arXiv subjects

Shota Shibagaki

Publications and source records attributed to Shota Shibagaki.

7 recordsLinked to original sources

arXiv2026

Circular polarization of gravitational waves from magnetorotational supernovae

Context. Gravitational waves (GWs) provide a unique probe of the explosion mechanism of massive stars and the evolution of nascent proto-neutron stars (PNSs). Magnetorotational explosions are one of the promising noncanonical core-collapse supernova scenarios, possibly linked to magnetar formation and energetic supernova explosions. However, the GW signatures of such events remain incompletely understood. Aims. We investigate the origin and nature of GW polarization arising from a magnetorotational core-collapse model and examine its potential detectability by current GW observatories. Methods. We performed a 3D GRMHD simulation of a rapidly rotating, strongly magnetized 20 $M_{\odot}$ progenitor, including multi-energy neutrino transport. The GW signals were extracted using the standard quadrupole formalism, and their polarization states were analyzed with Stokes parameters. Results. Strong circular polarization emerges along the rotation axis during the early post-bounce phase ($\lesssim$ 230 ms). The characteristic GW spectrum peaks at ~90 Hz, consistent with the emission at twice the local angular velocity (~45 Hz) around the PNS surface at cylindrical radii of ~50 km. These features are attributed to the low-$T/\vert{}W\vert{}$ instabilities and nonaxisymmetric motions near the PNS and not to the MHD jets themselves. The polarization signals lie within the sensitivity bands of current detectors such as Advanced LIGO, Advanced Virgo, and KAGRA. Conclusions. Models launching magnetorotationally driven jets can produce circularly polarized GW signals originating from the inner PNS region. This provides an observational signature that complements previous findings from nonmagnetized rotating models. Thus, GW polarization is a promising diagnostic of noncanonical core-collapse supernovae. Future third-generation detectors will be crucial to fully exploit this potential.

astro-ph.HE↗

arXiv2024

Neutron stars in accreting systems -- signatures of the QCD phase transition

Neutron stars (NS) that are born in binary systems with a main-sequence star companion can experience mass transfer, resulting in the accumulation of material at the surface of the NS. This, in turn, leads to the continuous growth of the NS mass and the associated steepening of the gravitational potential. Supposing the central density surpasses the onset for the phase transition from nuclear, generally hadronic matter to deconfined quark-gluon plasma, which is a quantity currently constrained solely from an upper limit by asymptotic freedom in quantum chromodynamics (QCD), the system may experience a dynamic response due to the appearance of additional degrees of freedom in the equation of state (EOS). This dynamical response might give rise to a rapid softening of the EOS during the transition in the hadron-quark matter co-existence region. While this phenomenon has long been studied in the context of hydrostatic configurations, the dynamical implications of this problem are still incompletely understood. It is the purpose of the present paper to simulate the dynamics of NSs with previously accreted envelopes caused by the presence of a first-order QCD phase transition. Therefore, we employed the neutrino radiation hydrodynamics treatment based on the fully general relativistic approach in spherical symmetry, implementing a three-flavor Boltzmann neutrino transport and a microscopic model EOS that contains a first-order hadron-quark phase transition. The associated neutrino signal shows a sudden rise in the neutrino fluxes and average energies, becoming observable for the present generation of neutrino detectors for a galactic event, and a gravitational wave mode analysis revealed the behaviors of the dominant $f$ mode and the first and the second gravity $g$ modes that are excited during the NS evolution across the QCD phase transition.

