arXiv subjects

Yudai Suwa

Publications and source records attributed to Yudai Suwa.

At least 37 records · Page 2Linked to original sources

arXiv2022

On the energy source of ultra-stripped supernovae

Ultra-stripped supernovae (USSN) with a relatively low ejecta mass of $\sim0.1M_\odot$ (e.g., iPTF14gqr and SN2019dge) are considered to originate from ultra-stripped carbon-oxygen (CO) cores in close binary systems and are likely to be progenitors of binary neutron stars (BNSs). Here we conduct long-term simulations of USSNe from ultra-stripped progenitors with various masses ($1.45\,M_\odot \leq M_\mathrm{CO} \leq 2.0\,M_\odot$) based on results of neutrino-radiation hydrodynamics simulations, and consistently calculate the nucleosynthesis and the SN light curves. We find that a USSN from a more massive progenitor has a larger ejecta mass but a smaller $^{56}$Ni mass mainly due to the fallback, which leads to the light curve being dimmer and slower. By comparing the synthetic light curves with the observed ones, we show that SN2019dge can be solely powered by $^{56}$Ni synthesized during the explosion of a progenitor with $M_\mathrm{CO} \lesssim 1.6\,M_\odot$ while iPTF14gqr cannot be explained by the $^{56}$Ni powered model; $\sim 0.05M_\odot$ of $^{56}$Ni inferred from the light curve fitting is argued to be difficult to synthesize for ultra-stripped progenitors. We consider fallback accretion onto and rotation-powered relativistic wind from the newborn NS as alternative energy sources and show that iPTF14gqr could be powered by a newborn NS with a magnetic field of $B_p \sim 10^{15}\,\mathrm{G}$ and an initial rotation period of $P_i \sim 0.1\,\mathrm{s}$.

astro-ph.HE↗

arXiv2022

Can Population III stars be major origins of both merging binary black holes and extremely metal poor stars?

Population (Pop) III stars, first stars, or metal-free stars are made of primordial gas. We have examined if they can be dominant origins of merging binary black holes (BHs) and extremely metal-poor stars. The abundance pattern of EMP stars is helpful to trace back the properties of Pop III stars. We have confirmed previous arguments that the observed BH merger rate needs Pop III star formation efficiency 10 times larger than theoretically predicted values, while the cosmic reionization history still permits such a high Pop III star formation efficiency. On the other hand, we have newly found that the elemental abundance pattern of EMP stars only allows the Pop III initial mass function with the minimum mass of $\sim 15 - 27$ $M_\odot$. In other words, the minimum mass must not deviate largely from the critical mass below and above which Pop III stars leave behind neutron stars and BHs, respectively. Pop III stars may be still a dominant origin of merging binary BHs but our study has reduced the allowed parameter space under a hypothesis that EMP stars are formed from primordial gas mixed with Pop III supernova ejecta.

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

Analytic solutions for neutrino-light curves of core-collapse supernovae

Neutrinos are a guaranteed signal from supernova explosions in the Milky Way, and a most valuable messenger that can provide us with information about the deepest parts of supernovae. In particular, neutrinos will provide us with physical quantities, such as the radius and mass of protoneutron stars (PNS), which are the central engine of supernovae. This requires a theoretical model that connects observables such as neutrino luminosity and average energy with physical quantities. Here, we show analytic solutions for the neutrino-light curve derived from the neutrino radiation transport equation by employing the diffusion approximation and the analytic density solution of the hydrostatic equation for a PNS. The neutrino luminosity and the average energy as functions of time are explicitly presented, with dependence on PNS mass, radius, the total energy of neutrinos, surface density, and opacity. The analytic solutions provide good representations of the numerical models from a few seconds after the explosion and allow a rough estimate of these physical quantities to be made from observational data.

