213 research outputs found
Advanced research of the impact of rotor bars anisotropic conductivity on starting torque of an induction motor with a die-cast copper cage
The paper deals with an optimal design of an induction motor with a die-cast copper cage rotor characterized by an anisotropic layer in the upper part of the rotor bars. 1D FEM optimization of an explosion-proof induction motor with a die-cast copper rotor with overall parameters of a commercial induction motor with a die-cast aluminum rotor (180 kW, 1140 V, 1500 rpm) is made with the help of numeric computations. In the paper, the design features and experimental results are given. It is shown that induction motors with a special "starting" layer placed in the upper part the diecast copper rotor bar have maximum value of starting torque for relative conductivity factors ranging from 0.3 to 0.5. The basic performance data obtained in both bench and service tests of a prototype 210kW induction motor are in close agreement with analytical data.Изложены результаты исследований влияния анизотропии проводимости стержней ротора на пусковой момент асинхронного двигателя, стендовых и эксплуатационных испытаний взрывозащищенного двигателя типа 2ЭКВ3,5-210 мощностью 210 кВт с литой анизотропной медной обмоткой ротора
Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303
We demonstrate that models of resonant cyclotron radiation transfer in a
strong field (i.e. cyclotron scattering) can account for spectral lines seen at
two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a
generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we
model line formation by injecting continuum photons into a static
plane-parallel slab of electrons threaded by a strong neutron star magnetic
field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the
slab normal. We examine two source geometries, which we denote "1-0" and "1-1,"
with the numbers representing the relative electron column densities above and
below the continuum photon source plane. We compare azimuthally symmetric
models, i.e. models in which the magnetic field is parallel to the slab normal,
with models having more general magnetic field orientations. If the bursting
source has a simple dipole field, these two model classes represent line
formation at the magnetic pole, or elsewhere on the stellar surface. We find
that the data of S1 and S2, considered individually, are consistent with both
geometries, and with all magnetic field orientations, with the exception that
the S1 data clearly favor line formation away from a polar cap in the 1-1
geometry, with the best-fit model placing the line-forming region at the
magnetic equator. Within both geometries, fits to the combined (S1+S2) data
marginally favor models which feature equatorial line formation, and in which
the observer's orientation with respect to the slab changes between the two
epochs. We interpret this change as being due to neutron star rotation, and we
place limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
We report observations with the Chandra X-ray Observatory of the single,
cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray
observations of single white dwarfs, we also re-analyzed archival ROSAT data
for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ
339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD
358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting
the most restrictive upper limit to the X-ray luminosity from any cool white
dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV
thermal-bremsstrahlung spectrum. The corresponding limit to the electron
density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data
confirmed the non-detections reported by the original investigators. We discuss
the implications of our and prior observations on models for coronal emission
from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent
constraints imposed by cyclotron radiation. In addition, we describe (in an
appendix) a statistical methodology for detecting a source and for constraining
the strength of a source, which applies even when the number of source or
background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa
On the Possibility of the Detection of Extinct Radio Pulsars
We explore the possibilities for detecting pulsars that have ceased to
radiate in the radio band. We consider two models: the model with hindered
particle escape from the pulsar surface (first suggested by Ruderman and
Sutherland 1975) and the model with free particle escape (Arons 1981; Mestel
1999). In the model with hindered particle escape, the number of particles that
leave the pulsar magnetosphere is small and their radiation cannot be detected
with currently available instruments. At the same time, for the free particle
escape model, both the number of particles and the radiation intensity are high
enough for such pulsars to be detectable with the presently available receivers
such as GLAST and AGILE spacecrafts. It is also possible that extinct radio
pulsars can be among the unidentified EGRET sources.Comment: 5 pages, 1 figure corrected version of the paper that was published
in Astronomy Letter
Annihilation Emission from the Galactic Black Hole
Both diffuse high energy gamma-rays and an extended electron-positron
annihilation line emission have been observed in the Galactic Center (GC)
region. Although X-ray observations indicate that the galactic black hole Sgr
A is inactive now, we suggest that Sgr A can become active when a
captured star is tidally disrupted and matter is accreted into the black hole.
