155 research outputs found
Theoretical and experimental stress-strain analysis of machining gas turbine engine parts, which made of the high energy structural efficiency alloys
Methods to improve accuracy of the gas turbine engine (GTE) blades machining by analyzing the stress-strain state were
reviewed. The new method to compensate for machining error was proposed. The model of parameters that affect cutting
mode and the radial component of cutting force during milling was developed
Sodium–Calcium Exchanger Can Account for Regenerative Ca2+ Entry in Thin Astrocyte Processes
Calcium transients in thin astrocytic processes can be important in synaptic plasticity, but their mechanism is not completely understood. Clearance of synaptic glutamate leads to increase in astrocytic sodium. This can electrochemically favor the reverse mode of the Na/Ca-exchanger (NCX) and allow calcium into the cell, accounting for activity-dependent calcium transients in perisynaptic astrocytic processes. However, cytosolic sodium and calcium are also allosteric regulators of the NCX, thus adding kinetic constraints on the NCX-mediated fluxes and providing for complexity of the system dynamics. Our modeling indicates that the calcium-dependent activation and also calcium-dependent escape from the sodium-mediated inactive state of the NCX in astrocytes can form a positive feedback loop and lead to regenerative calcium influx. This can result in sodium-dependent amplification of calcium transients from nearby locations or other membrane mechanisms. Prolonged conditions of elevated sodium, for example in ischemia, can also lead to bistability in cytosolic calcium levels, where a delayed transition to the high-calcium state can be triggered by a short calcium transient. These theoretical predictions call for a dedicated experimental estimation of the kinetic parameters of the astrocytic Na/Ca-exchanger
Magnetic fileds of coalescing neutron stars and the luminosity function of short gamma-ray burst
Coalescing neutron star binaries are believed to be the most reliable sources
for ground-based detectors of gravitational waves and likely progenitors of
short gamma-ray bursts. In the process of coalescence, magnetic fields of
neutron stars can induce interesting observational manifestations and affect
the form of gravitational wave signal. In this papaer we use the population
synthesis method to model the expected distribution of neutron star magnetic
fields during the coalescence under different assumptions on the initial
parameters of neutron stars and their magnetic field evolution. We discuss
possible elecotrmagnetic phenomena preceding the coalescence of magnetized
neutron star binaries and the effect of magnetic field on the gravitational
wave signal. We find that a log-normal (Gaussian in logarithms) distribution of
the initial magnetic fields of neutron stars, which agrees with observed
properties of radio pulsars, produces the distribution of the magnetic field
energy during the coalescence that adequately describes the observed luminosity
function of short gamma-ray bursts under different assumptions on the field
evolution and initial parameters of neutron stars. This agreement lends further
support to the model of coalescing neutron star binaries as progenitors of
gamma-ray bursts.Comment: v.2, LATEX, 25 pages, inc. 7 ps figures, Astron. Lett., in press.
Typos corrected, reference adde
Algebraic approach to time-delay data analysis for LISA
Cancellation of laser frequency noise in interferometers is crucial for
attaining the requisite sensitivity of the triangular 3-spacecraft LISA
configuration. Raw laser noise is several orders of magnitude above the other
noises and thus it is essential to bring it down to the level of other noises
such as shot, acceleration, etc. Since it is impossible to maintain equal
distances between spacecrafts, laser noise cancellation must be achieved by
appropriately combining the six beams with appropriate time-delays. It has been
shown in several recent papers that such combinations are possible. In this
paper, we present a rigorous and systematic formalism based on algebraic
geometrical methods involving computational commutative algebra, which
generates in principle {\it all} the data combinations cancelling the laser
frequency noise. The relevant data combinations form the first module of
syzygies, as it is called in the literature of algebraic geometry. The module
is over a polynomial ring in three variables, the three variables corresponding
to the three time-delays around the LISA triangle. Specifically, we list
several sets of generators for the module whose linear combinations with
polynomial coefficients generate the entire module. We find that this formalism
can also be extended in a straight forward way to cancel Doppler shifts due to
optical bench motions. The two modules are infact isomorphic.
