arXiv subjects

Kei Kotake

Publications and source records attributed to Kei Kotake.

At least 37 records · Page 2Linked to original sources

arXiv2022

3D template-based $Fermi$-LAT constraints on the diffuse supernova axion-like particle background

Axion-like particles (ALPs) may be abundantly produced in core-collapse (CC) supernovae (SNe), hence the cumulative signal from all past SN events can create a diffuse flux peaked at energies of about 25~MeV. We improve upon the modeling of the ALPs flux by including a set of CC SN models with different progenitor masses, as well as the effects of failed CC SNe -- which yield the formation of black holes instead of explosions. Relying on the coupling strength of ALPs to photons and the related Primakoff process, the diffuse SN ALP flux is converted into gamma rays while traversing the magnetic field of the Milky Way. The spatial morphology of this signal is expected to follow the shape of the Galactic magnetic field lines. We make use of this via a template-based analysis that utilizes 12 years of $Fermi$-LAT data in the energy range from 50 MeV to 500 GeV. In our benchmark case of the realization of astrophysical and cosmological parameters, we find an upper limit of $g_{aγ} \lesssim 3.76\times10^{-11}\;\mathrm{GeV}^{-1}$ at 95$\%$ confidence level for $m_a \ll 10^{-11}$ eV, while we find that systematic deviations from this benchmark scenario induce an uncertainty as large as about a factor of two. Our result slightly improves the CAST bound, while still being a factor of six (baseline scenario) weaker than the SN1987A gamma-ray burst limit.

astro-ph.HE↗

arXiv2022

Shock Revival in Core-collapse Supernovae Assisted by Heavy Axion-like Particles

Axion-like particles (ALPs) are a class of hypothetical pseudoscalar particles which feebly interact with ordinary matter. The hot plasma of core-collapse supernovae is a possible laboratory to explore physics beyond the standard model including ALPs. Once produced, some of the ALPs can be absorbed by the supernova matter and affect energy transfer. In this study, we calculate the ALP emission in core-collapse supernovae and the backreaction on supernova dynamics consistently. It is found that the stalled bounce shock can be revived if the coupling between ALPs and photons is as high as g_{ag}~10^{-9} GeV^{-1} and the ALP mass is 40-400 MeV. Most of the models result in more energetic explosions than the average observed supernova. While this can be used to place constraints on those ALPs, long-term simulations across multiple progenitors need to be further investigated to place robust limits.

astro-ph.HE↗

arXiv2021

Pre-supernova Ultra-light Axion-like Particles

We calculate the production of ultra-light axion-like particles (ALPs) in a nearby supernova progenitor. Once produced, ALPs escape from the star and a part of them is converted into photons during propagation in the Galactic magnetic field. It is found that the MeV photon flux that reaches Earth may be detectable by gamma ray telescopes for ALPs lighter than ~1 neV when Betelgeuse undergoes oxygen and silicon burning. The dependence of the gamma ray flux on the stellar mass is much smaller than the uncertainty that originates from the Galactic magnetic field. If ALPs are lighter than ~0.1 neV and the supernova progenitor is close enough to the Solar System, the gamma ray flux is insensitive to the distance d because the ALP-photon conversion probability is proportional to d^2. (Non-)detection of gamma rays from a supernova progenitor with next-generation gamma ray telescopes just after pre-supernova neutrino alerts would lead to an independent constraint on ALP parameters as stringent as a SN 1987A limit.

hep-ph↗

arXiv2021

The Next Generation Global Gravitational Wave Observatory: The Science Book

The next generation of ground-based gravitational-wave detectors will observe coalescences of black holes and neutron stars throughout the cosmos, thousands of them with exceptional fidelity. The Science Book is the result of a 3-year effort to study the science capabilities of networks of next generation detectors. Such networks would make it possible to address unsolved problems in numerous areas of physics and astronomy, from Cosmology to Beyond the Standard Model of particle physics, and how they could provide insights into workings of strongly gravitating systems, astrophysics of compact objects and the nature of dense matter. It is inevitable that observatories of such depth and finesse will make new discoveries inaccessible to other windows of observation. In addition to laying out the rich science potential of the next generation of detectors, this report provides specific science targets in five different areas in physics and astronomy and the sensitivity requirements to accomplish those science goals. This report is the second in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book (this report), iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network.

