arXiv subjects

Yudai Suwa

Publications and source records attributed to Yudai Suwa.

66 records · Page 4Linked to original sources

arXiv2011

Impacts of Collective Neutrino Oscillations on Supernova Explosions

By performing a series of one- and two-dimensional (1-, 2D) hydrodynamic simulations with spectral neutrino transport, we study possible impacts of collective neutrino oscillations on the dynamics of core-collapse supernovae. To model the spectral swapping which is one of the possible outcome of the collective neutrino oscillations, we parametrize the onset time when the spectral swap begins, the radius where the spectral swap occurs, and the threshold energy above which the spectral interchange between heavy-lepton neutrinos and electron/anti-electron neutrinos takes place, respectively. By doing so, we systematically study how the neutrino heating enhanced by the spectral swapping could affect the shock evolution as well as the matter ejection. We also investigate the progenitor dependence using a suite of progenitor models (13, 15, 20, and 25 $M_\odot$). We find that there is a critical heating rate induced by the spectral swapping to trigger explosions, which significantly differs between the progenitors. The critical heating rate is generally smaller for 2D than 1D due to the multidimensionality that enhances the neutrino heating efficiency. For the progenitors employed in this paper, the final remnant masses are estimated to range in 1.1-1.5$M_\odot$. For our 2D model of the $15M_\odot$ progenitor, we find a set of the oscillation parameters that could account for strong supernova explosions ($\sim 10^{51}$ erg), simultaneously leaving behind the remnant mass close to $\sim 1.4 M_\odot$.

astro-ph.HE↗

arXiv2011

Afterglow of binary neutron star merger

The merger of two neutron stars results often in a rapidly and differentially rotating hypermassive neutron star (HMNS). We show by numerical-relativity simulation that the magnetic-field profile around such HMNS is dynamically varied during its subsequent evolution, and as a result, electromagnetic radiation with a large luminosity ~ 0.1 B^2 R^3 Omega is emitted with baryon (B, R, and Omega are poloidal magnetic-field strength at stellar surface, stellar radius, and angular velocity of a HMNS). The predicted luminosity of electromagnetic radiation, which is primarily emitted along the magnetic-dipole direction, is ~ 10^{47} (B/10^{13} G)^2(R/10 km)^3(Omega/10^4 rad/s) ergs/s, that is comparable to the luminosity of quasars.

astro-ph.HE↗

arXiv2011

Truncated Moment Formalism for Radiation Hydrodynamics in Numerical Relativity

A truncated moment formalism for general relativistic radiation hydrodynamics, based on the Thorne's moment formalism, is derived. The fluid rest frame is chosen to be the fiducial frame for defining the radiation moments. Then, zeroth-, first-, and second-rank radiation moments are defined from the distribution function with a physically reasonable assumption for it in the optically thin and thick limits. The source terms are written, focusing specifically on the neutrino transfer and neglecting higher harmonic angular dependence of the reaction angle. Finally, basic equations for a truncated moment formalism for general relativistic radiation hydrodynamics in a closed covariant form are derived assuming a closure relation among the radiation stress tensor, energy density, and energy flux, and a variable Eddington factor, which works well.

astro-ph.HE↗

arXiv2010

Can Gamma-Ray Burst Jets Break Out the First Stars?

We show that a relativistic gamma-ray burst (GRB) jet can potentially pierce the envelope of very massive first generation star (Population III; Pop III) by using the stellar density profile to estimate both the jet luminosity (via accretion) and its penetrability. The jet breakout is possible even if the Pop III star has a supergiant hydrogen envelope without mass loss, thanks to the long-lived powerful accretion of the envelope itself. While the Pop III GRB is estimated to be energetic, E_{gamma,iso} ~ 10^{55} erg, the supergiant envelope hides the initial bright phase into the cocoon component, leading to a GRB with a long duration ~ 1000(1+z) sec and an ordinary isotropic luminosity ~ 10^{52} erg s^{-1} (~ 10^{-9} erg cm^{-2} s^{-1} at redshift z ~ 20). The neutrino-annihilation is not effective for Pop III GRBs because of a low central temperature, while the magnetic mechanism is viable. We also derive analytic estimates of the breakout conditions, which are applicable to various progenitor models. The GRB luminosity and duration are found to be very sensitive to the core and envelope mass, providing possible probes of the first luminous objects at the end of the high redshift dark ages.

