486 research outputs found
The nonlinear development of the relativistic two-stream instability
The two-stream instability has been mooted as an explanation for a range of
astrophysical applications from GRBs and pulsar glitches to cosmology. Using
the first nonlinear numerical simulations of relativistic multi-species
hydrodynamics we show that the onset and initial growth of the instability is
very well described by linear perturbation theory. In the later stages the
linear and nonlinear description match only qualitatively, and the instability
does not saturate even in the nonlinear case by purely ideal hydrodynamic
effects.Comment: 15 pages, 9 figure
Effects of tangential velocity in the reactive relativistic Riemann problem
Type I X-ray bursts are thermonuclear burning events which occur on the
surfaces of accreting neutron stars. Burning begins in a localised spot in the
star's ocean layer before propagating across the entire surface as a
deflagration. On the scale of the entire star, the burning front can be thought
of as discontinuity. To model this, we investigated the reactive Riemann
problem for relativistic deflagrations and detonations and developed a
numerical solver. Unlike for the Newtonian Riemann problem, where only the
velocity perpendicular to the interface is relevant, in the relativistic case
the tangential velocity becomes coupled through the Lorentz factor and can
alter the waves present in the solution. We investigated whether a fast
tangential velocity may be able to cause a deflagration wave to transition to a
detonation. We found that such a transition is possible, but only for
tangential velocities that are a significant fraction of the speed of light or
for systems already on the verge of transitioning. Consequently, it is highly
unlikely that this transition would occur for a burning front in a neutron star
ocean without significant contributions from additional multidimensional
effects.Comment: 16 pages, 10 figures, Accepted for publication in Ap
Local magneto-shear instability in Newtonian gravity
The magneto-rotational instability (MRI) - which is due to an interplay
between a sheared background and the magnetic field - is commonly considered a
key ingredient for developing and sustaining turbulence in the outer envelope
of binary neutron star merger remnants. To assess whether (or not) the
instability is active and resolved, criteria originally derived in the
accretion disk literature - thus exploiting the symmetries of such systems -
are often used. In this paper we discuss the magneto-shear instability as a
truly local phenomenon, relaxing common symmetry assumptions on the background
on top of which the instability grows. This makes the discussion well-suited
for highly dynamical environments such as binary mergers. We find that -
although this is somewhat hidden in the usual derivation of the MRI dispersion
relation - the instability crucially depends on the assumed symmetries.
Relaxing the symmetry assumptions on the background we find that the role of
the magnetic field is significantly diminished, as it affects the modes' growth
but does not drive it. This suggests that we should not expect the standard
instability criteria to provide a faithful indication/diagnostic of what "is
actually going on" in mergers. We conclude by making contact with a suitable
filtering operation, as this is key to separating background and fluctuations
in highly dynamical systems.Comment: 15 pages, 1 figur
Calculating the mass fraction of primordial black holes
We reinspect the calculation for the mass fraction of primordial black holes (PBHs) which are formed from primordial perturbations, finding that performing the calculation using the comoving curvature perturbation c in the standard way vastly overestimates the number of PBHs, by many orders of magnitude. This is because PBHs form shortly after horizon entry, meaning modes significantly larger than the PBH are unobservable and should not affect whether a PBH forms or not - this important effect is not taken into account by smoothing the distribution in the standard fashion. We discuss alternative methods and argue that the density contrast, Δ, should be used instead as super-horizon modes are damped by a factor k2. We make a comparison between using a Press-Schechter approach and peaks theory, finding that the two are in close agreement in the region of interest. We also investigate the effect of varying the spectral index, and the running of the spectral index, on the abundance of primordial black holes
Dynamics and gravitational wave signature of collapsar formation
We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formatio
Crustal failure during binary inspiral
We present the first fully relativistic calculations of the crustal strain
induced in a neutron star by a binary companion at the late stages of inspiral,
employing realistic equations of state for the fluid core and the solid crust.
We show that while the deep crust is likely to fail only shortly before
coalescence, there is a large variation in elastic strain, with the outermost
layers failing relatively early on in the inspiral. We discuss the significance
of the results for both electromagnetic and gravitational-wave astronomy.Comment: 5 pages, 3 eps figure
Dynamics and Gravitational Wave Signature of Collapsar Formation
We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation
Signatures of non-gaussianity in the isocurvature modes of primordial black hole dark matter
Primordial black holes (PBHs) are black holes which may have formed very
early on during the radiation dominated era in the early universe. We present
here a method by which the large scale perturbations in the density of
primordial black holes may be used to place tight constraints on
non-gaussianity if PBHs account for dark matter (DM). The presence of
local-type non-gaussianity is known to have a significant effect on the
abundance of primordial black holes, and modal coupling from the observed CMB
scale modes can significantly alter the number density of PBHs that form within
different regions of the universe, which appear as DM isocurvature modes. Using
the recent \emph{Planck} constraints on isocurvature perturbations, we show
that PBHs are excluded as DM candidates for even very small local-type
non-gaussianity, and remarkably the constraint on
is almost as strong. Even small non-gaussianity is excluded if DM is
composed of PBHs. If local non-Gaussianity is ever detected on CMB scales, the
constraints on the fraction of the universe collapsing into PBHs (which are
massive enough to have not yet evaporated) will become much tighter.Comment: 23 pages, 11 figures. V2: minor corrections and changes, matches
published versio
Spatial characteristics of species distributions as drivers in conservation prioritization
Peer reviewe
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