arXiv subjects

Ken'ichiro Nakazato

Publications and source records attributed to Ken'ichiro Nakazato.

At least 19 recordsLinked to original sources

arXiv2026

Influence of effective mass of the relativistic mean field theory on core collapse supernovae and compact objects

We study the influence of the effective mass in the relativistic mean field (RMF) theory on the properties of the central core of collapse-driven supernovae and the formation of compact objects. Influence of the effective mass has been so far studied within the non-relativistic frameworks. In order to clarify the role of the effective mass in the relativistic frameworks, which is different from non-relativistic ones, we adopt the set of equation of state (EOS) tables using the parameterizations TM1e and TM1m, which have different effective masses but with the same saturation properties, in the RMF theory. We show that choices of the effective mass in supernova matter affect both the stiffness of the EOS through pressure and the thermodynamical behavior through temperature under the RMF frameworks. We explore differences in matter evolution with neutrino emissions by performing a set of numerical simulations of the gravitational collapse and bounce of massive stars and the cooling of the proto-neutron stars. The EOS with large effective mass leads to compact proto-neutron stars and early collapse to black holes with high densities and temperatures due to the softness. It leads to high energy neutrinos in long emission from the proto-neutron star cooling and in short burst from the black hole formation.

astro-ph.HE↗

arXiv2026

Paleodetectors for neutrino signals from diverse Galactic stellar collapses

The detectability of neutrinos from past Galactic core collapse supernovae (SNe) is investigated in terms of nuclear recoil tracks in ancient minerals, known as paleodetectors. To account for the diversity of core-collapse outcomes, variations in neutron star (NS) masses as well as failed SNe leading to black hole (BH) formation are taken into account. The role of the nuclear equation of state (EOS) is also considered, as it determines NS radii and maximum masses. The enhancement of sensitivity is quantified for models with larger neutrino emission. This emission reflects the gravitational energy released during core collapse and is larger for more compact remnants in the NS-forming case and for EOS with larger maximum masses in the BH-forming case. Furthermore, motivated by the hypothesis that nearby SN activity may have contributed to the snowball Earth events, the sensitivity to SNe occuring in a burst-like manner is also investigated. The results indicate that burst-like SN activity involving several tens of events at a distance of 10 pc would be within the reach of paleodetectors.

astro-ph.HE↗

arXiv2026

From supernovae to neutron stars: crust formation time

A neutron star is born as a hot, lepton-rich protoneutron star (PNS) and cools via neutrino emission, eventually allowing heavy ions in the outer layers to crystallize into a solid crust. We develop a simple analytic estimate for the onset time of this crust formation during the late, post-convective PNS cooling phase. Using a diffusion-based neutrino luminosity and the resulting entropy evolution together with an approximately isentropic interior structure, we obtain the time-dependent density and temperature at the neutrinosphere. We then impose the Coulomb crystallization condition for heavy nuclei, expressed through the Coulomb coupling parameter, and determine when the neutrinosphere temperature first falls below the crystallization threshold evaluated at the neutrinosphere density. This procedure yields closed expressions for the entropy at crystallization and the corresponding crust-formation time, with explicit dependence on the PNS mass and radius, an effective diffusion/cooling normalization, and composition parameters such as the ionic charge $Z$ and heavy-nuclei mass fraction. For canonical microphysics, we find that the first solid phase typically appears at $t_{\mathrm{crust}}\sim 100$-$500\,\mathrm{s}$. These closed-form scalings provide a useful late-time analytic benchmark for the onset of crust formation and clarify its dependence on PNS and composition parameters.

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

Role of symmetry energy at subnuclear densities in protoneutron star crusts

The impact of matter properties at subnuclear densities on the evolution of protoneutron stars is investigated. Several models of nuclear equation of state (EOS) are constructed with varying saturation parameters, particularly the symmetry energy $S_0$ and its density slope $L$. Using the Thomas--Fermi approximation, the mass and proton numbers of heavy nuclei at subnuclear densities are systematically evaluated, along with their dependence on the EOS. Cooling simulations of protoneutron stars reveal that EOSs with smaller $L$ values lead to a longer cooling timescale and higher average neutrino energies. This behavior is attributed to the enhanced neutrino scattering caused by larger mass numbers, which increases the thermal insulation. Furthermore, the crystallization temperature, marking the onset of crust formation, is found to be higher for EOSs with smaller values of $L$. This is due to the enhanced Coulomb energy associated with larger proton numbers. As a result, despite slower cooling, crust formation occurs earlier for smaller-$L$ EOSs. These findings indicate that the timing of crust formation is sensitive to the EOS and highlight the importance of late-time neutrino observations as probes of the matter properties at subnuclear densities.

