arXiv subjects

Nobuya Nishimura

Publications and source records attributed to Nobuya Nishimura.

At least 19 recordsLinked to original sources

arXiv2026

Temperature-resolved sensitivities of $^{56}{\rm Ni}$ production to helium-burning reactions in pair-instability supernovae

We propose a temperature-resolved Monte Carlo (MC) approach to identify the temperature regimes in which low-energy helium-burning reaction rates most strongly affect nucleosynthesis in very massive stars that undergo pair-instability supernovae (PISNe). By performing MC simulations of PISNe, we quantify how temperature-dependent variations in key helium-burning reaction rates, i.e., the triple-$α$ and $^{12}{\rm C}(α,γ)^{16}{\rm O}$ rates, influence $^{56}{\rm Ni}$ synthesis. Thousands of stellar evolution calculations using $\texttt{MESA}$ reveal that both the $^{12}{\rm C}(α,γ)^{16}{\rm O}$ and triple-$α$ reactions exhibit their strongest sensitivity at $T \simeq 2.5 \times 10^{8}\,{\rm K}$, but with opposite correlation signs. We show that this temperature corresponds to the regime in which the ratio of the sampled rate multipliers is most clearly imprinted on the pre-carbon-burning C/O composition. This demonstrates that PISN nucleosynthesis can probe helium-burning reaction rates in specific low-temperature regimes.

astro-ph.SR↗

arXiv2026

Evidence of Nuclear Urca Process in the Ocean of Neutron-Star Superburst MAXI J1752$-$457

We propose that the rapid cooling of the neutron star following its X-ray superburst in MAXI J1752$-$457 over a period of 4 days, observed by two Japanese satellites, MAXI and NinjaSat, is due to enhanced neutrino emission from cycles of electron capture and $β^{-}$ decay involving odd-$A$ nuclei (or Urca pairs) in the ocean. Hence, this work provides the first indication of the possible existence of such a ``nuclear Urca process". The observation of MAXI J1752$-$457 implies a hot ignition layer with a maximum temperature of $\sim4~{\rm GK}$, located near the Urca shell in the ocean, such that the nuclear Urca process becomes dominant for up to $\sim2$ days after the superburst. This behavior is distinct from that of normal Type-I X-ray bursts, which are triggered by hydrogen or helium burning in much shallower layers than those of superbursts. Our findings enable probing of superburst ashes through Urca pairs via long-term monitoring of crust cooling on day-long timescales.

astro-ph.HE↗

arXiv2026

Uncertainties in the production of iron-group nuclides in core-collapse supernovae from Monte Carlo variations of reaction rates

Core-collapse supernovae, occurring at the end of massive star evolution, produce heavy elements, including those in the iron peak. Although the explosion mechanism is not yet fully understood, theoretical models can reproduce optical observations and observed elemental abundances. However, many nuclear reaction rates involved in explosive nucleosynthesis have large uncertainties, impacting the reliability of abundance predictions. To address this, we have previously developed a Monte Carlo-based nucleosynthesis code that accounts for reaction rate uncertainties and has been applied to nucleosynthesis processes beyond iron. Our framework is also well suited for studying explosive nucleosynthesis in supernovae. In this paper, we investigate 1D explosion models using the "PUSH method", focusing on progenitors with varying metallicities and initial masses around $M_\mathrm{ZAMS} = 16 M_{\odot}$. Detailed post-process nucleosynthesis calculations and Monte Carlo analyses are used to explore the effects of reaction rate uncertainties and to identify key reaction rates in explosive nucleosynthesis. We find that many reactions have little impact on the production of iron-group nuclei, as these elements are primarily synthesized in the nuclear statistical equilibrium. However, we identify a few "key reactions" that significantly influence the production of radioactive nuclei, which may affect astrophysical observables. In particular, for the production of ${}^{44}$Ti, we confirm that several traditionally studied nuclear reactions have a strong impact. However, determining a single reaction rate is insufficient to draw a definitive conclusion.

astro-ph.SR↗

arXiv2025

Return of the Clocked Burster: Exceptionally Short Recurrence Time in GS 1826-238

