2,336 research outputs found
Rich-club network topology to minimize synchronization cost due to phase difference among frequency-synchronized oscillators
Functions of some networks, such as power grids and large-scale brain
networks, rely on not only frequency synchronization, but also phase
synchronization. Nevertheless, even after the oscillators reach to
frequency-synchronized status, phase difference among oscillators often shows
non-zero constant values. Such phase difference potentially results in
inefficient transfer of power or information among oscillators, and avoid
proper and efficient functioning of the network. In the present study, we newly
define synchronization cost by the phase difference among the
frequency-synchronized oscillators, and investigate the optimal network
structure with the minimum synchronization cost through rewiring-based
optimization. By using the Kuramoto model, we demonstrate that the cost is
minimized in a network topology with rich-club organization, which comprises
the densely-connected center nodes and peripheral nodes connecting with the
center module. We also show that the network topology is characterized by its
bimodal degree distribution, which is quantified by Wolfson's polarization
index. Furthermore, we provide analytical interpretation on why the rich-club
network topology is related to the small amount of synchronization cost.Comment: 4 figures + one appendix figur
Driving the Growth of the Earliest Supermassive Black Holes with Major Mergers of Host Galaxies
The formation mechanism of supermassive black holes (SMBHs) in general, and
of SMBHs observed as luminous quasars at redshifts
in particular, remains an open fundamental question. The presence of
such massive BHs at such early times, when the Universe was less than a billion
years old, implies that they grew via either super-Eddington accretion, or
nearly uninterrupted gas accretion near the Eddington limit; the latter, at
first glance, is at odds with empirical trends at lower redshifts, where quasar
episodes associated with rapid BH growth are rare and brief. In this work, I
examine whether and to what extent the growth of the quasar SMBHs can be
explained within the standard quasar paradigm, in which major mergers of host
galaxies trigger episodes of rapid gas accretion below or near the Eddington
limit. Using a suite of Monte Carlo merger tree simulations of the assembly
histories of the likely hosts of the quasars, I investigate (i) their
growth and major merger rates out to , and (ii) how long the feeding
episodes induced by host mergers must last in order to explain the observed quasar population without super-Eddington accretion. The halo major merger
rate scales roughly as , with quasar hosts typically
experiencing major mergers between (), compared to for typical massive galaxies at
(). An example of a viable sub-Eddington SMBH growth
model is one where a host merger triggers feeding for a duration comparable to
the halo dynamical time. These findings suggest that the growth mechanisms of
the earliest quasar SMBHs need not have been drastically different from their
counterparts at lower redshifts.Comment: 24 pages, 5 figures, invited submission to a focus issue on galactic
centers in Classical and Quantum Gravit
Recurring flares from supermassive black hole binaries: implications for tidal disruption candidates and OJ 287
I discuss the possibility that accreting supermassive black hole (SMBH)
binaries with sub-parsec separations produce periodically recurring luminous
outbursts that interrupt periods of relative quiescence. This hypothesis is
motivated by two characteristics found generically in simulations of binaries
embedded in prograde accretion discs: (i) the formation of a central,
low-density cavity around the binary, and (ii) the leakage of gas into this
cavity, occurring once per orbit via discrete streams on nearly radial
trajectories. The first feature would reduce the emergent optical/UV flux of
the system relative to active galactic nuclei powered by single SMBHs, while
the second can trigger quasiperiodic fluctuations in luminosity. I argue that
the quasiperiodic accretion signature may be much more dramatic than previously
thought, because the infalling gas streams can strongly shock-heat via
self-collision and tidal compression, thereby enhancing viscous accretion. Any
optically thick gas that is circularized about either SMBH can accrete before
the next pair of streams is deposited, fueling transient, luminous flares that
recur every orbit. Due to the diminished flux in between accretion episodes,
such cavity-accretion flares could plausibly be mistaken for the tidal
disruptions of stars in quiescent nuclei. The flares could be distinguished
from tidal disruption events if their quasiperiodic recurrence is observed, or
if they are produced by very massive SMBHs that cannot disrupt solar-type
stars. They may be discovered serendipitously in surveys such as LSST or
eROSITA. I present a heuristic toy model as a proof of concept for the
production of cavity-accretion flares, and generate mock light curves and
specta. I also apply the model to the active galaxy OJ 287, whose production of
quasiperiodic pairs of optical flares has long fueled speculation that it hosts
a SMBH binary.Comment: 15 pages, 5 figures. Accepted for publication in MNRA
A note on testing parameter constancy in cointegrated vector autoregression: the case of near I(2) processes
This note investigates the behaviour of a parameter-constancy test statistic when near I(2) (integrated of order 2) variables are incorporated in a cointegrated vector autoregressive system. Simulation studies indicate that the presence of such variables has a significant impact on size properties of the constancy test.Parameter Constancy, Cointegraed Vector Autoregression, Near I(2) Variable.
A note on small-sample correction for hypothesis testing on cointegrating vectors: recursive Monte Carlo analysis
This note conducts recursive Monte Carlo experiments on the Bartlett correction for a likelihood-based test on cointegrating vectors. The experiments show that the correction can reduce size distortions even in situations where regularity conditions for I(1) cointegration analysis are satisfied only marginally.Cointegrating Vector, Small Sample, Bartlett Correction, Recursive Monte Carlo Experiment.
Investigating time series properties of a dynamic system for Japan's import demand
This note aims to investigate time series properties of a dynamic system for Japan's aggregate import demand. A multivariate cointegration analysis of Japanese data reveals a stable economic linkage interpretable as a long-run import demand function. A vector equilibrium correction system is then estimated, which exhibits short-run and long-run interdependent relationships between aggregate import demand and the ratio of import price to domestic price level.Aggregate Import Demand, Cointegration, Vector Equilibrium Correction System.
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