astro-ph.HE↗

arXiv2020

Developing an end-to-end simulation framework of supernova neutrino detection

Massive stars can explode as supernovae at the end of their life cycle, releasing neutrinos whose total energy reaches $10^{53}$ erg. Moreover, neutrinos play key roles in supernovae, heating and reviving the shock wave as well as cooling the resulting protoneutron star. Therefore, neutrino detectors are waiting to observe the next galactic supernova and several theoretical simulations of supernova neutrinos are underway. While these simulation concentrate mainly on only the first one second after the supernova bounce, the only observation of a supernova with neutrinos, SN 1987A, revealed that neutrino emission lasts for more than 10 seconds. For this reason, long-time simulation and analysis tools are needed to compare theories with the next observation. Our study is to develop an integrated supernova analysis framework to prepare an analysis pipeline for treating galactic supernovae observations in the near future. This framework deals with the core-collapse, bounce and proto-neutron star cooling processes, as well as with neutrino detection on earth in a consistent manner. We have developed a new long-time supernova simulation in one dimension that explodes successfully and computes the neutrino emission for up to 20 seconds. Using this model we estimate the resulting neutrino signal in the Super-Kamiokande detector to be about 1,800 events for an explosion at 10 kpc and discuss its implications in this paper. We compare this result with the SN 1987A observation to test its reliability.

astro-ph.HE↗

arXiv2020

A Consistent Modeling of Neutrino-driven Wind with Accretion Flow onto a Protoneutron Star and its Implications for $^{56}$Ni Production

Details of the explosion mechanism of core-collapse supernovae (CCSNe) are not yet fully understood. There is now an increasing number of successful examples of reproducing explosions in the first-principles simulations, which have shown a slow increase of explosion energy. However, it was recently pointed out that the growth rates of the explosion energy of these simulations are insufficient to produce enough $^{56}$Ni mass to account for observations. We refer to this issue as the `nickel mass problem' (Ni problem, hereafter) in this paper. The neutrino-driven wind is suggested as one of the most promising candidates for the solution to the Ni problem in previous literature, but a multi-dimensional simulation for this is computationally too expensive to allow long-term investigations. In this paper, we first built a consistent model of the neutrino-driven wind with an accretion flow onto a protoneutron star (PNS), by connecting a steady-state solution of the neutrino-driven wind and a phenomenological mass accretion model. Comparing the results of our model with the results of first-principles simulations, we find that the total ejectable amount of the neutrino-driven wind is roughly determined within $\sim$ 1 sec from the onset of the explosion and the supplementable amount at a late phase ($t_e \gtrsim 1$ sec) remains $M_\mathrm{ej} \lesssim 0.01M_\odot$ at most. Our conclusion is that it is difficult to solve the Ni problem, by continuous injection of $^{56}$Ni by the neutrino-driven wind. We suggest that the total amount of synthesized $^{56}$Ni can be estimated robustly if simulations are followed up to $\sim 2$ seconds.

astro-ph.HE↗

arXiv2019

Kompaneets equation for neutrinos: Application to neutrino heating in supernova explosions

We derive a `Kompaneets equation' for neutrinos, which describes how the distribution function of neutrinos interacting with matter deviates from a Fermi-Dirac distribution with zero chemical potential. To this end, we expand the collision integral in the Boltzmann equation of neutrinos up to the second order in energy transfer between matter and neutrinos. The distortion of the neutrino distribution function changes the rate at which neutrinos heat matter, as the rate is proportional to the mean square energy of neutrinos, $E_ν^2$. For electron-type neutrinos the enhancement in $E_ν^2$ over its thermal value is given approximately by $E_ν^2/E_{ν,\rm thermal}^2=1+0.086(V/0.1)^2$ where $V$ is the bulk velocity of nucleons, while for the other neutrino species the enhancement is $(1+δ_v)^3$, where $δ_v=mV^2/3k_BT$ is the kinetic energy of nucleons divided by the thermal energy. This enhancement has a significant implication for supernova explosions, as it would aid neutrino-driven explosions.

astro-ph.HE↗

arXiv2019

Observing Supernova Neutrino Light Curves with Super-Kamiokande: Expected Event Number over 10 s