We report the discovery of an exceptionally short burst recurrence time in the well-known clocked burster GS 1826$-$238, observed with the CubeSat X-ray observatory NinjaSat. In 2025 May, GS 1826$-$238 underwent a soft-to-hard state transition for the first time in 10 years. On June 23, NinjaSat began monitoring GS 1826$-$238 in the hard state and continued until it returned to a steady soft state. During this period, we detected 19 X-ray bursts: 14 during the hard state, 4 in the transitional state, and 1 in the soft state. In the hard state, we identified a new clocked bursting epoch, during which the burst recurrence time remained highly stable and unprecedentedly short among the clocked bursting phases of GS 1826$-$238, with $t_{\rm rec} = 1.603 \pm 0.040$ hr ($1σ$ error). Previous observations showed that the burst recurrence time in GS 1826$-$238 decreased with increasing mass accretion rate, reached its minimum value of $t_{\rm rec} \sim 3$ hr, and then increased again. The observed 1.6 hr recurrence time is therefore exceptionally short, indicating anomalous ignition conditions. We propose that this phenomenon reflects fuel accumulation over a smaller fraction of the neutron star surface, resulting in a higher local accretion rate compared to earlier epochs. This scenario naturally accounts for the exceptionally short recurrence time, together with the observed reductions during bursts in blackbody normalization (proportional to the emitting area) and fluence. We also discuss possible contributions from residual heat in the neutron star crust or the presence of an additional soft spectral component.

astro-ph.HE↗

arXiv2025

Thermonuclear superburst of MAXI J1752$-$457 observed with NinjaSat and MAXI

An uncatalogued bright X-ray transient was detected with MAXI on November 9, 2024, named MAXI J1752$-$457. The NinjaSat X-ray observatory promptly observed the source from November 10 to 18 while the small angular separation from the Sun hampered follow-up campaigns by other X-ray observatories. The MAXI and NinjaSat light curves in the 2-10 keV band showed first and second decaying components at the early and late phases, approximated by exponential functions with e-folding constants of 1.2 $\pm$ 0.2 and 14.9 $\pm$ 0.9 hours (1$σ$ errors), respectively. A single blackbody model reproduces the X-ray spectrum with a softening trend of its temperature decreasing from 1.8 $\pm$ 0.1 keV to 0.59 $\pm$ 0.06 keV. Assuming the unknown source distance at 8 kpc, at which the initial X-ray luminosity roughly corresponds to the Eddington limit, the shrinking blackbody radius was estimated at 5-11 km. This X-ray brightening is interpreted as a superburst in a Galactic low-mass X-ray binary, which is powered by thermonuclear burning triggered presumably by carbon ignition close to the outer crust of the neutron star. The transition between two decaying components occurred at 5.5-7.7 hours, corresponding to the thermal time scale of the burning layer. The ignition column density is estimated to be (2.8-5.1)$\times 10^{12}$ g cm$^{-2}$.

astro-ph.HE↗

arXiv2025

Systematic study of the composition of Type I X-ray burst ashes: Neutron star structure v.s. Reaction rate uncertainties

In this study, we calculate for the first time the impacts of neutron star(NS) structure on the type I X-ray burst ashes using the \texttt{MESA} code. We find an increased mass fraction of the heavier elements with increasing surface gravity (increase mass or decrease radius), resulting in a higher average mass number ($A_{\rm ash}$) of burst ashes (except for higher mass NS due to the competition between the envelope temperature and the recurrence time). The burst strength ($α$) increases as surface gravity increases, which indicates the positive correlation between $A_{\rm ash}$ and $α$ with changes in surface gravity. If the $α$ value is higher, heavier $p$-nuclei should be produced by the type I X-ray burst nucleosynthesis. Besides, the effects of various burst input parameters, e.g. base heating ($Q_{\rm b}$), metallicity ($Z$) and some new reaction rates are calculated for comparison. We find that the heavier nuclei synthesis is inversely correlated to the base heating/metallicity, the smaller the base heating/metallicity, the greater the mass fraction of the heavier elements. The $α$ value decreases as $Q_{\rm b}$ or $Z$ decreases, which also indicates the positive correlation between $A_{\rm ash}$ and $α$ with variation in $Q_{\rm b}$ or $Z$. The new reaction rates from the $(p,γ)$ reactions on $^{17}\rm{F}$, $^{19}\rm{F}$, $^{26}\rm{P}$, $^{56}\rm{Cu}$, $^{65}\rm{As}$, and $(α,p)$ reaction on $^{22}\rm{Mg}$ have only minimal effects on burst ashes. In hydrogen-rich X-ray binary systems, nuclei heavier than $^{64}\rm{Ge}$ are fertile produced with larger NS mass, smaller NS radius, smaller base heating and smaller metallicity.

