399 research outputs found
Monitoring RXTE Observations of Markarian 348: the origin of the column density variations
We analyze 37 RXTE observations of the type 2 Seyfert galaxy Mrk348 obtained
during a period of 14 months. We confirm the spectral variability previous
reported by Smith et al., in the sense that thecolumn density decreases by a
factor of ~3 as the count rate increases. Column density variations could
possibly originate either due to the random drift of clouds within the
absorption screen, or due to photoionization processes. Our modeling of the
observed variations implies that the first scenario is more likely. These
clouds should lie in a distance of >2 light years from the source, having a
diameter of a few light days and a density of >10^7 cm^(-3), hence probably
residing outside the Broad Line Region.Comment: 6 pages, 3 figures, to appear in MNRA
Dengue disease, basic reproduction number and control
Dengue is one of the major international public health concerns. Although
progress is underway, developing a vaccine against the disease is challenging.
Thus, the main approach to fight the disease is vector control. A model for the
transmission of Dengue disease is presented. It consists of eight mutually
exclusive compartments representing the human and vector dynamics. It also
includes a control parameter (insecticide) in order to fight the mosquito. The
model presents three possible equilibria: two disease-free equilibria (DFE) and
another endemic equilibrium. It has been proved that a DFE is locally
asymptotically stable, whenever a certain epidemiological threshold, known as
the basic reproduction number, is less than one. We show that if we apply a
minimum level of insecticide, it is possible to maintain the basic reproduction
number below unity. A case study, using data of the outbreak that occurred in
2009 in Cape Verde, is presented.Comment: This is a preprint of a paper whose final and definitive form has
appeared in International Journal of Computer Mathematics (2011), DOI:
10.1080/00207160.2011.55454
Reaction-Diffusion System in a Vesicle with Semi-Permeable Membrane
We study the Schloegl model in a vesicle with semi-permeable membrane. The
diffusion constant takes a smaller value in the membrane region, which prevents
the outflow of self-catalytic product. A nonequilibrium state is stably
maintained inside of the vesicle. Nutrients are absorbed and waste materials
are exhausted through the membrane by diffusion. It is interpreted as a model
of primitive metabolism in a cell.Comment: 8 pages, 6 figure
Universal Texture of Quark and Lepton Mass Matrices and a Discrete Symmetry Z_3
Recent neutrino data have been favourable to a nearly bimaximal mixing, which
suggests a simple form of the neutrino mass matrix. Stimulated by this matrix
form, a possibility that all the mass matrices of quarks and leptons have the
same form as in the neutrinos is investigated. The mass matrix form is
constrained by a discrete symmetry Z_3 and a permutation symmetry S_2. The
model, of course, leads to a nearly bimaximal mixing for the lepton sectors,
while, for the quark sectors, it can lead to reasonable values of the CKM
mixing matrix and masses.Comment: 24 pages, RevTEX, no figure, some references and comments were adde
Stability of cluster solutions in a cooperative consumer chain model
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ Springer-Verlag Berlin Heidelberg 2012.We study a cooperative consumer chain model which consists of one producer and two consumers. It is an extension of the Schnakenberg model suggested in Gierer and Meinhardt [Kybernetik (Berlin), 12:30-39, 1972] and Schnakenberg (J Theor Biol, 81:389-400, 1979) for which there is only one producer and one consumer. In this consumer chain model there is a middle component which plays a hybrid role: it acts both as consumer and as producer. It is assumed that the producer diffuses much faster than the first consumer and the first consumer much faster than the second consumer. The system also serves as a model for a sequence of irreversible autocatalytic reactions in a container which is in contact with a well-stirred reservoir. In the small diffusion limit we construct cluster solutions in an interval which have the following properties: The spatial profile of the third component is a spike. The profile for the middle component is that of two partial spikes connected by a thin transition layer. The first component in leading order is given by a Green's function. In this profile multiple scales are involved: The spikes for the middle component are on the small scale, the spike for the third on the very small scale, the width of the transition layer for the middle component is between the small and the very small scale. The first component acts on the large scale. To the best of our knowledge, this type of spiky pattern has never before been studied rigorously. It is shown that, if the feedrates are small enough, there exist two such patterns which differ by their amplitudes.We also study the stability properties of these cluster solutions. We use a rigorous analysis to investigate the linearized operator around cluster solutions which is based on nonlocal eigenvalue problems and rigorous asymptotic analysis. The following result is established: If the time-relaxation constants are small enough, one cluster solution is stable and the other one is unstable. The instability arises through large eigenvalues of order O(1). Further, there are small eigenvalues of order o(1) which do not cause any instabilities. Our approach requires some new ideas: (i) The analysis of the large eigenvalues of order O(1) leads to a novel system of nonlocal eigenvalue problems with inhomogeneous Robin boundary conditions whose stability properties have been investigated rigorously. (ii) The analysis of the small eigenvalues of order o(1) needs a careful study of the interaction of two small length scales and is based on a suitable inner/outer expansion with rigorous error analysis. It is found that the order of these small eigenvalues is given by the smallest diffusion constant Īµ22.RGC of Hong Kon
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