As a consequence the galactic black hole could be a powerful source of
relativistic protons. We are able to explain the current observed diffuse
gamma-rays and the very detailed 511 keV annihilation line of secondary
positrons by collisions of such protons, with appropriate injection times
and energy. Relativistic protons could have been injected into the ambient
material if the black hole captured a 50M star at several tens million
years ago. An alternative possibility is that the black hole continues to
capture stars with 1M every hundred thousand years. Secondary
positrons produced by collisions at energies \ga 30 MeV are cooled down
to thermal energies by Coulomb collisions, and annihilate in the warm neutral
and ionized phases of the interstellar medium with temperatures about several
eV, because the annihilation cross-section reaches its maximum at these
temperatures. It takes about ten million years for the positrons to cool down
to thermal temperatures so they can diffuse into a very large extended region
around the Galactic center. A much more recent star capture may be also able to
account for recent TeV observations within 10 pc of the galactic center as well
as for the unidentified GeV gamma-ray sources found by EGRET at GC. The
spectral difference between the GeV flux and the TeV flux could be explained
naturally in this model as well.Comment: Accepted by ApJ on March 24, 200
Collective processes in relativistic plasma and their implications for gamma-ray burst afterglows
We consider the effects of collective plasma processes on synchrotron
emission from highly relativistic electrons. We find, in agreement with Sazonov
(1970), that strong effects are possible also in the absence of a
non-relativistic plasma component, due to the relativistic electrons (and
protons) themselves. In contrast with Sazonov, who infers strong effects only
in cases where the ratio of plasma frequency to cyclotron frequency is much
larger than the square of the characteristic electron Lorentz factor, nu_p/nu_B
>> gamma^2, we find strong effects also for 1 << nu_p/nu_B << gamma^2. The
modification of the spectrum is prominent at frequencies nu < nu_{R*} = nu_p
min[gamma, (nu_p/nu_B)^(1/2)], where nu_{R*} generalizes the Razin-Tsytovich
frequency, nu_R = gamma nu_p, to the regime nu_p/nu_B << gamma^2. Applying our
results to gamma-ray burst (GRB) plasmas, we predict a strong modification of
the radio spectrum on minute time scale following the GRB, at the onset of
fireball interaction with its surrounding medium, in cases where the ratio of
the energy carried by the relativistic electrons to the energy carried by the
magnetic field exceeds ~ 10^5. Plausible electron distribution functions may
lead to negative synchrotron reabsorption, i.e to coherent radio emission,
which is characterized by a low degree of circular polarization. Detection of
these effects would constrain the fraction of energy in the magnetic field,
which is currently poorly determined by observations, and, moreover, would
provide a novel handle on the properties of the environment into which the
fireball expands.Comment: 28 pages, 1 figure, submitted to Ap
A number-conserving linear response study of low-velocity ion stopping in a collisional magnetized classical plasma
The results of a theoretical investigation on the low-velocity stopping power
of the ions moving in a magnetized collisional plasma are presented. The
stopping power for an ion is calculated employing linear response theory using
the dielectric function approach. The collisions, which leads to a damping of
the excitations in the plasma, is taken into account through a
number-conserving relaxation time approximation in the linear response
function. In order to highlight the effects of collisions and magnetic field we
present a comparison of our analytical and numerical results obtained for a
nonzero damping or magnetic field with those for a vanishing damping or
magnetic field. It is shown that the collisions remove the anomalous friction
obtained previously [Nersisyan et al., Phys. Rev. E 61, 7022 (2000)] for the
collisionless magnetized plasmas at low ion velocities. One of major objectives
of this study is to compare and contrast our theoretical results with those
obtained through a novel diffusion formulation based on Dufty-Berkovsky
relation evaluated in magnetized one-component plasma models framed on target
ions and electrons.Comment: Submitted to Phys. Rev. E, 17 pages, 4 figure
Completing the puzzle of the 2004-2005 outburst in V0332+53: the brightening phase included
Analysis of the data obtained with the RXTE observatory during a powerful
outburst of the X-ray pulsar V0332+53 in 2004-2005 is presented. Observational
data covering the outburst brightening phase are analysed in detail for the
first time. A comparison of source parameters and their evolution during the
brightening and fading phases shows no evidence for any hysteresis behaviour.
It is found that the dependences of the energy of the cyclotron absorption line
on the luminosity during the brightening and fading phases are almost
identical. The complete data sequence including the outburst brightening and
fading phases makes it possible to impose the more stringent constraints on the
magnetic field in the source. The pulse profile and pulsed fraction are studied
as functions of the luminosity and photon energy.Comment: 9 pages, 10 figures, accepted for publication in MNRA
Atomic wave packet dynamics in finite time-dependent optical lattices
Atomic wave packets in optical lattices which are both spatially finite and
time-dependent exhibit many striking similarities with light pulses in photonic
crystals. We analytically characterize the transmission properties of such a
potential geometry for an ideal gas in terms of a position-dependent band
structure. In particular, we find that at specific energies, wave packets at
the center of the finite lattice may be enclosed by pairs of band gaps. These
act as mirrors between which the atomic wave packet is reflected, thereby
effectively yielding a matter wave cavity. We show that long trapping times may
be obtained in such a resonator and investigate the collapse and revival
dynamics of the atomic wave packet by numerical evaluation of the Schr\"odinger
equation
Observations of Non-radial Pulsations in Radio Pulsars
We introduce a model for pulsars in which non-radial oscillations of high
spherical degree (l) aligned to the magnetic axis of a spinning neutron star
reproduce the morphological features of pulsar beams. In our model, rotation of
the pulsar carries a pattern of pulsation nodes underneath our sightline,
reproducing the longitude stationary structure seen in average pulse profiles,
while the associated time-like oscillations reproduce "drifting
subpulses"--features that change their longitude between successive pulsar
spins. We will show that the presence of nodal lines can account for observed
180 degree phase jumps in drifting subpulses and their otherwise poor phase
stability, even if the time-like oscillations are strictly periodic. Our model
can also account for the "mode changes" and "nulls" observed in some pulsars as
quasiperiodic changes between pulsation modes of different l or radial overtone
n, analogous to pulsation mode changes observed in oscillating white dwarf
stars. We will discuss other definitive and testable requirements of our model
and show that they are qualitatively supported by existing data. While
reserving judgment until the completion of quantitative tests, we are inspired
enough by the existing observational support for our model to speculate about
the excitation mechanism of the non-radial pulsations, the physics we can learn
from them, and their relationship to the period evolution of pulsars.Comment: 28 pages, 9 figures (as separate png files), Astrophysical Journal,
in pres
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