1,096 research outputs found
Gravitational Wave Background from Neutrino-Driven Gamma-Ray Bursts
We discuss the gravitational wave background (GWB) from a cosmological
population of gamma-ray bursts (GRBs). Among various emission mechanisms for
the gravitational waves (GWs), we pay a particular attention to the vast
anisotropic neutrino emissions from the accretion disk around the black hole
formed after the so-called failed supernova explosions. The produced GWs by
such mechanism are known as burst with memory, which could dominate over the
low-frequency regime below \sim 10Hz. To estimate their amplitudes, we derive
general analytic formulae for gravitational waveform from the axisymmetric
jets. Based on the formulae, we first quantify the spectrum of GWs from a
single GRB. Then, summing up its cosmological population, we find that the
resultant value of the density parameter becomes roughly \Omega_{GW} \approx
10^{-20} over the wide-band of the low-frequency region, f\sim 10^{-4}-10^1Hz.
The amplitude of GWB is sufficiently smaller than the primordial GWBs
originated from an inflationary epoch and far below the detection limit.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Three-Dimensional Simulations of Standing Accretion Shock Instability in Core-Collapse Supernovae
We have studied non-axisymmetric standing accretion shock instability, or
SASI, by 3D hydrodynamical simulations. This is an extention of our previous
study on axisymmetric SASI. We have prepared a spherically symmetric and steady
accretion flow through a standing shock wave onto a proto-neutron star, taking
into account a realistic equation of state and neutrino heating and cooling.
This unperturbed model is supposed to represent approximately the typical
post-bounce phase of core-collapse supernovae. We then have added a small
perturbation (~1%) to the radial velocity and computed the ensuing evolutions.
Not only axisymmetric but non-axisymmetric perturbations have been also
imposed. We have applied mode analysis to the non-spherical deformation of the
shock surface, using the spherical harmonics. We have found that (1) the growth
rates of SASI are degenerate with respect to the azimuthal index m of the
spherical harmonics Y_l^m, just as expected for a spherically symmetric
background, (2) nonlinear mode couplings produce only m=0 modes for the
axisymmetric perturbations, whereas m=!0 modes are also generated in the
non-axisymmetric cases according to the selection rule for the quadratic
couplings, (3) the nonlinear saturation level of each mode is lower in general
for 3D than for 2D because a larger number of modes are contributing to
turbulence in 3D, (4) low l modes are dominant in the nonlinear phase, (5) the
equi-partition is nearly established among different m modes in the nonlinear
phase, (6) the spectra with respect to l obey power laws with a slope slightly
steeper for 3D, and (7) although these features are common to the models with
and without a shock revival at the end of simulation, the dominance of low l
modes is more remarkable in the models with a shock revival.Comment: 37 pages, 16 figures, and 1 table, submitted to Ap
Inelastic Neutrino-Helium Scatterings and Standing Accretion Shock Instability in Core-Collapse Supernovae
We present the results of numerical experiments, in which we have
investigated the influence of the inelastic neutrino-helium interactions on the
standing accretion shock instability supposed to occur in the post-bounce
supernova core. The axisymmetric hydrodynamical simulations of accretion flows
through the standing accretion shock wave onto the protoneutron star show that
the interactions are relatively minor and the linear growth of the shock
instability is hardly affected. The extra heating given by the inelastic
reactions becomes important for the shock revival after the instability enters
the non-linear regime, but only when the neutrino luminosity is very close to
the critical value, at which the shock would be revived without the
interactions. We have also studied the dependence of the results on the initial
amplitudes of perturbation and the temperatures of mu and tau neutrinos.Comment: 19 pages, 6 figures, submitted to Ap
Neutrino oscillations in magnetically driven supernova explosions
We investigate neutrino oscillations from core-collapse supernovae that
produce magnetohydrodynamic (MHD) explosions. By calculating numerically the
flavor conversion of neutrinos in the highly non-spherical envelope, we study
how the explosion anisotropy has impacts on the emergent neutrino spectra
through the Mikheyev-Smirnov-Wolfenstein effect. In the case of the inverted
mass hierarchy with a relatively large theta_(13), we show that survival
probabilities of electron type neutrinos and antineutrinos seen from the
rotational axis of the MHD supernovae (i.e., polar direction), can be
significantly different from those along the equatorial direction. The event
numbers of electron type antineutrinos observed from the polar direction are
predicted to show steepest decrease, reflecting the passage of the
magneto-driven shock to the so-called high-resonance regions. Furthermore we
point out that such a shock effect, depending on the original neutrino spectra,
appears also for the low-resonance regions, which leads to a noticeable
decrease in the electron type neutrino signals. This reflects a unique nature
of the magnetic explosion featuring a very early shock-arrival to the resonance
regions, which is in sharp contrast to the neutrino-driven delayed supernova
models. Our results suggest that the two features in the electron type
antineutrinos and neutrinos signals, if visible to the Super-Kamiokande for a
Galactic supernova, could mark an observational signature of the magnetically
driven explosions, presumably linked to the formation of magnetars and/or
long-duration gamma-ray bursts.Comment: 25 pages, 21 figures, JCAP in pres
Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields
We study generation of magnetic fields by the Biermann mechanism in the
pair-instability supernovae explosions of first stars. The Biermann mechanism
produces magnetic fields in the shocked region between the bubble and
interstellar medium (ISM), even if magnetic fields are absent initially. We
perform a series of two-dimensional magnetohydrodynamic simulations with the
Biermann term and estimate the amplitude and total energy of the produced
magnetic fields. We find that magnetic fields with amplitude
G are generated inside the bubble, though the amount of
magnetic fields generated depend on specific values of initial conditions. This
corresponds to magnetic fields of erg per each supernova
remnant, which is strong enough to be the seed magnetic field for galactic
and/or interstellar dynamo.Comment: 12 pages, 3 figure
Parametrized 3D models of neutrino-driven supernova explosions: Neutrino emission asymmetries and gravitational-wave signals
Time-dependent and direction-dependent neutrino and gravitational-wave (GW)
signatures are presented for a set of 3D hydrodynamic models of parametrized,
neutrino-driven supernova explosions of non-rotating 15 and 20 solar mass
stars. We employ an approximate treatment of neutrino transport. Due to the
excision of the high-density core of the proto-neutron star and the use of an
axis-free overset grid, the models can be followed from the post-bounce
accretion phase for more than one second without imposing any symmetry
restrictions. GW and neutrino emission exhibit the generic time-dependent
features known from 2D models. Non-radial hydrodynamic mass motions in the
accretion layer and their interaction with the outer layers of the
proto-neutron star together with anisotropic neutrino emission give rise to a
GW signal with an amplitude of ~5-20 cm and frequencies 100--500 Hz. The GW
emission from mass motions reaches a maximum before the explosion sets in.
