1,250 research outputs found
Evaluation of quasi-square wave inverter as a power source for induction motors
The relative merits of quasi-square wave inverter-motor technology versus a sine wave inverter-motor system were investigated. The empirical results of several tests on various sizes of wye-wound induction motors are presented with mathematical analysis to support the conclusions of the study. It was concluded that, within the limitations presented, the quasi-square wave inverter-motor system is superior to the more complex sine wave system for most induction motor applications in space
Uniform Approximation from Symbol Calculus on a Spherical Phase Space
We use symbol correspondence and quantum normal form theory to develop a more
general method for finding uniform asymptotic approximations. We then apply
this method to derive a result we announced in an earlier paper, namely, the
uniform approximation of the -symbol in terms of the rotation matrices. The
derivation is based on the Stratonovich-Weyl symbol correspondence between
matrix operators and functions on a spherical phase space. The resulting
approximation depends on a canonical, or area preserving, map between two pairs
of intersecting level sets on the spherical phase space.Comment: 18 pages, 5 figure
A tree-decomposed transfer matrix for computing exact Potts model partition functions for arbitrary graphs, with applications to planar graph colourings
Combining tree decomposition and transfer matrix techniques provides a very
general algorithm for computing exact partition functions of statistical models
defined on arbitrary graphs. The algorithm is particularly efficient in the
case of planar graphs. We illustrate it by computing the Potts model partition
functions and chromatic polynomials (the number of proper vertex colourings
using Q colours) for large samples of random planar graphs with up to N=100
vertices. In the latter case, our algorithm yields a sub-exponential average
running time of ~ exp(1.516 sqrt(N)), a substantial improvement over the
exponential running time ~ exp(0.245 N) provided by the hitherto best known
algorithm. We study the statistics of chromatic roots of random planar graphs
in some detail, comparing the findings with results for finite pieces of a
regular lattice.Comment: 5 pages, 3 figures. Version 2 has been substantially expanded.
Version 3 shows that the worst-case running time is sub-exponential in the
number of vertice
The X-ray outburst of the Galactic Center magnetar over six years of Chandra observations
The magnetar SGR J1745-2900 discovered at parsecs distance from the Milky Way
central black hole, Sagittarius A*, represents the closest pulsar to a
supermassive black hole ever detected. Furthermore, its intriguing radio
emission has been used to study the environment of the black hole, as well as
to derive a precise position and proper motion for this object. The discovery
of SGR J1745-2900 has opened interesting debates about the number, age and
nature of pulsars expected in the Galactic center region. In this work, we
present extensive X-ray monitoring of the outburst of SGR J1745-2900 using the
Chandra X-ray Observatory, the only instrument with the spatial resolution to
distinguish the magnetar from the supermassive black hole (2.4" angular
distance). It was monitored from its outburst onset in April 2013 until August
2019, collecting more than fifty Chandra observations for a total of more than
2.3 Ms of data. Soon after the outburst onset, the magnetar emission settled
onto a purely thermal emission state that cooled from a temperature of about
0.9 to 0.6 keV over 6 years. The pulsar timing properties showed at least two
changes in the period derivative, increasing by a factor of about 4 during the
outburst decay. We find that the long-term properties of this outburst
challenge current models for the magnetar outbursts.Comment: 11 pages, 6 figures. Accepted by Ap
The X-ray outburst of the Galactic Centre magnetar SGR J1745-2900 during the first 1.5 year
In 2013 April a new magnetar, SGR 1745-2900, was discovered as it entered an
outburst, at only 2.4 arcsec angular distance from the supermassive black hole
at the Centre of the Milky Way, Sagittarius A*. SGR 1745-2900 has a surface
dipolar magnetic field of ~ 2x10^{14} G, and it is the neutron star closest to
a black hole ever observed. The new source was detected both in the radio and
X-ray bands, with a peak X-ray luminosity L_X ~ 5x10^{35} erg s^{-1}. Here we
report on the long-term Chandra (25 observations) and XMM-Newton (8
observations) X-ray monitoring campaign of SGR 1745-2900, from the onset of the
outburst in April 2013 until September 2014. This unprecedented dataset allows
