17,279 research outputs found
Gravitational Collapse in One Dimension
We simulate the evolution of one-dimensional gravitating collisionless
systems from non- equilibrium initial conditions, similar to the conditions
that lead to the formation of dark- matter halos in three dimensions. As in the
case of 3D halo formation we find that initially cold, nearly homogeneous
particle distributions collapse to approach a final equilibrium state with a
universal density profile. At small radii, this attractor exhibits a power-law
behavior in density, {\rho}(x) \propto |x|^(-{\gamma}_crit), {\gamma}_crit
\simeq 0.47, slightly but significantly shallower than the value {\gamma} = 1/2
suggested previously. This state develops from the initial conditions through a
process of phase mixing and violent relaxation. This process preserves the
energy ranks of particles. By warming the initial conditions, we illustrate a
cross-over from this power-law final state to a final state containing a
homogeneous core. We further show that inhomogeneous but cold power-law initial
conditions, with initial exponent {\gamma}_i > {\gamma}_crit, do not evolve
toward the attractor but reach a final state that retains their original
power-law behavior in the interior of the profile, indicating a bifurcation in
the final state as a function of the initial exponent. Our results rely on a
high-fidelity event-driven simulation technique.Comment: 14 Pages, 13 Figures. Submitted to MNRA
Influence of adaptive mesh refinement and the hydro solver on shear-induced mass stripping in a minor-merger scenario
We compare two different codes for simulations of cosmological structure
formation to investigate the sensitivity of hydrodynamical instabilities to
numerics, in particular, the hydro solver and the application of adaptive mesh
refinement (AMR). As a simple test problem, we consider an initially spherical
gas cloud in a wind, which is an idealized model for the merger of a subcluster
or galaxy with a big cluster. Based on an entropy criterion, we calculate the
mass stripping from the subcluster as a function of time. Moreover, the
turbulent velocity field is analyzed with a multi-scale filtering technique. We
find remarkable differences between the commonly used PPM solver with
directional splitting in the Enzo code and an unsplit variant of PPM in the Nyx
code, which demonstrates that different codes can converge to systematically
different solutions even when using uniform grids. For the test case of an
unbound cloud, AMR simulations reproduce uniform-grid results for the mass
stripping quite well, although the flow realizations can differ substantially.
If the cloud is bound by a static gravitational potential, however, we find
strong sensitivity to spurious fluctuations which are induced at the cutoff
radius of the potential and amplified by the bow shock. This gives rise to
substantial deviations between uniform-grid and AMR runs performed with Enzo,
while the mass stripping in Nyx simulations of the subcluster is nearly
independent of numerical resolution and AMR. Although many factors related to
numerics are involved, our study indicates that unsplit solvers with advanced
flux limiters help to reduce grid effects and to keep numerical noise under
control, which is important for hydrodynamical instabilities and turbulent
flows.Comment: 23 pages, 18 figures, accepted for publication by Astronomy and
Computin
Transport in the random Kronig-Penney model
The Kronig-Penney model with random Dirac potentials on the lattice \ZM has
critical energies at which the Lyapunov exponent vanishes and the density of
states has a van Hove singularity. This leads to a non-trivial quantum
diffusion even though the spectrum is known to be pure-point
Evidence for a parsec scale X-ray jet from the accreting neutron star Circinus X-1
We analyzed the zero-order image of a 50 ks Chandra gratings observation of
Circinus X-1, taken in 2005 during the source's low-flux state. Circinus X-1 is
an accreting neutron star that exhibits ultra-relativistic arcsecond-scale
radio jets and diffuse arcminute-scale radio jets and lobes. The image shows a
clear excess along the general direction of the north-western counter-jet,
coincident with the radio emission, suggesting that it originates either in the
jet itself or in the shock the jet is driving into its environment. This makes
Circinus X-1 the first neutron star for which an extended X-ray jet has been
detected. The kinetic jet power we infer is significantly larger than the
minimum power required for the jet to inflate the large scale radio nebula.Comment: Added journal reference, corrected on reference and typo in labels
for Fig. 1; 5 pages, 3 figures, ApJ Letter, in pres
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