16,250 research outputs found
Observers in Kerr spacetimes: the ergoregion on the equatorial plane
We perform a detailed analysis of the properties of stationary observers
located on the equatorial plane of the ergosphere in a Kerr spacetime,
including light-surfaces. This study highlights crucial differences between
black hole and the super-spinner sources. In the case of Kerr naked
singularities, the results allow us to distinguish between "weak" and "strong"
singularities, corresponding to spin values close to or distant from the
limiting case of extreme black holes, respectively. We derive important
limiting angular frequencies for naked singularities. We especially study very
weak singularities as resulting from the spin variation of black holes. We also
explore the main properties of zero angular momentum observers for different
classes of black hole and naked singularity spacetimes.Comment: 20 pages, 13 multi-panels figures, 2 table
Squeezing of toroidal accretion disks
Accretion disks around very compact objects such as very massive Black hole
can grow according to thick toroidal models. We face the problem of defining
how does change the thickness of a toroidal accretion disk spinning around a
Schwarzschild Black hole under the influence of a toroidal magnetic field and
by varying the fluid angular momentum. We consider both an hydrodynamic and a
magnetohydrodynamic disk based on the Polish doughnut thick model. We show that
the torus thickness remains basically unaffected but tends to increase or
decrease slightly depending on the balance of the magnetic, gravitational and
centrifugal effects which the disk is subjected to.Comment: 6 pages, 17 figures, to appear in EP
Thresholds for macroparasite infections
We analyse here the equilibria of an infinite system of partial differential equations modelling the dynamics of a population infected by macroparasites. We find that it is possible to define a reproduction number R0 that satisfies the intuitive definition, and yields a sharp threshold in the behaviour of the system: when R0 1, the PFE is unstable and there exists a unique endemic equilibrium. The results mainly confirm what had been obtained in simplified models, except for the fact that no backwards bifurcation occur in this model. The stability of equilibria is established by extending an abstract linearization principle and by analysing the spectra of appropriate operators
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