron neutrino sector, a signature of the fast flavor conversion. We find them near the PNS surface in all models.

astro-ph.HE↗

arXiv2025

Observing Supernova Neutrino Light Curves with Super-Kamiokande.VI. A Practical Data Analysis Technique Considering Realistic Experimental Backgrounds

Neutrinos from supernovae, especially those emitted during the late phase of core collapse, are essential for understanding the final stages of massive star evolution. We have been dedicated to developing methods for the analysis of neutrinos emitted during the late phase and observed at Super-Kamiokande (SK). Our previous studies have successfully demonstrated the potential of various analysis methods in extracting essential physical properties; however, the lack of background consideration has limited their practical application. In this study, we address this issue by incorporating a realistic treatment of the experimental signal and background events with the on-going SK experiment. We therefore optimize our analysis framework to reflect realistic observational conditions, including both signal and background events. Using this framework we study several long-time supernova models, simulating the late phase neutrino observation in SK and focusing in particular on the identification of the last observed event. We discuss the possibility of model discrimination methods using timing information from this last observed event.

astro-ph.HE↗

arXiv2025

Comparative Testing of Subgrid Models for Fast Neutrino Flavor Conversions in Core-collapse Supernova Simulations

We investigate key methodologies of Bhatnagar-Gross-Krook subgrid modeling for neutrino fast flavor conversions (FFC) in core-collapse supernova based on spherically symmetric Boltzmann radiation hydrodynamics simulations. We first examine time integration methods (explicit, implicit, or semi-implicit) and time step control for the subgrid term, and then compare various approaches in the literature approximating FFCs in two aspects: (1) angular dependent survival probability of neutrinos versus simple equipartition condition with a certain baryon mass density threshold, and (2) 4-species treatment versus 3-species assumption ($ν_x=\barν_x$). We find that the equipartition condition is reasonable for out-going neutrinos, but large deviations emerge in the incoming neutrinos, that has an influence on matter profiles. We also find that the 3-species model, in which flavor conversions evolve towards erasing electron neutrino lepton number (ELN) crossings, behave differently from the 4-species models where heavy leptonic neutrino number (XLN) are appropriately treated in FFC subgrid modeling. In 4-species models, we commonly observe noticeable differences between $ν_x$ and $\barν_x$, highlighting the limitation in 3-species treatments to study impacts of flavor conversion on neutrino signals. Our result also suggests that FFC models yield lower neutrino heating rate and smaller shock radii compared to cases with no FFC, in agreement with earlier studies employing quantum kinetic neutrino transport. This work provides valuable information towards robust implementation of FFC subgrid model into classical transport, and serves as a pilot study for future multi-dimensional simulations.

astro-ph.HE↗

arXiv2025

Observing Supernova Neutrino Light Curves with Super-Kamiokande. V. Distance Estimation with Neutrinos

Neutrinos are pivotal signals in multi-messenger observations of supernovae (SNe). Recent advancements in the analysis method of supernova (SN) neutrinos, especially in quantitative analysis, have significantly broadened scientific possibilities. This study demonstrates the feasibility of estimating distances to SNe using neutrinos. This estimation utilizes the direct relationship between the radius of a neutron star (NS) and the distance to the SN, which is analogous to main-sequence fitting. The radius of an NS is determined with an approximate uncertainty of 10% through observations such as X-rays and gravitational waves. By integrating this information, the distance to the SN can be estimated with an uncertainty of within 15% at a 95% confidence level. It has been established that neutrinos can pinpoint the direction of SNe, and when combined with distance estimates, three-dimensional localization becomes achievable. This capability is vital for follow-up observations using multi-messenger approaches. Moreover, more precise distance determinations to SNe through follow-up observations, such as optical observations, allow for accurate measurements of NS radii. This data, via the NS mass-radius relationship, could provide various insights into nuclear physics.