astro-ph.CO↗

arXiv2010

Explosion geometry of a rotating 13 $M_{\odot}$ star driven by the SASI-aided neutrino-heating supernova mechanism

By performing axisymmetric hydrodynamic simulations of core-collapse supernovae with spectral neutrino transport based on the isotropic diffusion source approximation scheme, we support the assumption that the neutrino-heating mechanism aided by the standing accretion shock instability and convection can initiate an explosion of a 13 $M_{\odot}$ star. Our results show that bipolar explosions are more likely to be associated with models which include rotation. We point out that models, which form a north-south symmetric bipolar explosion, can lead to larger explosion energies than for the corresponding unipolar explosions.

astro-ph.HE↗

arXiv2009

Probing the central engine of long gamma-ray bursts and hypernovae with gravitational waves and neutrinos

There are the two common candidates as the viable energy source for the central engine of long gamma-ray bursts (GRBs) and hypernovae (HNe), neutrino annihilation and magnetic fields. We investigate gravitational wave (GW) emission accompanied by these two mechanisms. Especially, we focus on GW signals produced by neutrinos from a hyper-accreting disk around a massive black hole. We show that neutrino-induced GWs are detectable for $\sim$1 Mpc events by LISA and $\sim$ 100 Mpc by DECIGO/BBO, if the central engine is powered by neutrinos. The GW signals depend on the viewing angle and they are anti-correlated with neutrino ones. But, simultaneous neutrino detections are also expected, and helpful for diagnosing the explosion mechanism when later electromagnetic observations enable us to identify the source. GW and neutrino observations are potentially useful for probing choked jets that do not produce prompt emission, as well as successful jets. Even in non-detection cases, observations of GWs and neutrinos could lead to profitable implications for the central engine of GRBs and HNe.

astro-ph.HE↗

arXiv2008

Non-thermal neutrinos from supernovae leaving a magnetar

Under the fossil field hypothesis of the origin of magnetar magnetic fields, the magnetar inherits its magnetic field from its progenitor. We show that during the supernova of such a progenitor, protons may be accelerated to \sim 10^4 GeV as the supernova shock propagates in the magnetic stellar envelope. Inelastic nuclear collisions of these protons produce a flash of high-energy neutrinos arriving a few hours after thermal (10 MeV) neutrinos. The neutrino flash is characterized by energies up to O(100) GeV and durations seconds to hours, depending on the progenitor: those from smaller Type Ibc progenitors are typically shorter in duration and reach higher energies compared to those from larger Type II progenitors. A Galactic Type Ib supernova leaving behind a magnetar remnant will yield up to \sim 160 neutrino induced muon events in Super-Kamiokande, and up to \sim 7000 in a km^3 class detector such as IceCube, providing a means of probing supernova models and the presence of strong magnetic fields in the stellar envelope.

astro-ph↗

arXiv2008

Probing reheating temperature of the universe with gravitational wave background

Thermal history of the universe after big-bang nucleosynthesis (BBN) is well understood both theoretically and observationally, and recent cosmological observations also begin to reveal the inflationary dynamics. However, the epoch between inflation and BBN is scarcely known. In this paper we show that the detection of the stochastic gravitational wave background around 1Hz provides useful information about thermal history well before BBN. In particular, the reheating temperature of the universe may be determined by future space-based laser interferometer experiments such as DECIGO and/or BBO if it is around 10^{6-9} GeV, depending on the tensor-to-scalar ratio $r$ and dilution factor $F$.