us to refine the timing properties of the source, as well as to study the
outburst spectral evolution as a function of time and rotational phase. Our
timing analysis confirms the increase in the spin period derivative by a factor
of ~2 around June 2013, and reveals that a further increase occurred between
2013 Oct 30 and 2014 Feb 21. We find that the period derivative changed from
6.6x10^{-12} s s^{-1} to 3.3x10^{-11} s s^{-1} in 1.5 yr. On the other hand,
this magnetar shows a slow flux decay compared to other magnetars and a rather
inefficient surface cooling. In particular, starquake-induced crustal cooling
models alone have difficulty in explaining the high luminosity of the source
for the first ~200 days of its outburst, and additional heating of the star
surface from currents flowing in a twisted magnetic bundle is probably playing
an important role in the outburst evolution.Comment: 16 pages, 12 figures; accepted for publication on MNRA
A strongly magnetized pulsar within grasp of the Milky Way's supermassive black hole
The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr)
A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of
intense star formation near the black hole a few Myr ago, which might have left
behind a young neutron star traveling deep into SgrA*'s gravitational
potential. On 2013 April 25, a short X-ray burst was observed from the
direction of the Galactic center. Thanks to a series of observations with the
Chandra and the Swift satellites, we pinpoint the associated magnetar at an
angular distance of 2.4+/-0.3 arcsec from SgrA*, and refine the source spin
period and its derivative (P=3.7635537(2) s and \dot{P} = 6.61(4)x10^{-12}
s/s), confirmed by quasi simultaneous radio observations performed with the
Green Bank (GBT) and Parkes antennas, which also constrain a Dispersion Measure
of DM=1750+/-50 pc cm^{-3}, the highest ever observed for a radio pulsar. We
have found that this X-ray source is a young magnetar at ~0.07-2 pc from SgrA*.
Simulations of its possible motion around SgrA* show that it is likely (~90%
probability) in a bound orbit around the black hole. The radiation front
produced by the past activity from the magnetar passing through the molecular
clouds surrounding the Galactic center region, might be responsible for a large
fraction of the light echoes observed in the Fe fluorescence features.Comment: ApJ Letters in pres
Asymptotes in SU(2) Recoupling Theory: Wigner Matrices, Symbols, and Character Localization
In this paper we employ a novel technique combining the Euler Maclaurin
formula with the saddle point approximation method to obtain the asymptotic
behavior (in the limit of large representation index ) of generic Wigner
matrix elements . We use this result to derive asymptotic
formulae for the character of an SU(2) group element and for
Wigner's symbol. Surprisingly, given that we perform five successive
layers of approximations, the asymptotic formula we obtain for is
in fact exact. This result provides a non trivial example of a
Duistermaat-Heckman like localization property for discrete sums.Comment: 36 pages, 3 figure
Rider Haggard and Rural England : Methods of social enquiry in the English countryside
Identification campaign of supernova remnant candidates in the Milky Way - I: Chandra observation of G308.3-1.4
ROSAT all-sky survey (RASS) data have provided another window to search for
supernova remnants (SNRs). In reexamining this data archive, a list of
unidentified extended X-ray objects have been suggested as promising SNR
candidate. However, most of these targets have not yet been fully explored by
the state-of-art X-ray observatories. For selecting a pilot target for a
long-term identification campaign, we have observed the brightest candidate,
G308.3-1.4, with Chandra X-ray observatory. An incomplete shell-like X-ray
structure which well-correlated with the radio shell emission at 843 MHz has
been revealed. The X-ray spectrum suggests the presence of a shock-heated
plasma. All these evidences confirm G308.3-1.4 as a SNR. The brightest X-ray
point source detected in this field-of-view is also the one locates closest to
the geometrical center of G308.3-1.4, which has a soft spectrum. The intriguing
temporal variability and the identification of optical/infrared counterpart
rule out the possibility of an isolated neutron star. On the other hand, the
spectral energy distribution from Ks band to R band suggests a late-type star.
Together with a putative periodicity of \sim1.4 hrs, the interesting excesses
in V, B bands and H-alpha suggest this source as a promising candidate of a
compact binary survived in a supernova explosion (SN).Comment: 19 pages, 10 figures, 2 tables, accepted for publication in Ap
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