1,789 research outputs found
An upper limit on nickel overabundance in the supercritical accretion disk wind of SS 433 from X-ray spectroscopy
We take advantage of a long (with a total exposure time of 120 ks) X-ray
observation of the unique Galactic microquasar SS 433, carried out with the
XMM-Newton space observatory, to search for a fluorescent line of neutral (or
weakly ionized) nickel at the energy 7.5 keV. We consider two models of the
formation of fluorescent lines in the spectrum of SS 433: 1) due to reflection
of hard X-ray radiation from a putative central source on the optically thick
walls of the accretion disk "funnel"; and 2) due to scattering of the radiation
coming from the hottest parts of the jets in the optically thin wind of the
system. It is shown, that for these cases, the photon flux of Ni I K
fluorescent line is expected to be 0.45 of the flux of Fe I K
fluorescent line at 6.4 keV, for the relative nickel overabundance , as observed in the jets of SS 433. For the continuum model without the
absorption edge of neutral iron, we set a 90 per cent upper limit on the flux
of the narrow Ni I K line at the level of ph
s cm. For the continuum model with the absorption edge, the
corresponding upper limit is ph s cm. At the
same time, for the Fe I K line, we measure the flux of
ph s cm. Taken at the face
value, the results imply that the relative overabundance of nickel in the wind
of the accretion disc should be at least 1.5 times less than the corresponding
excess of nickel observed in the jets of SS 433.Comment: 17 pages, 12 figures, 4 tables, Astronomy Letters, in press, 2018,
Volume 44, Issue
The shape evolution of cometary nuclei via anisotropic mass loss
Context. Breathtaking imagery recorded during the European Space Agency's
Rosetta mission confirmed the bilobate nature of comet
67P/Churyumov-Gerasimenko's nucleus. Its peculiar appearance is not unique
among comets. The majority of cometary cores imaged at high resolution exhibit
a similar build. Various theories have been brought forward as to how cometary
nuclei attain such peculiar shapes.
Aims. We illustrate that anisotropic mass loss and local collapse of
subsurface structures caused by non-uniform exposure of the nucleus to solar
irradiation can transform initially spherical comet cores into bilobed ones.
Methods. A mathematical framework to describe the changes in morphology
resulting from non-uniform insolation during a nucleus' spin-orbit evolution is
derived. The resulting partial differential equations that govern the change in
the shape of a nucleus subject to mass loss and consequent collapse of depleted
subsurface structures are solved analytically for simple insolation
configurations and numerically for more realistic scenarios.
Results. The here proposed mechanism is capable of explaining why a large
fraction of periodic comets appear to have peanut-shaped cores and why
light-curve amplitudes of comet nuclei are on average larger than those of
typical main belt asteroids of the same size.Comment: 4 pages of the main text, 2 pages of appendix, 4 figure
Chaos-Order Transition in Matrix Theory
Classical dynamics in SU(2) Matrix theory is investigated. A classical
chaos-order transition is found. For the angular momentum small enough (even
for small coupling constant) the system exhibits a chaotic behavior, for
angular momentum large enough the system is regular.Comment: 14 pages, Latex, 10 figure
Superbroad Component in Emission Lines of SS 433
We have detected new components in stationary emission lines of SS 433; these
are the superbroad components that are low-contrast substrates with a width of
2000--2500 km s-1 in He I and H and 4000--5000 km s-1 in
He II . Based on 44 spectra taken during four years of
observations from 2003 to 2007, we have found that these components in the He
II and He I lines are eclipsed by the donor star; their behavior with
precessional and orbital phases is regular and similar to the behavior of the
optical brightness of SS 433. The same component in H shows neither
eclipses nor precessional variability. We conclude that the superbroad
components in the helium and hydrogen lines are different in origin. Electron
scattering is shown to reproduce well the superbroad component of H at a
gas temperature of 20--35 kK and an optical depth for Thomson scattering 0.25--0.35. The superbroad components of the helium lines are probably
formed in the wind from the supercritical accretion disk. We have computed a
wind model based on the concept of Shakura-Sunyaev supercritical disk
accretion. The main patterns of the He II line profiles are well reproduced in
this model: not only the appearance of the superbroad component but also the
evolution of the central two-component part of the profile of this line during
its eclipse by the donor star can be explained.Comment: 17 pages, 13 figures, 2 tables, published in Astronomy Letters, 2013,
vol. 39, N 12, pp. 826 - 84
The geometry of spontaneous spiking in neuronal networks
The mathematical theory of pattern formation in electrically coupled networks
of excitable neurons forced by small noise is presented in this work. Using the
Freidlin-Wentzell large deviation theory for randomly perturbed dynamical
systems and the elements of the algebraic graph theory, we identify and analyze
the main regimes in the network dynamics in terms of the key control
parameters: excitability, coupling strength, and network topology. The analysis
reveals the geometry of spontaneous dynamics in electrically coupled network.