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

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

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

Study of the neutrino-oxygen cross sections of the charged-current reaction 16O($\barν_e,e^+$)16N(0 MeV,$2^-$) and the neutral-current reaction 16O($ν,ν$')16O(12.97/12.53 MeV,$2^-$), producing high-energy gamma rays

In the previous work, we discussed the cross section and the detection of 4.4-MeV $γ$ rays produced in the neutrino neutral-current (NC) reaction $^{16}$O($ν, ν^{\prime}$)$^{16}$O(12.97 MeV and 12.53 MeV, $2^-$) in a water Cherenkov detector at the low energy below 100 MeV. In this report, we further investigated both the charged-current (CC) reaction $^{16}$O($\barν_e, e^+$)$^{16}$N(0 MeV, $2^-$) and the NC reaction$^{16}$O($ν, ν^{\prime}$)$^{16}$O(12.97 MeV and 12.53 MeV, $2^-$), producing high-energy $γ$ rays, in which the more solid identification of the reactions can be applied via the coincidence method.

nucl-th↗

arXiv2023

Diffuse Neutrino Flux Based on the Rates of Core-collapse Supernovae and Black Hole Formation Deduced from a Novel Galactic Chemical Evolution Model

Fluxes of the diffuse supernova neutrino background (DSNB) are calculated based on a new modeling of galactic chemical evolution, where a variable stellar initial mass function (IMF) depending on the galaxy type is introduced and black hole (BH) formation from the failed supernova is considered for progenitors heavier than 18$M_{\odot}$. The flux calculations are performed for different combinations of the star formation rate, nuclear equation of state, and neutrino mass hierarchy to examine the systematic effects from these factors. In any case, our new model predicts the enhanced DSNB $\barν_{e}$ flux at $E_ν\gtsim 30$~MeV and $E_ν\ltsim 10$~MeV due to more frequent BH formation and a larger core-collapse rate at high redshifts in early-type galaxies, respectively. Event rate spectra of the DSNB $\barν_{e}$ at a detector from the new model are shown and the detectability at water-based Cherenkov detectors, Super-Kamiokande with a gadolinium dissolution and Hyper-Kamiokande, is discussed. In order to investigate the impacts of the assumptions in the new model, we prepare alternative models, based on different IMF forms and treatments of BH formation, and estimate the discrimination capabilities between the new and alternative models at these detectors.

astro-ph.HE↗

arXiv2023

Neutrino-induced reactions on 18O and implications of 18O mixture in water Cherenkov detectors on supernova neutrino events

Neutrino-nucleus reaction cross sections on 18O are evaluated by shell-model calculations and compared with those on 16O. Important contributions from Gamow-Teller transitions are noticed for 18O (ν_e, e^{-})18F in contrary to the case for 16O, where spin-dipole transitions are dominant contributions. Calculated cross sections for 18O(ν_e, e^{-})18F are shown to be larger than for 16O at low neutrino energies below 20 MeV in natural water with the 0.205% admixture of 18O due to the lower threshold energy (1.66 MeV) for 18O than that for 16O (15.42 MeV). The resulting electron spectra, that is, the cross sections as functions of emitted electron energy Te, are also shown to be quite different, reflecting the different threshold energies. The electron spectra from (ν_e, e^{-}) reactions on 18O and 16O in water Cherenkov detectors for supernova neutrino detection are investigated for both the cases with and without the neutrino oscillation and compared with those of the neutrino-electron scattering. It has been shown that the contribution from 18O (0.205% mixture) enhances the rates from 16O by 60% for the case without the oscillation and by 20-30% for the case with the oscillation below Te =20 MeV. For the case with the neutrino oscillation, the event rates for 18O and 16O become comparable to those of the neutrino-electron scattering. However, their rates at low energy (Te < 20 MeV) are much smaller than those of the neutrino-electron scattering, which is important for the pointing accuracy to the supernova direction.

nucl-th↗

arXiv2023

Observing Supernova Neutrino Light Curves with Super-Kamiokande. IV. Development of SPECIAL BLEND: a New Public Analysis Code for Supernova Neutrinos

Supernova neutrinos are invaluable signals that offer information about the interior of supernovae. Because a nearby supernova can occur at any time, preparing for future supernova neutrino observation is an urgent task. For the prompt analysis of supernova neutrinos, we have developed a new analysis code, ``Supernova Parameter Estimation Code based on Insight on Analytic Late-time Burst Light curve at Earth Neutrino Detector (SPECIAL BLEND)''. This code estimates the parameters of supernova based on an analytic model of supernova neutrinos from the proto-neutron star cooling phase. For easy availability to the community, this code is public and easily runs on web environments. SPECIAL BLEND can estimate the parameters better than the analysis pipeline we developed in the previous paper. By using SPECIAL BLEND, we can estimate the supernova parameters within $10\%$ precision up to $\sim 20\,{\rm kpc}$ and $\sim 60\,{\rm kpc}$ (Large Magellanic Cloud contained) with Super Kamiokande and Hyper Kamiokande, respectively.