astro-ph.HE↗

arXiv2024

Quasi-steady evolution of fast neutrino-flavor conversions

In astrophysical environments such as core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs), neutrinos potentially experience substantial flavor mixing due to the refractive effects of neutrino self-interactions. Determining the survival probability of neutrinos in asymptotic states is paramount to incorporating flavor conversions' effects in the theoretical modeling of CCSN and BNSM. Some phenomenological schemes have shown good performance in approximating asymptotic states of fast neutrino-flavor conversions (FFCs), known as one of the collective neutrino oscillation modes induced by neutrino self-interactions. However, a recent study showed that they would yield qualitatively different asymptotic states of FFC if the neutrino number is forced to evolve. It is not yet fully understood why the canonical phenomenological models fail to predict asymptotic states. In this paper, we perform detailed investigations through numerical simulations and then provide an intuitive explanation with a quasi-homogeneous analysis. Based on the analysis, we propose a new phenomenological model, in which the quasi-steady evolution of FFCs is analytically determined. The model also allows us to express the convolution term of spatial wave number as a concise form, which corresponds to useful information on analyses for the non-linear feedback from small-scale flavor conversions to large-scale ones. Our model yields excellent agreement with numerical simulations, which lends support to our interpretation.

astro-ph.HE↗

arXiv2024

Impacts of Black-Hole-Forming Supernova Explosions on the Diffuse Neutrino Background

Flux spectrum, event rate, and experimental sensitivity are investigated for the diffuse supernova neutrino background (DSNB), which originates from past stellar collapses and is also known as a supernova relic neutrino background. For this purpose, the contribution of collapses that lead to successful supernova (SN) explosion and black hole (BH) formation simultaneously, which are suggested to be a non-negligible population from the perspective of Galactic chemical evolution, is taken into account. If the BH-forming SNe involve the matter fallback onto the protoneutron star for the long term, their total emitted neutrino energy becomes much larger than that of ordinary SNe and failed SNe (BH formation without explosion). Then, in the case of the normal mass hierarchy in neutrino oscillations and with half of all core-collapse SNe being BH-forming SNe, the expected event rate according to the current DSNB model is enhanced by up to a factor of two due to the BH-forming SNe. While substantial uncertainties exist regarding the duration of the matter fallback, which determines the total amount of emitted neutrinos, and the fraction of BH-forming SNe, the operation time required to detect the DSNB at Hyper-Kamiokande would be reduced by such contribution in any case.

astro-ph.HE↗

arXiv2024

Muon-induced collisional flavor instability in core-collapse supernova

Neutrinos are known to undergo flavor conversion among their three flavors. In the theoretical modeling of core-collapse supernova (CCSN), there has been a great deal of attention to recent discoveries of a new type of neutrino flavor conversions, namely collisional flavor instability (CFI), in which the instability is induced by the flavor-dependent decoherence due to the disparity of neutrino-matter interactions among flavors. In this paper, we study how the appearance of on-shell muons and associated neutrino-matter interactions can impact CFIs based on linear stability analysis of flavor conversions. Some striking results emerge from the present study. First, we analytically show that breaking beta- and pair equilibrium is a necessary condition to trigger CFIs. This also indicates that CFIs with on-shell muons could appear in $e τ$ and $μτ$ neutrino mixing sectors in very high-density region ($\gtrsim 10^{13} {\rm g/cm^{3}}$), exhibiting a possibility of large impacts of CFIs on CCSN. Second, resonance-like CFIs, having a much higher growth rate than normal CFIs, can be triggered by muons. The resonance point of CFIs is different between $e τ$ and $μτ$ sectors; the former (latter) occurs at $μ_{e (μ)} = μ_{n} - μ_{p}$, where $μ_{i}$ denotes the chemical potential of $i$ constitute ($n$ and $p$ represent neutrons and protons, respectively). Our result suggests that the non-linear evolution of CFI with on-shell muons would induce flavor conversions with the complex interplay among all three different neutrino-mixing sectors.

hep-ph↗

arXiv2024

Collisional and Fast Neutrino Flavor Instabilities in Two-dimensional Core-collapse Supernova Simulation with Boltzmann Neutrino Transport