astro-ph↗

arXiv2008

Impact of Rotation on Neutrino Emission and Relic Neutrino Background from Population III Stars

We study the effects of rotation on the neutrino emission from Population III (Pop III) stars by performing a series of two-dimensional rotational collapse simulations of Pop III stellar cores. Our results show that rotation enhances the neutrino luminosities and the average energies of emitted neutrinos. This is because the thermalized inner core, which is the dominant neutrino source from Pop III stars, can be enlarged, due to rotational flattening, enough to extend the inner core outside the neutrinospheres. This is in sharp contrast to the case of spherical collapse, in which the case of inner core shrinks deeper inside the neutrinospheres before black hole formation, which hinders the efficient neutrino emission. In the case of rotational core-collapse, the emitted neutrino energies are found to become larger in the vicinity near the pole than the ones near the equatorial plane. These factors make the emergent neutrino spectrum broader and harder than the spherical collapse case. By computing the overall neutrino signals produced by the ensemble of individual rotating Pop III stars, we find that the amplitudes of the relic neutrinos, depending on their star formation rates, can dominate over the contributions from ordinary core-collapse supernovae below a few MeV. A detection of this signal could be an important tool to probe star formation history in the early universe.

astro-ph↗

arXiv2008

Space laser interferometers can determine the thermal history of the early Universe

It is shown that space-based gravitational wave detectors such as DECIGO and/or Big Bang Observer (BBO) will provide us with invaluable information on the cosmic thermal history after inflation and they will be able to determine the reheat temperature $T_R$ provided that it lies in the range preferred by the cosmological gravitino problem, $T_R\sim 10^{5-9}$ GeV. Therefore it is strongly desired that they will be put into practice as soon as possible.

hep-ph↗

arXiv2007

Gravitational Wave Background from Population III Stars

We estimate the gravitational wave (GW) background from Population III (Pop III) stars using the results from our hydrodynamic simulations (Suwa et al. 2007). We calculate gravitational waveforms from matter motions and anisotropic neutrino emissions for single Pop III stars. We find that the GW amplitudes from matter motions are dominant until just after bounce, but those from neutrinos dominate later on at frequencies below $\sim 10$ Hz in the GW spectrum. Computing the overall signal produced by the ensemble of such Pop III stars, we find that the resultant density parameter of the GW background peaks at the amplitude of $Ω_\mathrm{GW}\sim 10^{-10}$ in the frequency interval $\sim 1-10$ Hz. We show that such signals, depending on the formation rate of Pop III stars, can be within the detection limits of future planned interferometers such as DECIGO and BBO in the frequency interval of $\sim 0.1-1$ Hz. Our results suggest that the detection of the GW background from Pop III stars can be an important tool to supply the information about the star formation history in the early universe.

astro-ph↗

arXiv2007

Magnetorotational Collapse of Population III Stars

We perform a series of two-dimensional magnetorotational core-collapse simulations of Pop III stars. Changing the initial distributions of rotation and magnetic fields prior to collapse in a parametric manner, we compute 19 models. By so doing, we systematically investigate how rotation and magnetic fields affect the collapse dynamics and explore how the properties of the black-hole formations and neutrino emissions could be affected. As for the microphysics, we employ a realistic equation of state and approximate the neutrino transfer by a multiflavour leakage scheme. With these computations, we find that the jet-like explosions are obtained by the magnetodriven shock waves if the initial magnetic field is as large as $10^{12}$ G. We point out that the black-hole masses at the formation decrease with the initial field strength, on the other hand, increase with the initial rotation rates. As for the neutrino properties, we point out that the degree of the differential rotation plays an important role to determine which species of the neutrino luminosity is more dominant than the others. Furthermore, we find that the stronger magnetic fields make the peak neutrino luminosities smaller, because the magnetic pressure acts to halt the collapse in the central regions, leading to the suppression of the releasable gravitational binding energies.

astro-ph↗