gr-qc↗

arXiv2021

Convection and Dynamo in Newly-born Neutron Stars

To study properties of magneto-hydrodynamic (MHD) convection and resultant dynamo activities in proto-neutron stars (PNSs), we construct a "PNS in a box" simulation model with solving compressible MHD equation coupled with a nuclear equation of state (EOS) and a simplified leptonic transport. As a demonstration, we apply it to two types of PNS models with different internal structures: fully-convective model and spherical-shell convection model. By varying the spin rate of models, the rotational dependence of convection and dynamo that operate inside the PNS is investigated. We find that, as a consequence of turbulent transport by rotating stratified convection, large-scale structures of flow and thermodynamic fields are developed in all models. Depending on the spin rate and the convection zone depth, various profiles of the large-scale structures are obtained, which can be physically understood as steady-state solutions to the "mean-field" equation of motion. Additionally to those hydrodynamic structures, the large-scale magnetic component with $\mathcal{O}(10^{15})$ G is also spontaneously organized in disordered tangled magnetic fields in all models. The higher the spin rate, the stronger the large-scale magnetic component is built up. Intriguingly, as an overall trend, the fully-convective models have a stronger large-scale magnetic component than that in the spherical-shell convection models. The deeper the convection zone extends, the larger the size of the convection eddies becomes. As a result, the rotationally-constrained convection seems to be more easily achieved in the fully-convective model, resulting in the higher efficiency of the large-scale dynamo there. To gain a better understanding of the origin of the diversity of NS's magnetic field, we need to study the PNS dynamo in a wider parameter range.

astro-ph.HE↗

arXiv2021

Core-collapse supernova simulations and the formation of neutron stars, hybrid stars, and black holes

We investigate observable signatures of a first-order quantum chromodynamics (QCD) phase transition in the context of core collapse supernovae. To this end, we conduct axially symmetric numerical relativity simulations with multi-energy neutrino transport, using a hadron-quark hybrid equation of state (EOS). We consider four non-rotating progenitor models, whose masses range from $9.6$ to $70$ M$_\odot$. We find that the two less massive progenitor stars (9.6 and 11.2 M$_\odot$) show a successful explosion, which is driven by the neutrino heating. They do not undergo the QCD phase transition and leave behind a neutron star (NS). As for the more massive progenitor stars (50 and 70 M$_\odot$), the proto-neutron star (PNS) core enters the phase transition region and experiences the second collapse. Because of a sudden stiffening of the EOS entering to the pure quark matter regime, a strong shock wave is formed and blows off the PNS envelope in the 50 M$_\odot$ model. Consequently, the remnant becomes a quark core surrounded by hadronic matters, leading to the formation of the hybrid star. However for the 70 M$_\odot$ model, the shock wave cannot overcome the continuous mass accretion and it readily becomes a black hole. We find that the neutrino and gravitational wave (GW) signals from supernova explosions driven by the hadron-quark phase transition are detectable for the present generation of neutrino and GW detectors. Furthermore, the analysis of the GW detector response reveals unique kHz signatures, which will allow us to distinguish this class of supernova explosions from failed and neutrino-driven explosions.

astro-ph.HE↗

arXiv2021

Insights into non-axisymmetric instabilities in three-dimensional rotating supernova models with neutrino and gravitational-wave signatures