We present a comprehensive study on the occurrences of the collisional flavor instability (CFI) and the fast flavor instability (FFI) of neutrinos based on a two-dimensional (2D) core-collapse supernova (CCSN) simulation performed with a Boltzmann radiation hydrodynamics code. We find that CFI occurs in a region with the baryon-mass density of $10^{10}\lesssim ρ\lesssim 10^{12}\,\mathrm{g}\,\mathrm{cm}^{-3}$, which is similar to the previous results in one-dimensional (1D) CCSN models. In contrast to 1D, however, the CFI region varies with time vigorously in the 2D model, whereas it had a quiescent structure in 1D. This is attributed to the fact that the turbulent flows advected from a gain region account for the temporal variations. Another noticeable difference from the 1D models is the appearance of resonance-like CFI where number densities of $ν_e$, $\barν_e$ nearly coincide each other. The CFI growth rate there is enhanced and can reach $\sim10^{-3}\,\mathrm{cm}^{-1}$. As for FFI, on the other hand, it appears in three different regions; (1) the region overlapped with the resonance-like CFI, (2) neutrino decoupling regions where $\barν_e$'s are strongly emitted, and (3) optically thin regions where neutral-current scatterings dominate over charged-current reactions. Although overall properties for FFI are consistent with previous studies, we find that the number of electron-neutrinos lepton number crossing (ELN crossing) temporary becomes multiple, which can be assessed accurately only by multi-angle treatments in neutrino transport. We find that the growth rate of FFI is always higher than CFI if both of them occur, which suggests that the former is dominant for the linear evolution.

astro-ph.HE↗

arXiv2024

Detectability of Late-time Supernova Neutrinos with Fallback Accretion onto Protoneutron star

We investigate the late-time neutrino emission powered by fallback mass accretion onto proto-neutron star (PNS), using neutrino radiation-hydrodynamic simulations with full Boltzmann neutrino transport. We follow the time evolution of accretion flow onto PNS until the system reaches a steady state. A standing shock wave is commonly formed in the accretion flow, whereas the shock radius varies depending on mass accretion rate and PNS mass. A sharp increase in temperature emerges in the vicinity of PNS ($\sim 10$ km), which characterizes neutrino emission. Both neutrino luminosity and average energy become higher with increasing mass accretion rate and PNS mass. The mean energy of emitted neutrinos is in the range of $10\lesssimε\lesssim20\,\mathrm{MeV}$, which is higher than that estimated from PNS cooling models ($\lesssim10\,\mathrm{MeV}$). Assuming a distance to core-collapse supernova of $10\,\mathrm{kpc}$, we quantify neutrino event rates for Super-Kamiokande (Super-K) and DUNE. The estimated detection rates are well above the background, and their energy-dependent features are qualitatively different from those expected from PNS cooling models. Another notable feature is that the neutrino emission is strongly flavor dependent, exhibiting that the neutrino event rate hinges on the neutrino oscillation model. We estimate them in the case with adiabatic Mikheev-Smirnov-Wolfenstein model, and show that the normal- and inverted mass hierarchy offer the large number of neutrino detection in Super-K and DUNE, respectively. Hence the simultaneous observation with Super-K and DUNE of the fallback neutrinos will provide a strong constraint on neutrino mass hierarchy.

astro-ph.HE↗

arXiv2023

Universality of the Neutrino Collisional Flavor Instability in Core Collapse Supernovae

Neutrinos are known to undergo flavor conversion processes among the three flavors. The fast flavor conversion (FFC) has been the central piece of flavor conversions taking place in core-collapse supernovae (CCSNe) due to its shorter timescale to the completion of flavor conversion compared to other types of flavor conversion. Although the ordinary collisions between neutrinos and matter were once thought to decohere neutrinos and thus damp flavor conversions, it was recently realized that they can also induce the flavor conversion. The linear analysis showed that the so-called collisional flavor instability or CFI occurs in the absence of FFC. In this paper, we investigate if CFI takes place in of the post-bounce core of CCSNe, using the results of spherically symmetric Boltzmann simulations of CCSNe for four progenitor models with different masses. We also provide a necessary (but not sufficient) condition of matter properties for the occurrence of CFI in optically thick and semi-transparent regions; baryon mass density ($ρ$), electron fraction ($Y_e$), and the degeneracy of electron-type neutrinos ($η_{ν_e}$) need to be $10^{10} {\rm g/cm^3} \lesssim ρ\lesssim 10^{12} {\rm g/cm^3}$, $Y_e\lesssim 0.4$, and $η_{ν_e} \lesssim 0.5$, respectively. This condition allows us to easily locate the place of possible CFI occurence without detailed stability analyses, which is useful for analyzing CFI in CCSN models phenomenologically

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

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↗