2,643 research outputs found
On the development of the Papaloizou-Pringle instability of the black hole-torus systems and quasi-periodic oscillations
We present the numerical study of dynamical instability of a
pressure-supported relativistic torus, rotating around the black hole with a
constant specific angular momentum on a fixed space-time background, in case of
perturbation by a matter coming from the outer boundary. Two dimensional
hydrodynamical equations are solved at equatorial plane using the HRSCS to
study the effect of perturbation on the stable systems. We have found that the
perturbed torus creates an instability which causes the gas falling into the
black hole in a certain dynamical time. All the models indicate an oscillating
torus with certain frequency around their instant equilibrium. The dynamic of
the accreted torus varies with the size of initial stable torus, black hole
spin and other variables, such as Mach number, sound speed, cusp location of
the torus, etc. The mass accretion rate is slightly proportional to the
torus-to-hole mass ratio in the black hole-torus system, but it strongly
depends on the cusp location of the torus. The cusp located in the
equipotential surfaces of the effective potential moves outwards into the
torus. The dynamical change of the torus increases the mass accretion rate and
triggers the Papaloizou-Pringle instability. It is also observed that the
growth of the mode of the Papaloizou-Pringle instability occurs for a
wide range of fluid and hydrodynamical parameters and a black hole spin. We
have also computed the QPOs from the oscillating relativistic torus.Comment: 14 Pages, 14 Figures, High resolution version of Figs.1 and 7 can be
found in the original manuscript. Substantial revision has been made. The
title, abstract and whole text are revise
Influence of osmotic and metal stresses on nitrogenase activity of cyanobacteria ýsolated from paddy fields
Samples were collected from paddy fields in Corum-Turk.ye. Nitrogen-free BG-11 medium was used for isolation of nitrogen fixing cyanobacteria. Acetylene reduction technique was used to determine the effects of different chemical agents on the nitrogenase activities of the cyanobacteria, which were identified at the genus level. Nostoc showed the highest nitrogenase activity (0.09 ethylene ƒÊl/mg.h) at 50 mM salt concentration. At 60 mM sucrose concentration, Nostoc showed the highest nitrogenase activity (0.08 ethylene ƒÊl/mg.h). The highest tolerances for the metals were present in Anabaena (0.006 ethylene ƒÊl/mg.h) for iron (20 ppm), Nodularia 0.1 ethylene ƒÊl/mg.h (for manganese 20 ppm) and Nostoc 0.96 ethylene ƒÊl/mg.h (for zinc 5 ppm)
Optimal Investment Under Transaction Costs: A Threshold Rebalanced Portfolio Approach
We study optimal investment in a financial market having a finite number of
assets from a signal processing perspective. We investigate how an investor
should distribute capital over these assets and when he should reallocate the
distribution of the funds over these assets to maximize the cumulative wealth
over any investment period. In particular, we introduce a portfolio selection
algorithm that maximizes the expected cumulative wealth in i.i.d. two-asset
discrete-time markets where the market levies proportional transaction costs in
buying and selling stocks. We achieve this using "threshold rebalanced
portfolios", where trading occurs only if the portfolio breaches certain
thresholds. Under the assumption that the relative price sequences have
log-normal distribution from the Black-Scholes model, we evaluate the expected
wealth under proportional transaction costs and find the threshold rebalanced
portfolio that achieves the maximal expected cumulative wealth over any
investment period. Our derivations can be readily extended to markets having
more than two stocks, where these extensions are pointed out in the paper. As
predicted from our derivations, we significantly improve the achieved wealth
over portfolio selection algorithms from the literature on historical data
sets.Comment: Submitted to IEEE Transactions on Signal Processin
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