336 research outputs found
On the number of limit cycles of the Lienard equation
In this paper, we study a Lienard system of the form dot{x}=y-F(x),
dot{y}=-x, where F(x) is an odd polynomial. We introduce a method that gives a
sequence of algebraic approximations to the equation of each limit cycle of the
system. This sequence seems to converge to the exact equation of each limit
cycle. We obtain also a sequence of polynomials R_n(x) whose roots of odd
multiplicity are related to the number and location of the limit cycles of the
system.Comment: 10 pages, 5 figures. Submitted to Physical Review
Galaxy Clusters as Reservoirs of Heavy Dark Matter and High-Energy Cosmic Rays: Constraints from Neutrino Observations
Galaxy Clusters (GCs) are the largest reservoirs of both dark matter and
cosmic rays (CRs). Dark matter self-annihilation can lead to a high luminosity
in gamma rays and neutrinos, enhanced by a strong degree of clustering in dark
matter substructures. Hadronic CR interactions can also lead to a high
luminosity in gamma rays and neutrinos, enhanced by the confinement of CRs from
cluster accretion/merger shocks and active galactic nuclei. We show that
IceCube/KM3Net observations of high-energy neutrinos can probe the nature of
GCs and the separate dark matter and CR emission processes, taking into account
how the results depend on the still-substantial uncertainties. Neutrino
observations are relevant at high energies, especially at >10 TeV. Our results
should be useful for improving experimental searches for high-energy neutrino
emission. Neutrino telescopes are sensitive to extended sources formed by dark
matter substructures and CRs distributed over large scales. Recent observations
by Fermi and imaging atmospheric Cherenkov telescopes have placed interesting
constraints on the gamma-ray emission from GCs. We also provide calculations of
the gamma-ray fluxes, taking into account electromagnetic cascades inside GCs,
which can be important for injections at sufficiently high energies. This also
allows us to extend previous gamma-ray constraints to very high dark matter
masses and significant CR injections at very high energies. Using both
neutrinos and gamma rays, which can lead to comparable constraints, will allow
more complete understandings of GCs. Neutrinos are essential for some dark
matter annihilation channels, and for hadronic instead of electronic CRs. Our
results suggest that the multi-messenger observations of GCs will be able to
give useful constraints on specific models of dark matter and CRs. [Abstract
abridged.]Comment: 31 pages, 20 figures, 1 table, accepted for publication in JCAP,
references and discussions adde
HST Observations of the Stellar Distribution Near Sgr A*
We present HST/NICMOS data to study the surface brightness distribution of
stellar light within the inner 10" of Sgr A* at 1.4, 1.7 and 1.9 microns. We
use these data to independently examine the surface brightness distribution
that had been measured previously with NICMOS and to determine whether there is
a drop in the surface density of stars very near Sgr A*. Our analysis confirms
that a previously reported drop in the surface brightness within 0.8" of Sgr A*
is an artifact of bright and massive stars near that radius. We also show that
the surface brightness profile within 5" or ~0.2 pc of Sgr A* can be fitted
with broken power laws. The power laws are consistent with previous
measurements, in that the profile becomes shallower at small radii. For radii >
0.7" the slope is beta=-0.34\pm0.04 where Sigma is proportional to r^beta and
becomes flatter at smaller radii with beta=-0.13\pm0.04. Modeling of the
surface brightness profile gives a stellar density that increases roughly as
r^-1 within the inner 1" of Sgr A*. This slope confirms earlier measurements in
that it is not consistent with that expected from an old, dynamically-relaxed
stellar cluster with a central supermassive black hole. Assuming that the
diffuse emission is not contaminated by a faint population of young stars down
to the 17.1 magnitude limit of our imaging data at 1.70, the shallow cusp
profile is not consistent with a decline in stellar density in the inner
arcsecond. In addition, converting our measured diffuse light profile to a
stellar mass profile, with the assumption that the light is dominated by K0
dwarfs, the enclosed stellar mass within radius r < 0.1 pc of Sgr A* is ~
3.2x10^4 M_solar (r/0.1 {pc})^2.1.Comment: 16 pages, 7 figures, ApJ, in pres
Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts
It is only now, with low-frequency radio telescopes, long exposures with
high-resolution X-ray satellites and gamma-ray telescopes, that we are
beginning to learn about the physics in the periphery of galaxy clusters. In
the coming years, Sunyaev-Zeldovich telescopes are going to deliver further
great insights into the plasma physics of these special regions in the
Universe. The last years have already shown tremendous progress with detections