We present a detailed analysis to clarify what determines the growth of the low-$T/|W|$ instability in the context of rapidly rotating core-collapse of massive stars. To this end, we perform three-dimensional core-collapse supernova (CCSN) simulations of a $27 M_{\odot}$ star including several updates in the general relativistic correction to gravity, the multi-energy treatment of heavy-lepton neutrinos, and the nuclear equation of state. Non-axisymmetric deformations are analyzed from the point of view of the time evolution of the pattern frequency and the corotation radius. The corotation radius is found to coincide with the convective layer in the proto neutron star (PNS). We propose a new mechanism to account for the growth of the low-$T/|W|$ instability in the CCSN environment. Near the convective boundary where a small Brunt-Väisälä frequency is expected, Rossby waves propagating in the azimuthal direction at mid latitude induce non-axisymmetric unstable modes, in both hemispheres. They merge with each other and finally become the spiral arm in the equatorial plane. We also investigate how the growth of the low-$T/|W|$ instability impacts the neutrino and gravitational-wave signatures.

astro-ph.HE↗

arXiv2021

A three-dimensional hydrodynamics simulation of oxygen-shell burning in the final evolution of a fast-rotating massive star

We perform for the first time a 3D hydrodynamics simulation of the evolution of the last minutes pre-collapse of the oxygen shell of a fast-rotating massive star. This star has an initial mass of 38 M$_\odot$, a metallicity of $\sim$1/50 Z$_\odot$, an initial rotational velocity of 600 km s$^{-1}$, and experiences chemically homogeneous evolution. It has a silicon- and oxygen-rich (Si/O) convective layer at (4.7-17)$\times 10^{8}$ cm, where oxygen-shell burning takes place. The power spectrum analysis of the turbulent velocity indicates the dominance of the large-scale mode ($\ell \sim 3$), which has also been seen in non-rotating stars that have a wide Si/O layer. Spiral arm structures of density and silicon-enriched material produced by oxygen-shell burning appear in the equatorial plane of the Si/O shell. Non-axisymmetric, large-scale ($m \le 3$) modes are dominant in these structures. The spiral arm structures have not been identified in previous non-rotating 3D pre-supernova models. Governed by such a convection pattern, the angle-averaged specific angular momentum becomes constant in the Si/O convective layer, which is not considered in spherically symmetrical stellar evolution models. Such spiral arms and constant specific angular momentum might affect the ensuing explosion or implosion of the star.

astro-ph.SR↗

arXiv2021

Axion-like Particles from Hypernovae

It was recently pointed out that very energetic subclasses of supernovae (SNe), like hypernovae and superluminous SNe, might host ultra-strong magnetic fields in their core. Such fields may catalyze the production of feebly interacting particles, changing the predicted emission rates. Here we consider the case of axion-like particles (ALPs) and show that the predicted large scale magnetic fields in the core contribute significantly to the ALP production, via a coherent conversion of thermal photons. Using recent state-of-the-art SN simulations including magnetohydrodynamics, we find that if ALPs have masses $m_a \sim {\mathcal O}(10)\, \rm MeV$, their emissivity via magnetic conversions is over two orders of magnitude larger than previously estimated. Moreover, the radiative decay of these massive ALPs would lead to a peculiar delay in the arrival times of the daughter photons. Therefore, high-statistics gamma-ray satellites can potentially discover MeV ALPs in an unprobed region of the parameter space and shed light on the magnetohydrodinamical nature of the SN explosion.

hep-ph↗

arXiv2021

Gravitational Wave Physics and Astronomy in the nascent era

The detections of gravitational waves (GW) by LIGO/Virgo collaborations provide various possibilities to physics and astronomy. We are quite sure that GW observations will develop a lot both in precision and in number owing to the continuous works for the improvement of detectors, including the expectation to the newly joined detector, KAGRA, and the planned detector, LIGO-India. In this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics.

gr-qc↗

arXiv2021

Three-flavor collective neutrino conversions with multi-azimuthal-angle instability in an electron-capture supernova model