astro-ph.HE↗

arXiv2025

NinjaSat monitoring of Type-I X-ray bursts from the clocked burster SRGA J144459.2$-$604207

The CubeSat X-ray observatory NinjaSat was launched on 2023 November 11 and has provided opportunities for agile and flexible monitoring of bright X-ray sources. On 2024 February 23, the NinjaSat team started long-term observation of the new X-ray source SRGA J144459.2$-$604207 as the first scientific target, which was discovered on 2024 February 21 and recognized as the sixth clocked X-ray burster. Our 25-day observation covered almost the entire decay of this outburst from two days after the peak at $\sim$100 mCrab on February 23 until March 18 at a few mCrab level. The Gas Multiplier Counter onboard NinjaSat successfully detected 12 Type-I X-ray bursts with a typical burst duration of $\sim$20 s, shorter than other clocked burster systems. As the persistent X-ray emission declined by a factor of five, X-ray bursts showed a notable change in its morphology: the rise time became shorter from 4.4(7) s to 0.3(3) s (1$σ$ errors), and the peak amplitude increased by 44%. The burst recurrence time $Δt_{\rm rec}$ also became longer from 2 hr to 10 hr, following the relation of $Δt_{\rm rec} \propto F_{\rm per}^{-0.84}$, where $F_{\rm per}$ is the persistent X-ray flux, by applying a Markov chain Monte Carlo method. The short duration of bursts is explained by the He-enhanced composition of accretion matter and the relation between $Δt_{\textrm{rec}}$ and $F_{\rm per}$ by a massive neutron star. This study demonstrated that CubeSat pointing observations can provide valuable astronomical X-ray data.

astro-ph.HE↗

arXiv2024

Evidence of non-Solar elemental composition in the clocked X-ray burster SRGA J144459.2$-$604207

In February and March 2024, a series of many Type I X-ray bursts from the accreting neutron star SRGA J144459.2$-$604207, which has been identified by multiple X-ray satellites, with the first reports coming from INTEGRAL and NinjaSat. These observations reveal that after exhibiting very regular behavior as a ``clocked'' burster, the peak luminosity of the SRGA J144459.2$-$604207 X-ray bursts shows a gradual decline. The observed light curves exhibit a short plateau feature, potentially with a double peak, followed by a rapid decay in the tail-features unlike those seen in previously observed clocked bursters. In this study, we calculate a series of multizone X-ray burst models with various compositions of accreted matter, specifically varying the mass fractions of hydrogen ($X$), helium ($Y$), and heavier CNO elements or metallicity ($Z_{\rm CNO}$). We demonstrate that a model with higher $Z_{\rm CNO}$ and/or lower $X/Y$ compared to the solar values can reproduce the observed behavior of SRGA J144459.2$-$604207. Therefore, we propose that this new X-ray burster is likely the first clocked burster with non-solar elemental compositions. Moreover, based on the X-ray burst light curve morphology in the decline phase observed by NinjaSat, a He-enhanced model with $X/Y \approx 1.5$ seems preferred over high-metallicity cases. We also give a brief discussion on the implications for the neutron star mass, binary star evolution, inclination angle, and the potential for a high-metallicity scenario, the last of which is closely related to the properties of the hot CNO cycle.

astro-ph.HE↗

arXiv2023

Postfission properties of uranium isotopes: A hybrid method with Langevin dynamics and the Hauser-Feshbach statistical model

