1,215 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
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
Gravitational waves from supernova matter
We have performed a set of 11 three-dimensional magnetohydrodynamical core
collapse supernova simulations in order to investigate the dependencies of the
gravitational wave signal on the progenitor's initial conditions. We study the
effects of the initial central angular velocity and different variants of
neutrino transport. Our models are started up from a 15 solar mass progenitor
and incorporate an effective general relativistic gravitational potential and a
finite temperature nuclear equation of state. Furthermore, the electron flavour
neutrino transport is tracked by efficient algorithms for the radiative
transfer of massless fermions. We find that non- and slowly rotating models
show gravitational wave emission due to prompt- and lepton driven convection
that reveals details about the hydrodynamical state of the fluid inside the
protoneutron stars. Furthermore we show that protoneutron stars can become
dynamically unstable to rotational instabilities at T/|W| values as low as ~2 %
at core bounce. We point out that the inclusion of deleptonization during the
postbounce phase is very important for the quantitative GW prediction, as it
enhances the absolute values of the gravitational wave trains up to a factor of
ten with respect to a lepton-conserving treatment.Comment: 10 pages, 6 figures, accepted, to be published in a Classical and
Quantum Gravity special issue for MICRA200
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
Gravitational Waves from Core Collapse Supernovae
We present the gravitational wave signatures for a suite of axisymmetric core
collapse supernova models with progenitors masses between 12 and 25 solar
masses. These models are distinguished by the fact they explode and contain
essential physics (in particular, multi-frequency neutrino transport and
general relativity) needed for a more realistic description. Thus, we are able
to compute complete waveforms (i.e., through explosion) based on
non-parameterized, first-principles models. This is essential if the waveform
amplitudes and time scales are to be computed more precisely. Fourier
decomposition shows that the gravitational wave signals we predict should be
observable by AdvLIGO across the range of progenitors considered here. The
fundamental limitation of these models is in their imposition of axisymmetry.
Further progress will require counterpart three-dimensional models.Comment: 10 pages, 5 figure
Equilibrium Configurations of Strongly Magnetized Neutron Stars with Realistic Equations of State
We investigate equilibrium sequences of magnetized rotating stars with four
kinds of realistic equations of state (EOSs) of SLy (Douchin et al.), FPS
(Pandharipande et al.), Shen (Shen et al.), and LS (Lattimer & Swesty).
Employing the Tomimura-Eriguchi scheme to construct the equilibrium
configurations. we study the basic physical properties of the sequences in the
framework of Newton gravity. In addition we newly take into account a general
relativistic effect to the magnetized rotating configurations. With these
computations, we find that the properties of the Newtonian magnetized stars,
e.g., structure of magnetic field, highly depends on the EOSs.
The toroidal magnetic fields concentrate rather near the surface for Shen and
LS EOSs than those for SLy and FPS EOSs. The poloidal fields are also affected
by the toroidal configurations. Paying attention to the stiffness of the EOSs,
we analyze this tendency in detail. In the general relativistic stars, we find
that the difference due to the EOSs becomes small because all the employed EOSs
become sufficiently stiff for the large maximum density, typically greater than
. The maximum baryon mass of the magnetized stars
with axis ratio increases about up to twenty percents for that of
spherical stars. We furthermore compute equilibrium sequences at finite
temperature, which should serve as an initial condition for the hydrodynamic
study of newly-born magnetars. Our results suggest that we may obtain
information about the EOSs from the observation of the masses of magnetars.Comment: submitted to MNRA
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
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