We investigate the multi-azimuthal angle (MAA) effect on collective neutrino oscillation by considering the three-dimensional neutrino momentum distribution in a realistic electron-capture supernova model with an $8.8 M_{\odot}$ progenitor. We find that the MAA effect induces collective flavor conversions at epochs when it is completely suppressed under the axial-symmetric approximation. This novel activity is switched on/off by the growth of the MAA instability and imprints additional time evolution in the expected neutrino event rate. We validate our results by extending the linear stability analysis into the three-flavor scheme including mixing angles, and confirm that the onset of collective neutrino oscillation matches the steep growth of flavor instability. We discuss how the MAA effect alters neutrino detection at Super-Kamiokande and DUNE.

astro-ph.HE↗

arXiv2021

Impact of binary interactions on the diffuse supernova neutrino background

Binary interactions, especially mass transfer and mergers, can strongly influence the evolution of massive stars and change their final properties and the occurrence of supernovae. Here, we investigate how binary interactions affect predictions of the diffuse flux of neutrinos. By performing stellar population syntheses including prescriptions for binary interactions, we show that the resulting detection rates of the diffuse supernova neutrino background is enhanced by 15%-20% compared to estimates without binary considerations. A source of significant uncertainty arises due to the presently sparse knowledge of the evolution of rapidly rotating carbon-oxygen cores, especially those created as a result of mergers near the white dwarf to core collapse boundary. The enhancement effect may be as small as a few percent if the effects of rotation in postmerger systems are neglected, or as large as 75% if trends are extrapolated. Our estimates serve to highlight that binary effects can be important.

astro-ph.HE↗

arXiv2021

Three-dimensional Hydrodynamics Simulations of Precollapse Shell Burning in the Si- and O-rich Layers

We present 3D hydrodynamics simulations of shell burning in two progenitors with zero-age main-sequence masses of 22 and 27 $M_{\odot}$ for $\sim$65 and 200 s up to the onset of gravitational collapse, respectively. The 22 and 27 $M_{\odot}$ stars are selected from a suite of 1D progenitors. The former and the latter have an extended Si- and O-rich layer with a width of $\sim$10$^9$ cm and $\sim$5$\times 10^9$ cm, respectively. Our 3D results show that turbulent mixing occurs in both of the progenitors with the angle-averaged turbulent Mach number exceeding $\sim$0.1 at the maximum. We observe that an episodic burning of O and Ne, which takes place underneath the convection bases, enhances the turbulent mixing in the 22 and 27 $M_\odot$ models, respectively. The distribution of nucleosynthetic yields is significantly different from that in 1D simulations, namely, in 3D more homogeneous and inhomogeneous in the radial and angular direction, respectively. By performing a spectrum analysis, we investigate the growth of turbulence and its role of material mixing in the convective layers. We also present a scalar spherical harmonics mode analysis of the turbulent Mach number. This analytical formula would be helpful for supernova modelers to implement the precollapse perturbations in core-collapse supernova simulations. Based on the results, we discuss implications for the possible onset of the perturbation-aided neutrino-driven supernova explosion.

astro-ph.SR↗

arXiv2021

Characteristic Time Variability of Gravitational-Wave and Neutrino Signals from Three-dimensional Simulations of Non-Rotating and Rapidly Rotating Stellar Core-Collapse