Background: Precise understanding of nuclear fission is crucial for experimental and theoretical nuclear physics, astrophysics, and industrial applications; however, the complete physical mechanics is unresolved due to the complexities. Purpose: In this study, we present a new method to describe the dynamical-fission process and following prompt-neutron emission, where we combine the dynamical fission calculation based on the Langevin method and the Hauser-Feshbach statistical model. Methods: Two methods are connected smoothly within the universal charge distribution and the energy conservation, allowing us to calculate a sequence of fission dynamics and post-fission phase, including prompt neutron emission. Results: Using a certain set of model parameters, we successfully reproduce the experimental primary-fission yields, total kinetic energy, independent-fission yields, and prompt neutron emissions for the neutron induced fission of ${}^{236}$U, a compound nucleus of ${\rm n} + {}^{235}{\rm U}$. We elucidate the physical mechanism of the characteristic features observed in previous experiments, such as shell properties. Additionally, we apply our calculation to two very neutron-rich uranium isotopes, i.e., ${}^{250}$U and ${}^{255}$U, which are not experimentally confirmed but are important for r-process nucleosynthesis. Theoretical results indicate that ${}^{250}$U exhibits an asymmetric multiple-peak fission yield distribution, while the neutron-rich ${}^{255}$U has a single peak due to symmetric fission. Our method predicts post-neutron emission fragments, where ${}^{250}$U shows a stronger neutron emissivity than ${}^{255}$U. Conclusions: Our framework is highly reproducible in the experiments and shows that the number of emitted neutrons after fission differs significantly in neutron-rich uranium fission depending on distributions of fission variables.

nucl-th↗

arXiv2023

Constraints on the Neutron-Star Structure from the Clocked X-Ray Burster 1RXS J180408.9$-$342058

Type-I X-ray bursts are rapid-brightening transient phenomena on the surfaces of accreting neutron stars (NSs). Some X-ray bursts, called {\it clocked bursters}, exhibit regular behavior with similar light curve profiles in their burst sequences. The periodic nature of clocked bursters has the advantage of constraining X-ray binary parameters and physics inside the NS. In the present study, we compute numerical models, based on different equations of state and NS masses, which are compared with the observation of a recently identified clocked burster, 1RXS J180408.9$-$342058. We find that the relation between accretion rate and recurrence time is highly sensitive to the NS mass and radius. We determine, in particular, that 1RXS J180408.9$-$342058 appears to possess a mass less than $1.7M_{\odot}$ and favors a stiffer nuclear equation of state (with an NS radius $\gtrsim12.7{\rm km}$). Consequently, the observations of this new clocked burster may provide additional constraints for probing the structure of NSs.

astro-ph.HE↗

arXiv2023

The Impacts of Neutron-Star Structure and Base Heating on Type I X-Ray Bursts and Code Comparison

Type I X-ray bursts are rapidly brightening phenomena triggered by thermonuclear burning on accreting layer of a neutron star (NS). The light curves represent the physical properties of NSs and the nuclear reactions on the proton-rich nuclei. The numerical treatments of the accreting NS and physics of the NS interior are not established, which shows uncertainty in modelling for observed X-ray light curves. In this study, we investigate theoretical X-ray-burst models, compared with burst light curves with GS~1826-24 observations. We focus on the impacts of the NS mass, the NS radius, and base-heating on the NS surface using the MESA code. We find a monotonic correlation between the NS mass and the parameters of the light curve. The higher the mass, the longer the recurrence time and the greater the peak luminosity. While the larger the radius, the longer the recurrence time, the peak luminosity remains nearly constant. In the case of increasing base heating, both the recurrence time and peak luminosity decrease. We also examine the above results using with a different numerical code, HERES, based on general relativity and consider the central NS. We find that the burst rate, burst energy and burst strength are almost same in two X-ray burst codes by adjusting the base-heat parameter in MESA (the relative errors $\lesssim5\%$), while the duration time and the rise time are significantly different between (the relative error is possibly $\sim50\%$). The peak luminosity and the e-folding time are ragged between two codes for different accretion rates.

astro-ph.HE↗

arXiv2023

Impacts of the direct URCA and Superfluidity inside a Neutron Star on Type-I X-Ray Bursts and X-Ray Superbursts

We investigate the impacts of neutrino cooling mechanism inside the neutron star (NS) core on the light curves of type-I X-ray bursts and X-ray superbursts. From several observations of NS thermal evolution, physical processes of fast neutrino cooling, such as the direct Urca (DU) process, are indicated. They significantly decrease the surface temperature of NSs, though the cooling effect could be suppressed by nucleon superfluidity. In the present study, focusing on the DU process and nucleon superfluidity, we investigate the effects of NS cooling on the X-ray bursts using a general-relativistic stellar-evolution code. We find that the DU process leads find the longer recurrence time and the higher peak luminosity, which could be obstructed by the neutrons superfluidity. We also apply our burst models to the comparison with {\it Clocked burster} GS 1826$-$24, and to the recurrence time of superburst triggered by carbon ignition. These effects are significant within a certain range of binary parameters and uncertainty of the NS equation of state.