Afterwards the GW signal exhibits a low-frequency modulation, in some cases
describing a quasi-monotonic growth, associated with the non-spherical
expansion of the explosion shock wave and the large-scale anisotropy of the
escaping neutrino flow. Variations of the mass-quadrupole moment due to
convective activity inside the nascent neutron star contribute a high-frequency
component to the GW signal during the post-explosion phase. The GW signals
exhibit strong variability between the two polarizations, different explosion
simulations and different observer directions, and does not possess any
template character. The neutrino emission properties show fluctuations over the
neutron star surface on spatial and temporal scales that reflect the different
types of non-spherical mass motions. The modulation amplitudes of the
measurable neutrino luminosities and mean energies are significantly smaller
than predicted by 2D simulations.Comment: revised version: 20 pages, 17 figures, Astronomy & Astrophysics in
pres
North-South Neutrino Heating Asymmetry in Strongly Magnetized and Rotating Stellar Cores
We perform a series of two-dimensional magnetohydrodynamic simulations of
supernova cores. Since the distributions of the angular momentum and the
magnetic fields of strongly magnetized stars are quite uncertain, we
systematically change the combinations of the strength of the angular momentum,
the rotations law, the degree of differential rotation, and the profiles of the
magnetic fields to construct the initial conditions. By so doing, we estimate
how the rotation-induced anisotropic neutrino heating are affected by the
strong magnetic fields through parity-violating effects and first investigate
how the north-south asymmetry of the neutrino heating in a strongly magnetized
supernova core could be. As for the microphysics, we employ a realistic
equation of state based on the relativistic mean field theory and take into
account electron captures and the neutrino transport via the neutrino leakage
scheme. With these computations, we find that the parity-violating corrections
reduce of the neutrino heating rate than that without the
magnetic fields in the vicinity of the north pole of a star, on the other hand,
enhance about in the vicinity of the south pole. If the
global asymmetry of the neutrino heating in the both of the poles develops in
the later phases, the newly born neutron star might be kicked toward the north
pole in the subsequent time.Comment: 25 pages, 6 figures, ApJ in press. A paper with higher-resolution
figures available at
http://www-utap.phys.s.u-tokyo.ac.jp/~kkotake/lonbun.htm
Equation-of-State Dependent Features in Shock-Oscillation Modulated Neutrino and Gravitational-Wave Signals from Supernovae
We present 2D hydrodynamic simulations of the long-time accretion phase of a
15 solar mass star after core bounce and before the launch of a supernova
explosion. Our simulations are performed with the Prometheus-Vertex code,
employing multi-flavor, energy-dependent neutrino transport and an effective
relativistic gravitational potential. Testing the influence of a stiff and a
soft equation of state for hot neutron star matter, we find that the non-radial
mass motions in the supernova core due to the standing accretion shock
instability (SASI) and convection impose a time variability on the neutrino and
gravitational-wave signals. These variations have larger amplitudes as well as
higher frequencies in the case of a more compact nascent neutron star. After
the prompt shock-breakout burst of electron neutrinos, a more compact accreting
remnant radiates neutrinos with higher luminosities and larger mean energies.
The observable neutrino emission in the direction of SASI shock oscillations
exhibits a modulation of several 10% in the luminosities and ~1 MeV in the mean
energies with most power at typical SASI frequencies of 20-100 Hz. At times
later than 50-100 ms after bounce the gravitational-wave amplitude is dominated
by the growing low-frequency (<200 Hz) signal associated with anisotropic
neutrino emission. A high-frequency wave signal is caused by nonradial gas
flows in the outer neutron star layers, which are stirred by anisotropic
accretion from the SASI and convective regions. The gravitational-wave power
then peaks at about 300-800 Hz with distinctively higher spectral frequencies
originating from the more compact and more rapidly contracting neutron star.
The detectability of the SASI effects in the neutrino and gravitational-wave
signals is briefly discussed. (abridged)Comment: 21 pages, 11 figures, 45 eps files; revised version including
discussion of signal detectability; accepted by Astronomy & Astrophysics;
high-resolution images can be obtained upon reques
- …