of shocks, estimates of magnetic field strengths and constraints on the
particle acceleration efficiency. X-ray observations have revealed shock fronts
in cluster outskirts which have allowed inferences about the microphysical
structure of shocks fronts in such extreme environments. The best indications
for magnetic fields and relativistic particles in cluster outskirts come from
observations of so-called radio relics, which are megaparsec-sized regions of
radio emission from the edges of galaxy clusters. As these are difficult to
detect due to their low surface brightness, only few of these objects are
known. But they have provided unprecedented evidence for the acceleration of
relativistic particles at shock fronts and the existence of muG strength fields
as far out as the virial radius of clusters. In this review we summarise the
observational and theoretical state of our knowledge of magnetic fields,
relativistic particles and shocks in cluster outskirts.Comment: 34 pages, to be published in Space Science Review
Statistical Mechanics of Canonical-Dissipative Systems and Applications to Swarm Dynamics
We develop the theory of canonical-dissipative systems, based on the
assumption that both the conservative and the dissipative elements of the
dynamics are determined by invariants of motion. In this case, known solutions
for conservative systems can be used for an extension of the dynamics, which
also includes elements such as the take-up/dissipation of energy. This way, a
rather complex dynamics can be mapped to an analytically tractable model, while
still covering important features of non-equilibrium systems. In our paper,
this approach is used to derive a rather general swarm model that considers (a)
the energetic conditions of swarming, i.e. for active motion, (b) interactions
between the particles based on global couplings. We derive analytical
expressions for the non-equilibrium velocity distribution and the mean squared
displacement of the swarm. Further, we investigate the influence of different
global couplings on the overall behavior of the swarm by means of
particle-based computer simulations and compare them with the analytical
estimations.Comment: 14 pages incl. 13 figures. v2: misprints in Eq. (40) corrected, ref.
updated. For related work see also:
http://summa.physik.hu-berlin.de/~frank/active.htm
The Swift BAT Perspective on Non-thermal Emission in HIFLUGCS Galaxy Clusters
The search for diffuse non-thermal, inverse Compton (IC) emission from galaxy
clusters at hard X-ray energies has been underway for many years, with most
detections being either of low significance or controversial. In this work, we
investigate 14-195 keV spectra from the Swift Burst Alert Telescope (BAT)
all-sky survey for evidence of non-thermal excess emission above the
exponentially decreasing tail of thermal emission in the flux-limited HIFLUGCS
sample. To account for the thermal contribution at BAT energies, XMM-Newton
EPIC spectra are extracted from coincident spatial regions so that both thermal
and non-thermal spectral components can be determined simultaneously. We find
marginally significant IC components in six clusters, though after closer
inspection and consideration of systematic errors we are unable to claim a
clear detection in any of them. The spectra of all clusters are also summed to
enhance a cumulative non-thermal signal not quite detectable in individual
clusters. After constructing a model based on single-temperature fits to the
XMM-Newton data alone, we see no significant excess emission above that
predicted by the thermal model determined at soft energies. This result also
holds for the summed spectra of various subgroups, except for the subsample of
clusters with diffuse radio emission. For clusters hosting a diffuse radio
halo, a relic, or a mini-halo, non-thermal emission is initially detected at
the \sim5-sigma confidence level - driven by clusters with mini-halos - but
modeling and systematic uncertainties ultimately degrade this significance. In
individual clusters, the non-thermal pressure of relativistic electrons is
limited to \sim10% of the thermal electron pressure, with stricter limits for
the more massive clusters, indicating that these electrons are likely not
dynamically important in the central regions of clusters.Comment: 25 pages, 15 figures; some figure and table numbering differs from
published ApJ version: please see that for superior formattin
Radio observations of ZwCl 2341.1+0000: a double radio relic cluster
Context: Hierarchal models of large scale structure (LSS) formation predict
that galaxy clusters grow via gravitational infall and mergers of (smaller)
mass concentrations, such as clusters and galaxy groups. Diffuse radio
emission, in the form of radio halos and relics, is found in clusters
undergoing a merger, indicating that shocks or turbulence associated with the
merger are capable of accelerating electrons to highly relativistic energies.