astro-ph.HE↗

arXiv2022

New constraints on the neutron-star mass and radius relation from terrestrial nuclear experiments

The determination of the equation of state (EOS) for nuclear matter has been one of the biggest problems in nuclear astrophysics, because the EOS is essential for determining the properties of neutron stars. To constrain the density-dependence of the nuclear symmetry energy, several nuclear experiments, e.g., reported by the S$π$RIT and PREX-II collaborations, have recently been performed. However, since their uncertainties are still large, additional constraints such as astronomical observations must be crucial. In addition, it is interesting to see the effect of their reported value on neutron star properties. In this study, focusing on the relatively lower density region, we investigate the allowed area of the neutron-star mass and radius relation by assuming the constraints from S$π$RIT and PREX-II. Each region predicted by these experiments is still consistent with the allowed area constrained by the various astronomical observations. Our results show that terrestrial nuclear experiments must provide further constraints on the EOS for neutron stars, complementing astronomical observations.

nucl-th↗

arXiv2021

Fission fragment distributions of neutron-rich nuclei based on Langevin calculations: toward r-process simulations

The nuclear fission of very neuron-rich nuclei related to the r-process is essential for the termination of nucleosynthesis flows on the nuclear chart and the final abundances. Nevertheless, most of the available fission data for the r-process calculations are based on theory predictions, including phenomenological treatments. In this study, we calculated a series of nuclear fission distribution for neutron-rich nuclei away from the beta-stability line. As most of these nuclei are experimentally unknown, we are based on theoretical calculations based on the dynamical fission model with the Langevin method. We performed fission distribution calculations for neutron-rich actinoid nuclei, applicable to the r-process nucleosynthesis simulations. In the present paper, we compared the obtained mass and charge distributions with experimental data. We also show the results of the systematic behaviour of mass distribution for neutron-rich U and Fm isotopes.

nucl-th↗

arXiv2019

r-Process enrichment in the Galactic halo characterized by nucleosynthesis variation in the ejecta of coalescing neutron star binaries

A large star-to-star variation in the abundances of r-process elements, as seen in the [Eu/Fe] ratio for Galactic halo stars, is a prominent feature that is distinguishable from other heavy elements. It is, in part, caused by the presence of highly r-process enriched stars, classified as r-II stars ([Eu/Fe]>+1). In parallel, halo stars show that the ratio of a light r-process element (Y) to Eu is tightly correlated with [Eu/Fe], giving the lowest [Y/Eu] ratio that levels off at r-II stars. On the other hand, recent hydrodynamical simulations of coalescing double neutron stars (cNSNSs) have suggested that r-process sites may be separated into two classes providing different electron-fraction distributions: tidally-driven dynamical ejecta and (dynamical or postmerger) non-tidal ejecta. Here, we show that a widely spanning feature of [Eu/Fe] can be reproduced by models that consider the different masses of tidally-driven dynamical ejecta from both cNSNSs and coalescing black hole/neutron star binaries (cBHNSs). In addition, the observed [Y/Eu] trend is explained by the combined nucleosynthesis in two kinds of ejecta with varying mass asymmetry in double NS systems. Our scenario suggests that massive tidally-driven dynamical ejecta accompanied by massive non-tidal part from cNSNSs or cBHNSs could alone accommodate r-II abundances, including an actinide boost in some cases. The event rate for cNSNSs estimated from our study agrees with the latest result of ~1000 (90% confidence interval of 110-3840) Gpc$^{-3}$yr$^{-1}$ by gravitational-wave detection, and a few events per Gpc$^3$ per year of cBHNSs associated with r-process production are predicted to emerge.

astro-ph.GA↗

arXiv2018

Early chemical evolution of Zn driven by magnetorotational supernovae and the pathway to the solar Zn composition

