7,308 research outputs found
Observation of a Spinning Top in a Bose-Einstein Condensate
Boundaries strongly affect the behavior of quantized vortices in
Bose-Einstein condensates, a phenomenon particularly evident in elongated
cigar-shaped traps where vortices tend to orient along a short direction to
minimize energy. Remarkably, contributions to the angular momentum of these
vortices are tightly confined to the region surrounding the core, in stark
contrast to untrapped condensates where all atoms contribute . We
develop a theoretical model and use this, in combination with numerical
simulations, to show that such localized vortices precess in an analogous
manner to that of a classical spinning top. We experimentally verify this
spinning-top behavior with our real-time imaging technique that allows for the
tracking of position and orientation of vortices as they dynamically evolve.
Finally, we perform an in-depth numerical investigation of our real-time
expansion and imaging method, with the aim of guiding future experimental
implementation, as well as outlining directions for its improvement.Comment: 10 pages, 7 figure
Plectoneme creation reduces the rotational friction of a polymer
The torsional dynamics of a semiflexible polymer with a contour length
larger than its persistence length L_p that is rotated at fixed frequency
omega_0 at one end is studied by scaling arguments and hydrodynamic
simulations. We find a non-equilibrium transition at a critical frequency
omega_*: In the linear regime, omega_0 < omega_*, axial spinning is the
dominant dissipation mode. In the non-linear regime, omega_0 > omega_*, the
twist-dissipation mode involves the continuous creation of plectonemes close to
the driven end and the rotational friction is substantially reduced
GW170817, General Relativistic Magnetohydrodynamic Simulations, and the Neutron Star Maximum Mass
Recent numerical simulations in general relativistic magnetohydrodynamics
(GRMHD) provide useful constraints for the interpretation of the GW170817
discovery. Combining the observed data with these simulations leads to a bound
on the maximum mass of a cold, spherical neutron star (the TOV limit): , where is the ratio of the maximum
mass of a uniformly rotating neutron star (the supramassive limit) over the
maximum mass of a nonrotating star. Causality arguments allow to be as
high as , while most realistic candidate equations of state predict
to be closer to , yielding in the range
. A minimal set of assumptions based on these simulations
distinguishes this analysis from previous ones, but leads to a similar
estimate. There are caveats, however, and they are enumerated and discussed.
The caveats can be removed by further simulations and analysis to firm up the
basic argument.Comment: 6 pages, 1 figure. Matches published versio
Black Hole Superradiance in Dynamical Spacetime
We study the superradiant scattering of gravitational waves by a nearly
extremal black hole (dimensionless spin ) by numerically solving the
full Einstein field equations, thus including backreaction effects. This allows
us to study the dynamics of the black hole as it loses energy and angular
momentum during the scattering process. To explore the nonlinear phase of the
interaction, we consider gravitational wave packets with initial energies up to
of the mass of the black hole. We find that as the incident wave energy
increases, the amplification of the scattered waves, as well as the energy
extraction efficiency from the black hole, is reduced. During the interaction
the apparent horizon geometry undergoes sizable nonaxisymmetric oscillations.
The largest amplitude excitations occur when the peak frequency of the incident
wave packet is above where superradiance occurs, but close to the dominant
quasinormal mode frequency of the black hole.Comment: 5 pages, 4 figures; revised to match PRD versio
Simulations of black-hole binaries with unequal masses or non-precessing spins: accuracy, physical properties, and comparison with post-Newtonian results
We present gravitational waveforms for the last orbits and merger of
black-hole-binary (BBH) systems along two branches of the BBH parameter space:
equal-mass binaries with equal non-precessing spins, and nonspinning
unequal-mass binaries. The waveforms are calculated from numerical solutions of
Einstein's equations for black-hole binaries that complete between six and ten
orbits before merger. Along the equal-mass spinning branch, the spin parameter
of each BH is , and along the unequal-mass
branch the mass ratio is . We discuss the construction of
low-eccentricity puncture initial data for these cases, the properties of the
final merged BH, and compare the last 8-10 GW cycles up to with
the phase and amplitude predicted by standard post-Newtonian (PN) approximants.
As in previous studies, we find that the phase from the 3.5PN TaylorT4
approximant is most accurate for nonspinning binaries. For equal-mass spinning
binaries the 3.5PN TaylorT1 approximant (including spin terms up to only 2.5PN
order) gives the most robust performance, but it is possible to treat TaylorT4
in such a way that it gives the best accuracy for spins . When
high-order amplitude corrections are included, the PN amplitude of the
modes is larger than the NR amplitude by between 2-4%.Comment: 21 pages, 9 figures, 6 tables. Version accepted by PR
Suitability of hybrid gravitational waveforms for unequal-mass binaries
This article studies sufficient accuracy criteria of hybrid post-Newtonian
(PN) and numerical relativity (NR) waveforms for parameter estimation of strong
binary black-hole sources in second- generation ground-based gravitational-wave
detectors. We investigate equal-mass non-spinning binaries with a new 33-orbit
NR waveform, as well as unequal-mass binaries with mass ratios 2, 3, 4 and 6.
For equal masses, the 33-orbit NR waveform allows us to recover previous
results and to extend the analysis toward matching at lower frequencies. For
unequal masses, the errors between different PN approximants increase with mass
ratio. Thus, at 3.5PN, hybrids for higher-mass-ratio systems would require NR
waveforms with many more gravitational-wave (GW) cycles to guarantee no adverse
impact on parameter estimation. Furthermore, we investigate the potential
improvement in hybrid waveforms that can be expected from 4th order
post-Newtonian waveforms, and find that knowledge of this 4th post-Newtonian
order would significantly improve the accuracy of hybrid waveforms.Comment: 11 pages, 14 figure
Tearing up the disc: how black holes accrete
We show that in realistic cases of accretion in active galactic nuclei or
stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central
regions of tilted accretion discs around spinning black holes into a set of
distinct planes with only tenuous flows connecting them. If the original
misalignment of the outer disc to the spin axis of the hole is , as in % of randomly oriented
accretion events, the continued precession of these discs sets up partially
counter-rotating gas flows. This drives rapid infall as angular momentum is
cancelled and gas attempts to circularize at smaller radii. Disc breaking close
to the black hole leads to direct dynamical accretion, while breaking further
out can drive gas down to scales where it can accrete rapidly. For smaller tilt
angles breaking can still occur, and may lead to other observable phenomena
such as QPOs. For such effects not to appear, the black hole spin must in
practice be negligibly small, or be almost precisely aligned with the disc.
Qualitatively similar results hold for any accretion disc subject to a forced
differential precession, such as an external disc around a misaligned black
hole binary.Comment: 5 pages, 3 figures. Accepted to ApJ Letter
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