We present results from full general relativistic three-dimensional hydrodynamics simulations of stellar core collapse of a 70 M$_\odot$ star with spectral neutrino transport. To investigate the impact of rotation on non-axisymmetric instabilities, we compute three models by parametrically changing the initial strength of rotation. The most rapidly rotating model exhibits a transient development of the low-$T/|W|$ instability with one-armed spiral flow at the early postbounce phase. Subsequently, the two-armed spiral flow appears, which persists during the simulation time. The moderately rotating model also shows the growth of the low-$T/|W|$ instability, but only with the two-armed spiral flow. In the nonrotating model, a vigorous activity of the standing accretion-shock instability (SASI) is only observed. The SASI is first dominated by the sloshing mode, which is followed by the spiral SASI until the black hole formation. We present a spectrogram analysis of the gravitational waves (GWs) and neutrinos, focusing on the time correlation. Our results show that characteristic time modulations in the GW and neutrino signals can be linked to the growth of the non-axisymmetric instabilities. We find that the degree of the protoneutron star (PNS) deformation, depending upon which modes of the non-axisymmetric instabilities develop, predominantly affects the characteristic frequencies of the correlated GW and neutrino signals. We point out that these signals would be simultaneously detectable by the current-generation detectors up to $\sim10$ kpc. Our findings suggest that the joint observation of GWs and neutrinos is indispensable for extracting information on the PNS evolution preceding the black hole formation.

astro-ph.HE↗

arXiv2020

Two-dimensional numerical study for magnetic field dependence of neutrino-driven core-collapse supernova models

We study the effects of the magnetic field on the dynamics of non-rotating stellar cores by performing two-dimensional (2D), magnetohydrodynamics (MHD) simulations. To this end, we have updated our neutrino-radiation-hydrodynamics supernova code to include MHD employing a divergence cleaning method with both careful treatments of finite volume and area reconstructions. By changing the initial strength of the magnetic field, the evolution of $15.0$, $18.4$ and $27.0$ $M_\odot$ presupernova progenitors is investigated. An intriguing finding in our study is that the neutrino-driven explosion occurs regardless of the strength of the initial magnetic field. For the 2D models presented in this work, the neutrino heating is the main driver for the explosion, whereas the magnetic field secondary contributes to the pre-explosion dynamics. Our results show that the strong magnetic field weakens the growth of the neutrino-driven turbulence in the small scale compared to the weak magnetic field. This results in the slower increase of the turbulent kinetic energy in the postshock region, leading to the slightly delayed onset of the shock revival for models with the stronger initial magnetic field.

astro-ph.HE↗

arXiv2020

Current status of space gravitational wave antenna DECIGO and B-DECIGO

Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy.

gr-qc↗

arXiv2020

Detectability of SASI activity in supernova neutrino signals

We introduce a novel methodology for establishing the presence of Standing Accretion Shock Instabilities (SASI) in the dynamics of a core collapse supernova from the observed neutrino event rate at water- or ice-based neutrino detectors. The methodology uses a likelihood ratio in the frequency domain as a test-statistics; it is also employed to assess the potential to estimate the frequency and the amplitude of the SASI modulations of the neutrino signal. The parameter estimation errors are consistent with the minimum possible errors as evaluated from the inverse of the Fisher information matrix, and close to the theoretical minimum for the SASI amplitude. Using results from a core-collapse simulation of a 15 solar-mass star by Kuroda $\it {et\, al.}$ (2017) as a test bed for the method, we find that SASI can be identified with high confidence for a distance to the supernova of up to $\sim 6$ kpc for IceCube and and up to $\sim 3$ kpc for a 0.4 Mt mass water Cherenkov detector. This methodology will aid the investigation of a future galactic supernova.

astro-ph.HE↗

arXiv2020

Time of Flight and Supernova Progenitor Effects on the Neutrino Halo

We argue that the neutrino halo, a population of neutrinos that have undergone direction-changing scattering in the stellar envelope of a core-collapse supernova (CCSNe), is sensitive to neutrino emission history through time of flight. We show that the constant time approximation, commonly used in calculating the neutrino halo, does not capture the spatiotemporal evolution of the halo neutrino population and that correcting for time of flight can produce conditions which may trigger fast neutrino flavor conversion. We also find that there exists a window of time early in all CCSNe where the neutrino halo population is sufficiently small that it may be negligible. This suggests that collective neutrino oscillation calculations which neglect the Halo may be well founded at sufficiently early times.

astro-ph.HE↗