9,563 research outputs found
Non polynomial conservation law densities generated by the symmetry operators in some hydrodynamical models
New extra series of conserved densities for the polytropic gas model and
nonlinear elasticity equation are obtained without any references to the
recursion operator or to the Lax operator formalism. Our method based on the
utilization of the symmetry operators and allows us to obtain the densities of
arbitrary homogenuity dimensions. The nonpolynomial densities with logarithmics
behaviour are presented as an example. The special attention is paid for the
singular case for which we found new non homogenious solutions
expressed in terms of the elementary functions.Comment: 11 pages, 1 figur
Spectral singularities for Non-Hermitian one-dimensional Hamiltonians: puzzles with resolution of identity
We examine the completeness of bi-orthogonal sets of eigenfunctions for
non-Hermitian Hamiltonians possessing a spectral singularity. The correct
resolutions of identity are constructed for delta like and smooth potentials.
Their form and the contribution of a spectral singularity depend on the class
of functions employed for physical states. With this specification there is no
obstruction to completeness originating from a spectral singularity.Comment: 25 pages, more refs adde
Search for the Optical Counterpart of the Vela Pulsar X-ray Nebula
Observations of the Vela pulsar region with the Chandra X-ray observatory
have revealed the fine structure of its synchrotron pulsar-wind nebula (PWN),
which showed an overall similarity with the Crab PWN. However, contrary to the
Crab, no firm detection of the Vela PWN in optical has been reported yet. To
search for the optical counterpart of the X-ray PWN, we analyzed deep optical
observations performed with different telescopes. We compared the optical
images with those obtained with the Chandra ACIS to search for extended
emission patterns which could be identified as counterparts of the X-ray nebula
elements. Although some features are seen in the optical images, we find no
correlation with the X-ray structure. Thus, we conclude that the diffuse
optical emission is more likely associated with filaments in the host Vela SNR.
The derived upper limits on the optical flux from the PWN are compatibile,
within the uncertainties, with the values expected on the basis of the
extrapolations of the X-ray data.Comment: 19 pages, 6 figures. Accepted for publication in Ap
Variability of the Vela Pulsar-wind Nebula Observed with Chandra
The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with
the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory,
taken on 2000 April 30 and November 30, reveal its complex morphology
reminiscent of that of the Crab PWN. Comparison of the two observations shows
changes up to 30% in the surface brightness of the PWN features. Some of the
PWN elements show appreciable shifts, up to a few arcseconds (about 10^{16}
cm), and/or spectral changes. To elucidate the nature of the observed
variations, further monitoring of the Vela PWN is needed.Comment: 7 pages (incl. 3 embedded PS figures), AASTEX, uses emulateapj5.sty.
Submitted to ApJ Lett. For a high-resolution color PS image of Figure 3 (6.3
Mby), see http://www.astro.psu.edu/users/divas/velaneb_fig3.p
Multicolor Photometry of the Vela Pulsar
Multicolor photometry of the Vela pulsar (PSR B0833-45), updated by recent
HST/WFPC2 observations obtained in the 555W, 675W and 814W filters, is
presented. The available data provide the best characterization so far of the
pulsar spectral shape, which is dominated by a flat power law continuum with
spectral index \alpha = -0.2 +/- 0.2, consistent with the extrapolation in the
optical domain of the power law component of the X-ray spectrum detected by
Chandra. In addition, a marginally significant dip (~ 3 sigma) seems to be
present at about 6500 AA. Spectroscopic observations with the VLT, now in
progress, will undoubtly provide a much better assessment of the reality and
characteristics of this feature.Comment: 4 pages, LaTex, 2 Postscript figures. Astronomy & Astrophysics,
accepte
Constrained Reductions of 2D dispersionless Toda Hierarchy, Hamiltonian Structure and Interface Dynamics
Finite-dimensional reductions of the 2D dispersionless Toda hierarchy,
constrained by the ``string equation'' are studied. These include solutions
determined by polynomial, rational or logarithmic functions, which are of
interest in relation to the ``Laplacian growth'' problem governing interface
dynamics. The consistency of such reductions is proved, and the Hamiltonian
structure of the reduced dynamics is derived. The Poisson structure of the
rationally reduced dispersionless Toda hierarchies is also derivedComment: 18 pages LaTex, accepted to J.Math.Phys, Significantly updated
version of the previous submissio
Space-Time Description of Scalar Particle Creation by a Homogeneous Isotropic Gravitational Field
We give the generalization of the method of the space-time description of
particle creation by a gravitational field for a scalar field with nonconformal
coupling to the curvature. The space-time correlation function is obtained for
a created pair of the quasi-particles, corresponding to a diagonal form of the
instantaneous Hamiltonian. The case of an adiabatic change of the metric of
homogeneous isotropic space is analyzed. We show that the created pairs of
quasi-particles in de Sitter space should be interpreted as pairs of virtual
particles.Comment: 7 pages, 3 figure
Polarization of Thermal X-rays from Isolated Neutron Stars
Since the opacity of a magnetized plasma depends on polarization of
radiation, the radiation emergent from atmospheres of neutron stars with strong
magnetic fields is expected to be strongly polarized. The degree of linear
polarization, typically ~10-30%, depends on photon energy, effective
temperature and magnetic field. The spectrum of polarization is more sensitive
to the magnetic field than the spectrum of intensity. Both the degree of
polarization and the position angle vary with the neutron star rotation period
so that the shape of polarization pulse profiles depends on the orientation of
the rotational and magnetic axes. Moreover, as the polarization is
substantially modified by the general relativistic effects, observations of
polarization of X-ray radiation from isolated neutron stars provide a new
method for evaluating the mass-to-radius ratio of these objects, which is
particularly important for elucidating the properties of the superdense matter
in the neutron star interiors.Comment: 7 figures, to be published in Ap
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