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

Detection of the 4.4-MeV gamma rays from $^{16}$O($ν, ν^{\prime}$)$^{16}$O(12.97 ${\rm MeV}, 2^-)$ with a water-Cherenkov detector in the supernova neutrino bursts

We first discuss and determine the isospin mixing of the two $2^-$ states (12.53 MeV and 12.97 MeV) of $^{16}$O nucleus using the inelastic electron scattering data. We then evaluate the cross section of 4.4-MeV $γ$ rays produced in the neutrino neutral-current (NC) reaction $^{16}$O($ν, ν^{\prime}$)$^{16}$O$(12.97 {\rm MeV}, 2^-$) in a water Cherenkov detector at the low energy below 100 MeV. The detection of $γ$ rays for $E_γ>5$ MeV from the NC reaction $^{16}$O($ν, ν^{\prime}$)$^{16}$O$(E_x>16\ {\rm MeV}, T=1$) with a water Cherenkov detector in the supernova neutrino bursts has been proposed and discussed by several authors previously. In this article, we discuss a new NC reaction channel from $^{16}$O(12.97 ${\rm MeV}, 2^-$) producing a 4.4-MeV $γ$ ray, the cross section of which is more robust and even larger at the low energy ($E_ν<25$ MeV) than the NC cross section from $^{16}$O$(E_x>16\ {\rm MeV}, T=1$). We also evaluate the number of such events induced by neutrinos from supernova explosion which can be observed by the Super-Kamiokande, a 32 kton water Cherenkov detector in the Earth.

astro-ph.HE↗

arXiv2022

Exploring the Fate of Stellar Core Collapse with Supernova Relic Neutrinos

Core collapse of massive stars leads to different fates for various physical factors, which gives different spectra of the emitted neutrinos. We focus on the supernova relic neutrinos (SRNs) as a probe to investigate the stellar collapse fate. We present the SRN fluxes and event rate spectra at a detector for three resultant states after stellar core collapse, the typical mass neutron star, the higher mass neutron star, or the failed supernova forming a black hole, based on different nuclear equations of state. Then possible SRN fluxes are formed as mixtures of the three components. We also show the expected sensitivities at the next-generation water-based Cherenkov detectors, SK-Gd and Hyper-Kamiokande, as constraining the mixture fractions. This study provides a practical example of extracting astrophysical constraints through SRN measurement.

astro-ph.HE↗

arXiv2022

Observing Supernova Neutrino Light Curves with Super-Kamiokande. III. Extraction of Mass and Radius of Neutron Stars from Synthetic Data

Neutrinos are guaranteed to be observable from the next galactic supernova (SN). Optical light and gravitational waves are also observable, but may be difficult to observe if the location of the SN in the galaxy or the details of the explosion are unsuitable. The key to observing the next supernova is to first use neutrinos to understand various physical quantities and then link them to other signals. In this paper, we present Monte Carlo sampling calculations of neutrino events from galactic supernova explosions observed with Super-Kamiokande. The analytical solution of neutrino emission, which represents the long-term evolution of neutrino-light curve from supernovae, is used as a theoretical template. It gives the event rate and event spectrum through inverse beta decay interactions with explicit model parameter dependence. Parameter estimation is performed on these simulated sample data by fitting least squares using the analytical solution. The results show that the mass, radius and total energy of a remnant neutron star produced by a SN can be determined with an accuracy of $\sim 0.1M_\odot$, $\sim 1$ km, and $\sim 10^{51}$ erg, respectively, for a galactic SN at 8 kpc.

astro-ph.HE↗

arXiv2022

Observing Supernova Neutrino Light Curves with Super-Kamiokande. II. Impact of the Nuclear Equation of State

The late-time evolution of the neutrino event rate from supernovae is evaluated for Super-Kamiokande using simulated results of proto-neutron star (PNS) cooling. In the present work we extend the result of Suwa et al. (2019) [arXiv:1904.09996], which studied the dependence on the PNS mass, but focus on the impact of the nuclear equation of state (EOS). We find that the neutrino event rate depends on both the high-density and low-density EOS, where the former determines the radius of the PNS and the latter affects its surface temperature. Based on the present evaluation of the neutrino event rate we propose a new analysis method to extract the time variability of the neutrino average energy taking into account the statistical error in the observation.

astro-ph.HE↗

arXiv2021

Numerical Study of Stellar Core Collapse and Neutrino Emission Using the Nuclear Equation of State Obtained by the Variational Method

Core-collapse simulations of massive stars are performed using the equation of state (EOS) based on the microscopic variational calculation with realistic nuclear forces. The progenitor models with the initial masses of $15M_\odot$, $9.6M_\odot$, and $30M_\odot$ are adopted as examples of the ordinary core-collapse supernova with a shock stall, the low-mass supernova with a successful explosion, and the black hole formation, respectively. Moreover, the neutrinos emitted from the stellar collapse are assessed. Then, the variational EOS is confirmed to work well in all cases. The EOS dependences of the dynamics, thermal structure, and neutrino emission of the stellar collapse are also investigated.

astro-ph.HE↗