astro-ph.HE↗

arXiv2023

Three-dimensional GRMHD Simulations of Rapidly Rotating Stellar Core-Collapse

We present results from fully general relativistic (GR), three-dimensional (3D), neutrino-radiation magneto-hydrodynamic (MHD) simulations of stellar core collapse of a 20 M$_\odot$ star with spectral neutrino transport. Our focus is to study the gravitational-wave (GW) signatures from the magnetorotationally (MR)-driven models. By parametrically changing the initial angular velocity and the strength of the magnetic fields in the core, we compute four models. Our results show that the MHD outflows are produced only for models (two out of four), to which magnetic field strengths of 10$^{12}$ G and rotation rates of 1 or 2 rad s$^{-1}$ are initially imposed in the core. Seen from the direction perpendicular to the rotational axis, a characteristic waveform is obtained exhibiting a monotonic time increase in the wave amplitude. As previously identified, this stems from the propagating MHD outflows along the axis. We show that the GW amplitude from anisotropic neutrino emission becomes more than one order-of-magnitude bigger than that from the matter contribution, whereas seen from the rotational axis, both of the two components are in the same order-of-magnitudes. Due to the memory effect, the frequency of the neutrino GW from our full-fledged 3D-MHD models is in the range less than $\sim$10 Hz. Toward the future GW detection for a Galactic core-collapse supernova, if driven by the MR mechanism, the planned next-generation detector as DECIGO is urgently needed to catch the low-frequency signals.

astro-ph.HE↗

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

A New Gravitational Wave Signature of Low-$T/|W|$ Instability in Rapidly Rotating Stellar Core Collapse

We present results from a full general relativistic three-dimensional hydrodynamics simulation of rapidly rotating core-collapse of a 70 M$_{\odot}$ star with three-flavor spectral neutrino transport. We find a strong gravitational wave (GW) emission that originates from the growth of the one- and two-armed spiral waves extending from the nascent proto-neutron star (PNS). The GW spectrogram shows several unique features that are produced by the non-axisymmetric instabilities. After bounce, the spectrogram first shows a transient quasi-periodic time modulation at $\sim$ 450 Hz. In the second active phase, it again shows the quasi-periodic modulation but with the peak frequency increasing with time, which continues until the final simulation time. From our detailed analysis, such features can be well explained by a combination of the so-called low-$T/|W|$ instability and the PNS core contraction.

astro-ph.HE↗

arXiv2019

The Recent Developmental Status of SNEGRAF: a Web-Based Gravitational Wave Signal Analyzer

Unveiling physical processes in a supernova is one of challenging topics of modern physics and astrophysics since that event is due to particle physics on a stellar scale and tightly related to nucleosynthesis in Universe. Multi-messenger astronomy, a combination, such as of electromagnetic-wave, gravitational-wave, and neutrino observations, will be a breakthrough to the puzzle. To boost the research, we released a web-based gravitational wave signal analyzer "SuperNova Event Gravitational-wave-display in Fukuoka (SNEGRAF)" last year (Eguchi et al. 2019). We are now working on an integration of the application with the RIDGE pipeline, which is for a coherent network analysis between the LIGO, VIRGO, and KAGRA observations (Hayama et al. 2007), and implemented in MATLAB. In the basic design phase, we decided to wrap RIDGE with a simple Python script and make it listen for connections from SNEGRAF. This design enables these two programs to be hosted on different servers independently, and minimizes the cyber risks of RIDGE. In this paper, we report the current developmental status of our system.

astro-ph.IM↗

arXiv2019

Prototype Implementation of a Web-Based Gravitational Wave Signal Analyzer: SNEGRAF

A direct detection of gravitational waves is one of the most exciting frontiers for modern astronomy and astrophysics. Gravitational wave signals combined with classical electro-magnetic observations, known as multi-messenger astronomy, promise newer and deeper insights about the cosmic evolution of astrophysical objects such as neutron starts and black holes. To this end, we have been developing an original data processing pipeline for KAGRA, a Japanese gravitational wave telescope, for optimal detections of supernova events. As a part of our project, we released a web application named SuperNova Event Gravitational-wave-display in Fukuoka (SNEGRAF) in autumn 2018. SNEGRAF accepts the users' theoretical waveforms as a plain text file consisting of a time series of $h_{+}$ and $h_{\times}$ (the plus and cross mode of gravitational waves, respectively), then displays the input, a corresponding spectrogram, and power spectrum together with KAGRA sensitivity curve and the signal-to-noise ratio; we adopt Google Visualization API for the interactive visualization of the input waveforms. However, it is a time-consuming task to draw more than $\sim 10^{5}$ data points directly with JavaScript, although the number can be typical for a supernova hunt by assuming a typical duration of the event and sampling rate of the detectors; a combination of recursive decimations of the original in the server-side program and an appropriate selection of them depending on the time duration requested by the user in a web browser achieves an acceptable latency. In this paper, we present the current design, implementation and optimization algorithms of SNEGRAF, and its future perspectives.

astro-ph.IM↗