Supernova neutrinos are crucially important to probe the final phases of massive star evolution. As is well known from observations of SN1987A, neutrinos provide information on the physical conditions responsible for neutron star formation and on the supernova explosion mechanism. However, there is still no complete understanding of the long-term evolution of neutrino emission in supernova explosions, although there are a number of modern simulations of neutrino radiation hydrodynamics, which study neutrino emission at times less than one second after the bounce. In the present work we systematically calculate the number of neutrinos that can be observed in Super-Kamiokande over periods longer than ten seconds using the database of Nakazato et al. (2013) anticipating that neutrinos from a Galactic supernova can be detected for several tens of seconds. We find that for a supernova at a distance of 10 kpc, neutrinos remain observable for longer than 30 s for a low-mass neutron star ($1.20M_\odot$ gravitational mass) and even longer than 100 s for a high-mass neutron star ($2.05M_\odot$). These scenarios are much longer than the observations of SN1987A and longer than the duration of existing numerical simulations. We propose a new analysis method based on the cumulative neutrino event distribution as a function of reverse time from the last observed event, as a useful probe of the neutron star mass. Our result demonstrates the importance of complete modeling of neutrino light curves in order to extract physical quantities essential for understanding supernova explosion mechanisms, such as the mass and radius of the resulting neutron star.

astro-ph.HE↗

arXiv2019

On the minimum mass of neutron stars

We investigate remnant neutron star masses (in particular, the minimum allowed mass) by performing advanced stellar evolution calculations and neutrino-radiation hydrodynamics simulations for core-collapse supernova explosions. We find that, based on standard astrophysical scenarios, low-mass carbon-oxygen cores can have sufficiently massive iron cores that eventually collapse, explode as supernovae, and give rise to remnant neutron stars that have a minimum mass of 1.17 M$_\odot$ --- compatible with the lowest mass of the neutron star precisely measured in a binary system of PSR J0453+1559.

astro-ph.HE↗

arXiv2017

Supernova forecast with strong lensing

In the coming LSST era, we will observe $\mathcal{O}(100)$ of lensed supernovae (SNe). In this paper, we investigate possibility for predicting time and sky position of a supernova using strong lensing. We find that it will be possible to predict the time and position of the fourth image of SNe which produce four images by strong lensing, with combined information from the three previous images. It is useful to perform multi-messenger observations of the very early phase of supernova explosions including the shock breakout.

astro-ph.HE↗

arXiv2017

Explosive nucleosynthesis of ultra-stripped Type Ic supernovae: application to light trans-iron elements

We investigate the explosive nucleosynthesis during two dimensional neutrino-driven explosion of ultra-stripped Type Ic supernovae evolved from 1.45 and 1.5 M$_\odot$ CO stars. These supernovae explode with the explosion energy of $\sim 10^{50}$ erg and release $\sim 0.1$ M$_\odot$ ejecta. The light trans-iron elements Ga-Zr are produced in the neutrino-irradiated ejecta. The abundance distribution of these elements has a large uncertainty because of the uncertainty of the electron fraction of the neutrino-irradiated ejecta. The yield of the elements will be less than 0.01 M$_\odot$. Ultra-stripped supernova and core-collapse supernova evolved from a light CO core can be main sources of the light trans-iron elements. They could also produce neutron-rich nuclei $^{48}$Ca. The ultra-stripped supernovae eject $^{56}$Ni of $\sim$ 0.006 - 0.01 M$_\odot$. If most of neutrino-irradiated ejecta is proton-rich, $^{56}$Ni will be produced more abundantly. The light curves of these supernovae indicate sub-luminous fast decaying explosion with the peak magnitude of about $-15$ - $-16$. Future observations of ultra-stripped supernovae could give a constraint to the event rate of a class of neutron star mergers.

astro-ph.HE↗

arXiv2017

Magnetar-Powered Supernovae in Two Dimensions. II. Broad-Line Supernovae Ic

Nascent neutron stars with millisecond periods and magnetic fields in excess of $10^{16}$ Gauss can drive highly energetic and asymmetric explosions known as magnetar-powered supernovae. These exotic explosions are one theoretical interpretation for supernovae Ic-BL which are sometimes associated with long gamma-ray bursts. Twisted magnetic field lines extract the rotational energy of the neutron star and release it as a disk wind or a jet with energies greater than 10$^{52}$ erg over $\sim 20$ sec. What fractions of the energy of the central engine go into the wind and the jet remain unclear. We have performed two-dimensional hydrodynamical simulations of magnetar-powered supernovae (SNe) driven by disk winds and jets with the CASTRO code to investigate the effect of the central engine on nucleosynthetic yields, mixing, and light curves. We find that these explosions synthesize less than 0.05 Msun of Ni and that this mass is not very sensitive to central engine type. The morphology of the explosion can provide a powerful diagnostic of the properties of the central engine. In the absence of a circumstellar medium these events are not very luminous, with peak bolometric magnitudes $M_b \sim -16.5 $ due to low Ni production.

