5,020 research outputs found
What if pulsars are born as strange stars?
The possibility and the implications of the idea, that pulsars are born as
strange stars, are explored. Strange stars are very likely to have atmospheres
with typical mass of but bare polar caps almost
throughout their lifetimes, if they are produced during supernova explosions. A
direct consequence of the bare polar cap is that the binding energies of both
positively and negatively charged particles at the bare quark surface are
nearly infinity, so that the vacuum polar gap sparking scenario as proposed by
Ruderman & Sutherland should operate above the cap, regardless of the sense of
the magnetic pole with respect to the rotational pole. Heat can not accumulate
on the polar cap region due to the large thermal conductivity on the bare quark
surface. We test this ``bare polar cap strange star'' (BPCSS) idea with the
present broad band emission data of pulsars, and propose several possible
criteria to distinguish BPCSSs from neutron stars.Comment: 31 pages in Latex. Accepted by AstroParticle Physic
Void Formation and Roughening in Slow Fracture
Slow crack propagation in ductile, and in certain brittle materials, appears
to take place via the nucleation of voids ahead of the crack tip due to plastic
yields, followed by the coalescence of these voids. Post mortem analysis of the
resulting fracture surfaces of ductile and brittle materials on the m-mm
and the nm scales respectively, reveals self-affine cracks with anomalous
scaling exponent in 3-dimensions and in
2-dimensions. In this paper we present an analytic theory based on the method
of iterated conformal maps aimed at modelling the void formation and the
fracture growth, culminating in estimates of the roughening exponents in
2-dimensions. In the simplest realization of the model we allow one void ahead
of the crack, and address the robustness of the roughening exponent. Next we
develop the theory further, to include two voids ahead of the crack. This
development necessitates generalizing the method of iterated conformal maps to
include doubly connected regions (maps from the annulus rather than the unit
circle). While mathematically and numerically feasible, we find that the
employment of the stress field as computed from elasticity theory becomes
questionable when more than one void is explicitly inserted into the material.
Thus further progress in this line of research calls for improved treatment of
the plastic dynamics.Comment: 15 pages, 20 figure
Nature and Nurture: a model for soft gamma-ray repeaters
During supernova explosions, strange stars with almost bare quark surfaces
may be formed. Under certain conditions, these stars could be rapidly spun down
by the torque exerted by the fossil disks formed from the fall-back materials.
They may also receive large kicks and hence, have large proper motion
velocities. When these strange stars pass through the spherical ``Oort'' comet
cloud formed during the pre-supernova era, they will capture some small-scale
comet clouds and collide with some comet-like objects occasionally. These
impacts can account for the repeating bursts as observed from the soft gamma
repeaters (SGRs). According to this picture, it is expected that SGR 1900+14
will become active again during 2004-2005.Comment: emulateapj, 5 pages, accepted by ApJ Letter
Topological regluing of rational functions
Regluing is a topological operation that helps to construct topological
models for rational functions on the boundaries of certain hyperbolic
components. It also has a holomorphic interpretation, with the flavor of
infinite dimensional Thurston--Teichm\"uller theory. We will discuss a
topological theory of regluing, and trace a direction in which a holomorphic
theory can develop.Comment: 38 page
Modeling the Broadband Spectral Energy Distribution of the Microquasars XTE J1550-564 and H 1743-322
We report results from a systematic study of the spectral energy distribution
(SED) and spectral evolution of XTE J1550--564 and H 1743--322 in outburst. The
jets of both sources have been directly imaged at both radio and X-ray
frequencies, which makes it possible to constrain the spectrum of the radiating
electrons in the jets. We modelled the observed SEDs of the jet `blobs' with
synchrotron emission alone and with synchrotron emission plus inverse Compton
scattering. The results favor a pure synchrotron origin of the observed jet
emission. Moreover, we found evidence that the shape of the electron spectral
distribution is similar for all jet `blobs' seen. Assuming that this is the
case for the jet as a whole, we then applied the synchrotron model to the radio
spectrum of the total emission and extrapolated the results to higher
