412 research outputs found
Early fragmentation in the adaptive voter model on directed networks
We consider voter dynamics on a directed adaptive network with fixed
out-degree distribution. A transition between an active phase and a fragmented
phase is observed. This transition is similar to the undirected case if the
networks are sufficiently dense and have a narrow out-degree distribution.
However, if a significant number of nodes with low out degree is present, then
fragmentation can occur even far below the estimated critical point due to the
formation of self-stabilizing structures that nucleate fragmentation. This
process may be relevant for fragmentation in current political opinion
formation processes.Comment: 9 pages, 8 figures as published in Phys. Rev.
A simple toy model of the advective-acoustic instability I. Perturbative approach
Some general properties of the advective-acoustic instability are described
and understood using a toy model which is simple enough to allow for analytical
estimates of the eigenfrequencies. The essential ingredients of this model, in
the unperturbed regime, are a stationary shock and a subsonic region of
deceleration. For the sake of analytical simplicity, the 2D unperturbed flow is
parallel and the deceleration is produced adiabatically by an external
potential. The instability mechanism is determined unambiguously as the
consequence of a cycle between advected and acoustic perturbations. The purely
acoustic cycle, considered alone, is proven to be stable in this flow. Its
contribution to the instability can be either constructive or destructive. A
frequency cut-off is associated to the advection time through the region of
deceleration. This cut-off frequency explains why the instability favours
eigenmodes with a low frequency and a large horizontal wavelength. The relation
between the instability occurring in this highly simplified toy model and the
properties of SASI observed in the numerical simulations of stellar
core-collapse is discussed. This simple set up is proposed as a benchmark test
to evaluate the accuracy, in the linear regime, of numerical simulations
involving this instability. We illustrate such benchmark simulations in a
companion paper.Comment: 14 pages, 10 figures, ApJ in pres
The outburst duration and duty-cycle of GRS 1915+105
The extraordinarily long outburst of GRS 1915+105 makes it one of the most
remarkable low-mass X-ray binaries (LMXBs). It has been in a state of constant
outburst since its discovery in 1992, an eruption which has persisted ~100
times longer than those of more typical LXMBs. The long orbital period of GRS
1915+105 implies that it contains large and massive accretion disc which is
able to fuel its extreme outburst. In this paper, we address the longevity of
the outburst and quiescence phases of GRS 1915+105 using Smooth Particle
Hydrodynamics (SPH) simulations of its accretion disc through many outburst
cycles. Our model is set in the two-alpha framework and includes the effects of
the thermo-viscous instability, tidal torques, irradiation by central X-rays
and wind mass loss. We explore the model parameter space and the examine the
impact of the various ingredients. We predict that the outburst of GRS 1915+105
should last a minimum of 20 years and possibly up to ~100 years if X-ray
irradiation is very significant. The predicted recurrence times are of the
order of 10^4 years, making the X-ray duty cycle a few 0.1%. Such a low duty
cycle may mean that GRS 1915+105 is not an anomaly among the more standard
LMXBs and that many similar, but quiescent, systems could be present in the
Galaxy.Comment: 10 pages, 9 figures, accepted for publication by MNRA
Momentum-space entanglement after smooth quenches
We compute the total amount of entanglement produced between momentum modes
at late times after a smooth mass quench in free bosonic and fermionic quantum
field theories. The entanglement and R\'enyi entropies are obtained in closed
form as a function of the parameters characterizing the quench protocol. For
bosons, we show that the entanglement production is more significant for light
modes and for fast quenches. In particular, infinitely slow or adiabatic
quenches do not produce any entanglement. Depending on the quench profile, the
decrease as a function of the quench rate can be either monotonic or
oscillating. In the fermionic case the situation is subtle and there is a
critical value for the quench amplitude above which this behavior is changed
and the entropies become peaked at intermediate values of momentum and of the
quench rate. We also show that the results agree with the predictions of a
Generalized Gibbs Ensemble and obtain explicitly its parameters in terms of the
quench data.Comment: 24 pages, 8 Figures; V2 matches published versio
Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers
We present the first comprehensive study of r-process element nucleosynthesis
in the ejecta of compact binary mergers (CBMs) and their relic black-hole
(BH)-torus systems. The evolution of the BH-accretion tori is simulated for
seconds with a Newtonian hydrodynamics code including viscosity effects,
pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment
closure scheme for the transport of electron neutrinos and antineutrinos. The
investigated cases are guided by relativistic double neutron star (NS-NS) and
NS-BH merger models, producing ~3-6 Msun BHs with rotation parameters of A~0.8
and tori of 0.03-0.3 Msun. Our nucleosynthesis analysis includes the dynamical
(prompt) ejecta expelled during the CBM phase and the neutrino and viscously
driven outflows of the relic BH-torus systems. While typically ~20-25% of the
initial accretion-torus mass are lost by viscously driven outflows,
neutrino-powered winds contribute at most another ~1%, but neutrino heating
enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide
distribution of electron fractions (0.1-0.6) and entropies, they produce heavy
elements from A~80 up to the actinides, with relative contributions of A>130
nuclei being subdominant and sensitively dependent on BH and torus masses and
the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus
phases can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 30 pages, 22 figures; revised version, accepted by MNRAS; appendix
added with test results for neutrino transpor
On Outage Probability and Diversity-Multiplexing Tradeoff in MIMO Relay Channels
Fading MIMO relay channels are studied analytically, when the source and
destination are equipped with multiple antennas and the relays have a single
one. Compact closed-form expressions are obtained for the outage probability
under i.i.d. and correlated Rayleigh-fading links. Low-outage approximations
are derived, which reveal a number of insights, including the impact of
correlation, of the number of antennas, of relay noise and of relaying
protocol. The effect of correlation is shown to be negligible, unless the
channel becomes almost fully correlated. The SNR loss of relay fading channels
compared to the AWGN channel is quantified. The SNR-asymptotic
diversity-multiplexing tradeoff (DMT) is obtained for a broad class of fading
distributions, including, as special cases, Rayleigh, Rice, Nakagami, Weibull,
which may be non-identical, spatially correlated and/or non-zero mean. The DMT
is shown to depend not on a particular fading distribution, but rather on its
polynomial behavior near zero, and is the same for the simple
"amplify-and-forward" protocol and more complicated "decode-and-forward" one
with capacity achieving codes, i.e. the full processing capability at the relay
does not help to improve the DMT. There is however a significant difference
between the SNR-asymptotic DMT and the finite-SNR outage performance: while the
former is not improved by using an extra antenna on either side, the latter can
be significantly improved and, in particular, an extra antenna can be
traded-off for a full processing capability at the relay. The results are
extended to the multi-relay channels with selection relaying and typical outage
events are identified.Comment: accepted by IEEE Trans. on Comm., 201
Confined bilayers passively regulate shape and stress
Lipid membranes are commonly confined to adjacent subcellular structures or to artificial substrates and particles. We develop an experimental and theoretical framework to investigate the mechanics of confined membranes, including the influence of adhesion, strain, and osmotic pressure. We find that supported lipid bilayers respond to stress by nucleating and evolving spherical and tubular protrusions. In cells, such transformations are generally attributed to proteins. Our results offer insights into the mechanics of cell membranes and can further extend the applications of supported bilayers.Peer ReviewedPostprint (author's final draft
Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori
This paper presents an initial survey of the properties of accretion flows in
the Kerr metric from three-dimensional, general relativistic
magnetohydrodynamic simulations of accretion tori. We consider three fiducial
models of tori around rotating, both prograde and retrograde, and nonrotating
black holes; these three fiducial models are also contrasted with axisymmetric
simulations and a pseudo-Newtonian simulation with equivalent initial
conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and
high-resolution version of figures available at
http://www.astro.virginia.edu/~jd5
Topologically biased random walk with application for community finding in networks
We present a new approach of topology biased random walks for undirected
networks. We focus on a one parameter family of biases and by using a formal
analogy with perturbation theory in quantum mechanics we investigate the
features of biased random walks. This analogy is extended through the use of
parametric equations of motion (PEM) to study the features of random walks {\em
vs.} parameter values. Furthermore, we show an analysis of the spectral gap
maximum associated to the value of the second eigenvalue of the transition
matrix related to the relaxation rate to the stationary state. Applications of
these studies allow {\em ad hoc} algorithms for the exploration of complex
networks and their communities.Comment: 8 pages, 7 figure
- …