Here we report on radio observations of ZwCl 2341.1+0000, a complex merging
structure of galaxies located at z=0.27, using Giant Metrewave Radio Telescope
(GMRT) observations.
Aims: The main aim of the observations is to study the nature of the diffuse
radio emission in the galaxy cluster ZwCl 2341.1+0000.
Methods: We have carried out GMRT 610, 241, and 157 MHz continuum
observations of ZwCl 2341.1+0000. The radio observations are combined with
X-ray and optical data of the cluster.
Results: The GMRT observations show the presence of a double peripheral radio
relic in the cluster ZwCl 2341.1+0000. The spectral index is -0.49 \pm 0.18 for
the northern relic and -0.76 \pm 0.17 for the southern relic respectively. We
have derived values of 0.48-0.93 microGauss for the equipartition magnetic
field strength. The relics are probably associated with an outwards traveling
merger shock waves.Comment: 14 pages, 10 figures, accepted for publication in A&A on July 30,
200
Singularly Perturbed Monotone Systems and an Application to Double Phosphorylation Cycles
The theory of monotone dynamical systems has been found very useful in the
modeling of some gene, protein, and signaling networks. In monotone systems,
every net feedback loop is positive. On the other hand, negative feedback loops
are important features of many systems, since they are required for adaptation
and precision. This paper shows that, provided that these negative loops act at
a comparatively fast time scale, the main dynamical property of (strongly)
monotone systems, convergence to steady states, is still valid. An application
is worked out to a double-phosphorylation ``futile cycle'' motif which plays a
central role in eukaryotic cell signaling.Comment: 21 pages, 3 figures, corrected typos, references remove
Intermediate Asymptotics of the Kerr Quasinormal Spectrum
We study analytically the quasinormal mode spectrum of near-extremal
(rotating) Kerr black holes. We find an analytic expression for these
black-hole resonances in terms of the black-hole physical parameters: its
Bekenstein-Hawking temperature T_{BH} and its horizon's angular velocity
\Omega, which is valid in the intermediate asymptotic regime
1<<\omega<<1/T_{BH}.Comment: 4 page
Indirect signals from light neutralinos in supersymmetric models without gaugino mass unification
We examine indirect signals produced by neutralino self-annihilations, in the
galactic halo or inside celestial bodies, in the frame of an effective MSSM
model without gaugino-mass unification at a grand unification scale. We compare
our theoretical predictions with current experimental data of gamma-rays and
antiprotons in space and of upgoing muons at neutrino telescopes. Results are
presented for a wide range of the neutralino mass, though our discussions are
focused on light neutralinos. We find that only the antiproton signal is
potentially able to set constraints on very low-mass neutralinos, below 20 GeV.
The gamma-ray signal, both from the galactic center and from high galactic
latitudes, requires significantly steep profiles or substantial clumpiness in
order to reach detectable levels. The up-going muon signal is largely below
experimental sensitivities for the neutrino flux coming from the Sun; for the
flux from the Earth an improvement of about one order of magnitude in
experimental sensitivities (with a low energy threshold) can make accessible
neutralino masses close to O, Si and Mg nuclei masses, for which resonant
capture is operative.Comment: 17 pages, 1 tables and 5 figures, typeset with ReVTeX4. The paper may
also be found at http://www.to.infn.it/~fornengo/papers/indirect04.ps.gz or
through http://www.astroparticle.to.infn.it/. Limit from BR(Bs--> mu+ mu-)
adde
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