frequencies. In spite of significant degeneracy in the fits, it seems clear
that, while the synchrotron radiation from the jets can account for nearly 100%
of the measured radio fluxes, it contributes little to the observed X-ray
emission, when the source is relatively bright. In this case, the X-ray
emission is most likely dominated by emission from the accretion flows. When
the source becomes fainter, however, the jet emission becomes more important,
even dominant, at X-ray energies. We also examined the spectral properties of
the sources during outbursts and the correlation between the observed radio and
X-ray variabilities. The implication of the results is discussed.Comment: 9 pages, 11 figures, MNRAS, accepted; the paper has been much
expanded (e.g., arguments strengthened, another source H 1743-322 added) and
rewritten (e.g., title changed, abstract revised); the main conclusions
remain unchange
Raman and Infra-red properties and layer dependence of the phonon dispersions in multi-layered graphene
The symmetry group analysis is applied to classify the phonon modes of
-stacked graphene layers (NSGL's) with AB- and AA-stacking, particularly
their infra-red and Raman properties. The dispersions of various phonon modes
are calculated in a multi-layer vibrational model, which is generalized from
the lattice vibrational potentials of graphene to including the inter-layer
interactions in NSGL's. The experimentally reported red shift phenomena in the
layer number dependence of the intra-layer optical C-C stretching mode
frequencies are interpreted. An interesting low frequency inter-layer optical
mode is revealed to be Raman or Infra-red active in even or odd NSGL's
respectively. Its frequency shift is sensitive to the layer number and
saturated at about 10 layers.Comment: enlarged versio
Comment on ``Scientific collaboration networks. II. Shortest paths, weighted networks, and centrality"
In this comment, we investigate a common used algorithm proposed by Newman
[M. E. J. Newman, Phys. Rev. E {\bf 64}, 016132(2001)] to calculate the
betweenness centrality for all vertices. The inaccurateness of Newman's
algorithm is pointed out and a corrected algorithm, also with O() time
complexity, is given. In addition, the comparison of calculating results for
these two algorithm aiming the protein interaction network of Yeast is shown.Comment: 3 pages, 2 tables, and 2 figure
Relation between repolarization and refractoriness in the human ventricle: Cycle length dependence and effect of procainamide
The cycle length dependence of the action potential duration and the effective refractory period of the right ventricular endocardium were investigated in 24 patients undergoing electrophysiologic studies for suspected ventricular tachycardia. The action potential duration at 90% repolarization and the effective refractory period at twice diastolic threshold strength were measured at the same catheter site at steady state cycle lengths of 350 to 600 ms. Both measurements decreased linearly with decreasing cycle length, maintaining a parallel relation. When the relation between action potential duration and effective refractory period was expressed as the effective refractory period-action potential duration difference, nearly constant values (range −12 to −15 ms) were obtained at all cycle lengths.To determine whether sodium channel blocking drugs influence the effective refractory period-action potential duration relation in humans, measurements of these two variables were obtained in 15 patients before and during the infusion of procainamide. Procainamide prolonged the action potential duration at each cycle length by a near constant amount over baseline values (p < 0.001). Procainamide also increased the effective refractory period at each cycle length but with a greater incremental increase at the shorter cycle lengths. The rate-dependent increase in the effective refractory period-action potential duration difference became significant at cycle lengths ≤400 ms; at these high rates, the effective refractory period-action potential duration difference became positive (1.6 ms, p < 0.01 compared with baseline).Thus, in the human ventricle, the action potential duration and the effective refractory period have a close relation that remains fixed over a wide range of cycle lengths. The cycle length-dependent increase in the effective refractory period relative to action potential duration induced by procainamide is consistent with use dependency of sodium channel blocking drugs observed in vitro and may be a useful marker for measuring antiarrhythmic drug activity in vivo
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