astro-ph.HE↗

arXiv2017

Magnetar Broadband X-ray Spectra Correlated with Magnetic Fields: Suzaku Archive of SGRs and AXPs Combined with NuSTAR, Swift, and RXTE

Studies were made of the 1-70 keV persistent spectra of fifteen magnetars as a complete sample observed with Suzaku from 2006 to 2013. Combined with early NuSTAR observations of four hard X-ray emitters, nine objects showed a hard power-law emission dominating at $\gtrsim$10 keV with the 15--60 keV flux of $\sim$1-$11\times 10^{-11}$ ergs s$^{-1}$ cm$^{-2}$. The hard X-ray luminosity $L_{\rm h}$, relative to that of a soft-thermal surface radiation $L_{\rm s}$, tends to become higher toward younger and strongly magnetized objects. Updated from the previous study, their hardness ratio, defined as $ξ=L_{\rm h}/L_{\rm s}$, is correlated with the measured spin-down rate $\dot{P}$ as $ξ=0.62 \times (\dot{P}/10^{-11}\,{\rm s}\,{\rm s}^{-1})^{0.72}$, corresponding with positive and negative correlations of the dipole field strength $B_{\rm d}$ ($ξ\propto B_{\rm d}^{1.41}$) and the characteristic age $τ_{\rm c}$ ($ξ\propto τ_{\rm c}^{-0.68}$), respectively. Among our sample, five transients were observed during X-ray outbursts, and the results are compared with their long-term 1-10 keV flux decays monitored with Swift/XRT and RXTE/PCA. Fading curves of three bright outbursts are approximated by an empirical formula used in the seismology, showing a $\sim$10-40 d plateau phase. Transients show the maximum luminosities of $L_{\rm s}$$\sim$$10^{35}$ erg s$^{-1}$, which is comparable to those of the persistently bright ones, and fade back to $\lesssim$$10^{32}$ erg s$^{-1}$. Spectral properties are discussed in a framework of the magnetar hypothesis.

astro-ph.HE↗

arXiv2017

Importance of $^{56}$Ni production on diagnosing explosion mechanism of core-collapse supernova

$^{56}$Ni is an important indicator of the supernova explosions, which characterizes light curves. Nevertheless, rather than $^{56}$Ni, the explosion energy has often been paid attention from the explosion mechanism community, since it is easier to estimate from numerical data than the amount of $^{56}$Ni. The final explosion energy, however, is difficult to estimate by detailed numerical simulations because current simulations cannot reach typical timescale of saturation of explosion energy. Instead, the amount of $^{56}$Ni converges within a short timescale so that it would be a better probe of the explosion mechanism. We investigated the amount of $^{56}$Ni synthesized by explosive nucleosynthesis in supernova ejecta by means of numerical simulations and an analytic model. For numerical simulations, we employ Lagrangian hydrodynamics code in which neutrino heating and cooling terms are taken into account by light-bulb approximation. Initial conditions are taken from Woosley & Hegel (2007), which have 12, 15, 20, and 25 $M_\odot$ in zero age main sequence. We additionally develop an analytic model, which gives a reasonable estimate of the amount of $^{56}$Ni. We found that, in order to produce enough amount of $^{56}$Ni, $\mathcal{O}(1)$ Bethe s$^{-1}$ of growth rate of the explosion energy is needed, which is much larger than that found in recent exploding simulations, typically $\mathcal{O}(0.1)$ Bethe s$^{-1}$.

astro-ph.HE↗

arXiv2016

Three-dimensional simulations of rapidly rotating core-collapse supernovae: finding a neutrino-powered explosion aided by non-axisymmetric flows