We use our formalism to obtain the transfer functions for the six beams and
for the generators. We specifically investigate monochromatic gravitational
wave sources in the LISA band and carry out the maximisiation over linear
combinations of the generators of the signal-to-noise ratios with the frequency
and source direction angles as parameters.Comment: 27 Pages, 6 figure
Potential use of antioxidants for the treatment of chronic inflammatory diseases
The excessive production of various reactive oxidant species over endogenous antioxidant defense mechanisms leads to the development of a state of oxidative stress, with serious biological consequences. The consequences of oxidative stress depend on the balance between the generation of reactive oxidant species and the antioxidant defense and include oxidative damage of biomolecules, disruption of signal transduction, mutation, and cell apoptosis. Accumulating evidence suggests that oxidative stress is involved in the physiopathology of various debilitating illnesses associated with chronic inflammation, including cardiovascular diseases, diabetes, cancer, or neurodegenerative processes, that need continuous pharmacological treatment. Oxidative stress and chronic inflammation are tightly linked pathophysiological processes, one of which can be simply promoted by another. Although, many antioxidant trials have been unsuccessful (some of the trials showed either no effect or even harmful effects) in human patients as a preventive or curative measure, targeting oxidative stress remains an interesting therapeutic approach for the development of new agents to design novel anti-inflammatory drugs with a reliable safety profile. In this regard, several natural antioxidant compounds were explored as potential therapeutic options for the treatment of chronic inflammatory diseases. Several metalloenzymes, such as superoxide dismutase, catalase, and glutathione peroxidase, are among the essential enzymes that maintain the low nanomolar physiological concentrations of superoxide (O2•−) and hydrogen peroxide (H2O2), the major redox signaling molecules, and thus play important roles in the alteration of the redox homeostasis. These enzymes have become a striking source of motivation to design catalytic drugs to enhance the action of these enzymes under pathological conditions related to chronic inflammation. This review is focused on several major representatives of natural and synthetic antioxidants as potential drug candidates for the treatment of chronic inflammatory diseases
Supernovae - Optical Precursors of Short Gamma-Ray Bursts
The probability of observing "supernova - gamma-ray burst" (GRB) pair events
and recurrent GRBs from one galaxy in a time interval of several years has been
estimated. Supernova explosions in binary systems accompanied by the formation
of a short-lived pair of compact objects can be the sources of such events. If
a short GRB is generated during the collision of a pair, then approximately
each of ~300 short GRBs with redshift z must have an optical precursor - a
supernova in the observer's time interval <2(1+z)yr. If the supernova explosion
has the pattern of a hypernova, then a successive observation of long and short
GRBs is possible. The scenario for the generation of multiple GRBs in
collapsing galactic nuclei is also discussed.Comment: 12 pages, 1 figure; this paper has the e-precursor arXiv:1101.3298
[astro-ph.HE
Observational Constraints on the Angular and Spectral Distributions of Photons in Gamma-Ray Burst Sources
The typical spectra of gamma-ray bursts (GRBs) are discussed in the context
of the compactness problem for GRB sources and how it is resolved in the
popular fireball model. In particular, observational (model-independent)
constraints on the collimation of the gamma-rays and the dependence of the
collimation angle on the photon energy are considered. The fact that the
threshold for the creation of pairs depends on the angle between
the momenta of the annihilating photons in the GRB source provides an
alternative solution to the compactness problem. A new approach to explaining
GRBs, taking into account the angular dependence for pair creation, is
proposed, and the main features of a scenario describing a GRB source with a
total (photon) energy smaller or of the order of erg are laid out.
Thus, we are dealing with an alternative to an ultra-relativistic fireball, if
it turns out (as follows from observations) that all "long" GRBs are associated
with normal (not peculiar) core-collapse supernovae. The effects of radiation
pressure and the formation of jets as a consequence of even a small amount of
anisotropy in the total radiation field in a (compact) GRB source are examined
in this alternative model. Possible energy release mechanisms acting in regions
smaller or of the order of cm in size (a compact model for a GRB) are
discussed. New observational evidence for such compact energy release in the
burst source is considered.Comment: 15 pages, no figures, no table
Coherence Resonance and Noise-Induced Synchronization in Globally Coupled Hodgkin-Huxley Neurons
The coherence resonance (CR) of globally coupled Hodgkin-Huxley neurons is
studied. When the neurons are set in the subthreshold regime near the firing
threshold, the additive noise induces limit cycles. The coherence of the system
is optimized by the noise. A bell-shaped curve is found for the peak height of
power spectra of the spike train, being significantly different from a
monotonic behavior for the single neuron. The coupling of the network can
enhance CR in two different ways. In particular, when the coupling is strong
enough, the synchronization of the system is induced and optimized by the
noise. This synchronization leads to a high and wide plateau in the local
measure of coherence curve. The local-noise-induced limit cycle can evolve to a
refined spatiotemporal order through the dynamical optimization among the
autonomous oscillation of an individual neuron, the coupling of the network,
and the local noise.Comment: five pages, five figure
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