Specifically, we show that the location of the minima of a certain continuous
function on the surface of the unit n-cube encodes the most likely activity
patterns generated by the network. By studying how the minima of this function
evolve under the variation of the coupling strength, we describe the principal
transformations in the network dynamics. The minimization problem is also used
for the quantitative description of the main dynamical regimes and transitions
between them. In particular, for the weak and strong coupling regimes, we
present asymptotic formulas for the network activity rate as a function of the
coupling strength and the degree of the network. The variational analysis is
complemented by the stability analysis of the synchronous state in the strong
coupling regime. The stability estimates reveal the contribution of the network
connectivity and the properties of the cycle subspace associated with the graph
of the network to its synchronization properties. This work is motivated by the
experimental and modeling studies of the ensemble of neurons in the Locus
Coeruleus, a nucleus in the brainstem involved in the regulation of cognitive
performance and behavior
Towards a Simple Model of Compressible Alfvenic Turbulence
A simple model collisionless, dissipative, compressible MHD (Alfvenic)
turbulence in a magnetized system is investigated. In contrast to more familiar
paradigms of turbulence, dissipation arises from Landau damping, enters via
nonlinearity, and is distributed over all scales. The theory predicts that two
different regimes or phases of turbulence are possible, depending on the ratio
of steepening to damping coefficient (m_1/m_2). For strong damping
(|m_1/m_2|<1), a regime of smooth, hydrodynamic turbulence is predicted. For
|m_1/m_2|>1, steady state turbulence does not exist in the hydrodynamic limit.
Rather, spikey, small scale structure is predicted.Comment: 6 pages, one figure, REVTeX; this version to be published in PRE. For
related papers, see http://sdphpd.ucsd.edu/~medvedev/papers.htm
Shaping bursting by electrical coupling and noise
Gap-junctional coupling is an important way of communication between neurons
and other excitable cells. Strong electrical coupling synchronizes activity
across cell ensembles. Surprisingly, in the presence of noise synchronous
oscillations generated by an electrically coupled network may differ
qualitatively from the oscillations produced by uncoupled individual cells
forming the network. A prominent example of such behavior is the synchronized
bursting in islets of Langerhans formed by pancreatic \beta-cells, which in
isolation are known to exhibit irregular spiking. At the heart of this
intriguing phenomenon lies denoising, a remarkable ability of electrical
coupling to diminish the effects of noise acting on individual cells.
In this paper, we derive quantitative estimates characterizing denoising in
electrically coupled networks of conductance-based models of square wave
bursting cells. Our analysis reveals the interplay of the intrinsic properties
of the individual cells and network topology and their respective contributions
to this important effect. In particular, we show that networks on graphs with
large algebraic connectivity or small total effective resistance are better
equipped for implementing denoising. As a by-product of the analysis of
denoising, we analytically estimate the rate with which trajectories converge
to the synchronization subspace and the stability of the latter to random
perturbations. These estimates reveal the role of the network topology in
synchronization. The analysis is complemented by numerical simulations of
electrically coupled conductance-based networks. Taken together, these results
explain the mechanisms underlying synchronization and denoising in an important
class of biological models
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