We report results from a series of three-dimensional (3D) rotational core-collapse simulations for $11.2$ and 27 $M_{/odot}$ stars employing neutrino transport scheme by the isotropic diffusion source approximation. By changing the initial strength of rotation systematically, we find a rotation-assisted explosion for the 27$M_{/odot}$ progenitor, which fails in the absence of rotation. The unique feature was not captured in previous two-dimensional (2D) self-consistent rotating models because the growing non-axisymmetric instabilities play a key role. In the rapidly rotating case, strong spiral flows generated by the so-called low $T/|W|$ instability enhance the energy transport from the proto-neutron star (PNS) to the gain region, which makes the shock expansion more energetic. The explosion occurs more strongly in the direction perpendicular to the rotational axis, which is different from previous 2D predictions.

astro-ph.HE↗

arXiv2016

Parametric initial conditions for core-collapse supernova simulations

We investigate a method to construct parametrized progenitor models for core-collapse supernova simulations. Different from all modern core-collapse supernova studies, which rely on progenitor models from stellar evolution calculations, we follow the methodology of Baron & Cooperstein (1990) to construct initial models. Choosing parametrized spatial distributions of entropy and electron fraction as a function of mass coordinate and solving the equation of hydrostatic equilibrium, we obtain the initial density structures of our progenitor models. First, we calculate structures with parameters fitting broadly the evolutionary model s11.2 of Woosley et al. (2002). We then demonstrate the reliability of our method by performing general relativistic hydrodynamic simulations in spherical symmetry with the isotropic diffusion source approximation to solve the neutrino transport. Our comprehensive parameter study shows that initial models with a small central entropy ($\lesssim 0.4\,k_B$ nucleon$^{-1}$) can explode even in spherically symmetric simulations. Models with a large entropy ($\gtrsim 6\,k_B$ nucleon$^{-1}$) in the Si/O layer have a rather large explosion energy ($\sim 4\times 10^{50}$ erg) at the end of the simulations, which is still rapidly increasing.

astro-ph.HE↗

arXiv2015

The criterion of supernova explosion revisited: the mass accretion history

By performing neutrino-radiation hydrodynamic simulations in spherical symmetry (1D) and axial symmetry (2D) with different progenitor models by Woosley & Heger (2007) from 12 $M_{\odot}$ to 100 $M_{\odot}$, we find that all 1D runs fail to produce an explosion and several 2D runs succeed. The difference in the shock evolutions for different progenitors can be interpreted by the difference in their mass accretion histories, which are in turn determined by the density structures of progenitors. The mass accretion history has two phases in the majority of the models: the earlier phase in which the mass accretion rate is high and rapidly decreasing and the later phase with a low and almost constant accretion rate. They are separated by the so-called turning point, the origin of which is a change of the accreting layer. We argue that shock revival will most likely occur around the turning point and hence that its location in the $\dot M$-$L_ν$ plane will be a good measure for the possibility of shock revival: if the turning point lies above the critical curve and the system stays there for a long time, shock revival will obtain. In addition, we develop a phenomenological model to approximately evaluate the trajectories in the $\dot M$-$L_ν$ plane, which, after calibrating free parameters by a small number of 1D simulations, reproduces the location of the turning point reasonably well by using the initial density structure of progenitor alone. We suggest the application of the phenomenological model to a large collection of progenitors in order to infer without simulations which ones are more likely to explode.

astro-ph.HE↗

arXiv2015

Neutrino-driven explosions of ultra-stripped type Ic supernovae generating binary neutron stars

We study explosion characteristics of ultra-stripped supernovae (SNe), which are candidates of SNe generating binary neutron stars (NSs). As a first step, we perform stellar evolutionary simulations of bare carbon-oxygen cores of mass from 1.45 to 2.0 $M_\odot$ until the iron cores become unstable and start collapsing. We then perform axisymmetric hydrodynamics simulations with spectral neutrino transport using these stellar evolution outcomes as initial conditions. All models exhibit successful explosions driven by neutrino heating. The diagnostic explosion energy, ejecta mass, Ni mass, and NS mass are typically $\sim 10^{50}$ erg, $\sim 0.1 M_\odot$, $\sim 0.01M_\odot$, and $\approx 1.3 M_\odot$, which are compatible with observations of rapidly-evolving and luminous transient such as SN 2005ek. We also find that the ultra-stripped SN is a candidate for producing the secondary low-mass NS in the observed compact binary NSs like PSR J0737-3039.

astro-ph.HE↗