The site of Zn production remains an elusive and challenging problem in astrophysics. A large enhancement of the [Zn/Fe] ratios of very metal-poor stars in the Galactic halo suggests the death of short-lived massive stars, i.e., core-collapse supernovae (CCSNe), as one major site for Zn production. Previous studies have claimed that some specific CCSNe can produce Zn in sufficient quantities. However, it remains unclear which models can withstand the critical test of observations. Using a Zn abundance feature similar to that of r-process elements in faint satellite galaxies, we find evidence that Zn production took place through much rarer events than canonical CCSNe. This finding can be unified with the implied decrease in the rate of Zn production with an increasing metallicity for Galactic halo stars, which narrows down the major site of Zn production in the early galaxy to magneto-rotational SNe (MR-SNe). On the other hand, in the later phase of galactic evolution, we predict that the major Zn-production site switched from MR-SNe to thermonuclear SNe (SNe Ia). According to this scenario, an accumulation of the contributions from two types of SNe eventually led to the solar isotope composition of Zn which mainly owes 66,68Zn to MR-SNe and 64Zn to SNe Ia triggered by He-detonation. The requirement of Zn production in SNe Ia sheds a new light on the hot debate on the scenario for SN Ia progenitors, suggesting that a He-detonation model might be one major channel for SNe Ia.

astro-ph.GA↗

arXiv2018

Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-Radiation Hydrodynamics Study

We perform long-term general relativistic neutrino radiation hydrodynamics simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a torus, which is a canonical remnant formed after the binary neutron star merger. We take into account the effects of viscosity, which is likely to arise in the merger remnant due to magnetohydrodynamical turbulence. As the initial condition, we employ the azimuthally averaged data of the MNS-torus system derived in a three-dimensional, numerical-relativity simulation for the binary neutron star merger. The viscous effect plays key roles for the remnant evolution and mass ejection from it in two phases of the evolution. In the first $t\lesssim10$ ms, a differential rotation state of the MNS is changed to a rigidly rotating state, and as a result, a sound wave, which subsequently becomes a shock wave, is formed in the vicinity of the MNS due to the variation of the quasi-equilibrium state of the MNS. The shock wave induces significant mass ejection of mass $\sim(0.5-2.0)\times 10^{-2}M_\odot$ for the alpha viscosity parameter of $0.01-0.04$. For the longer-term evolution with $\sim 0.1-10$ s, a significant fraction of the torus material is ejected. The ejecta mass is likely to be of order $10^{-2}M_\odot$, so that the total mass of the viscosity-driven ejecta could dominate that of the dynamical ejecta of mass $\lesssim 10^{-2}M_\odot$. The electron fraction, $Y_e$, of the ejecta is always high enough ($Y_e\gtrsim0.25$) that this post-merger ejecta is lanthanide-poor; hence, the opacity of the ejecta is likely to be $\sim 10-100$ times lower than that of the dynamical ejecta. This indicates that the electromagnetic signal from the ejecta would be rapidly evolving, bright, and blue if it is observed from a small viewing angle ($\lesssim 45^\circ$) for which the effect of the dynamical ejecta is minor.

astro-ph.HE↗

arXiv2017

Uncertainties in s-process nucleosynthesis in massive stars determined by Monte Carlo variations

The $s$-process in massive stars produces the weak component of the $s$-process (nuclei up to $A \sim 90$), in amounts that match solar abundances. For heavier isotopes, such as barium, production through neutron capture is significantly enhanced in very metal-poor stars with fast rotation. However, detailed theoretical predictions for the resulting final $s$-process abundances have important uncertainties caused both by the underlying uncertainties in the nuclear physics (principally neutron capture reaction and $β$-decay rates) as well as by the stellar evolution modeling. In this work, we investigated the impact of nuclear-physics uncertainties relevant to the $s$-process in massive stars. Using a Monte-Carlo based approach, we performed extensive nuclear reaction network calculations that include newly evaluated upper and lower limits for the individual temperature dependent reaction rates. We found that most of the uncertainty in the final abundances is caused by uncertainties in the neutron capture rates, while $β$-decay rate uncertainties affect only a few nuclei near $s$-process branchings. The $s$-process in rotating metal-poor stars shows quantitatively different uncertainties and key reactions, although the qualitative characteristics are similar. We confirmed that our results do not significantly change at different metallicities for fast rotating massive stars in the very low metallicity regime. We highlight which of the identified key reactions are realistic candidates for improved measurement by future experiments.

astro-ph.SR↗