8,689 research outputs found
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
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
Toxicological Patterns of Multicomponent Polymer Systems
Development of new polymer systems (PS) based on thermo-active resins is important not only from a chemical viewpoint but also because of the opportunity for programmable changing their properties and subsequent application in clinical practice. Some chemical features of a new composition for a multicomponent polymer system (MPS) based on hydrophilized unsaturated polyester resin (HUPR) are presented. Strength parameters of several resins of different compositions are examined. The analysis of the acute oral toxicity of these resins in white Wistar rats proves that they are low toxic. These PS do not demonstrate any harmful effects on human skin after the predicting irritation and sensitization tests using epicutaneous samples. Their main advantages include preliminary water-solubility and capacity to incorporate water, good dilution into water and resin water dispersions, porosity, low relative mass, high strength indices, improved ecological features, thin-founding capacity, good relief impressions in combination with high adhesion ability towards wet surfaces. A wide MPS usage in the manufacture of orthopedic plaster dressings, prostheses, splints, insoles, positive-sample dental imprints, and casts is recommended
Photon assisted tunneling in pairs of silicon donors
Shallow donors in silicon are favorable candidates for the implementation of solid-state quantum computer architectures because of the promising combination of atomiclike coherence properties and scalability from the semiconductor manufacturing industry. Quantum processing schemes require (among other things) controlled information transfer for readout. Here we demonstrate controlled electron tunneling at 10 K from P to Sb impurities and vice versa with the assistance of resonant terahertz photons
Particles with negative energies in black holes
The problem of the existence of particles with negative energies inside and
outside of Schwarzschild, charged and rotating black holes is investigated.
Different definitions of the energy of the particle inside the Schwarzschild
black hole are analyzed and it is shown in what cases this energy can be
negative. A comparison is made for the cases of rotating black holes described
by the Kerr metric when the energy of the particle can be negative in the
ergosphere and the Reissner-Nordstrom metric.Comment: 10 pages, 2 figures, typos correction to match published versio
Operator interpretation of resonances generated by some operator matrices
We consider the analytic continuation of the transfer function for a 2x2
matrix Hamiltonian into the unphysical sheets of the energy Riemann surface. We
construct a family of non-selfadjoint operators which reproduce certain parts
of the transfer-function spectrum including resonances situated on the
unphysical sheets neighboring the physical sheet. On this basis, completeness
and basis properties for the root vectors of the transfer function (including
those for the resonances) are proved.Comment: LaTeX, 15 pages, no figures; Contribution to Proceedings of the Mark
Krein International Conference on Operator Theory and Applications, Odessa,
August 18-22, 199
High-energy threshold reaction rates on 0.8 GeV proton-irradiated thick Pb-target
This works presents results of activation-aided determination of threshold
reaction rates in 92 209Bi, natPb, 197Au, 181Ta, 169Tm, natIn, 93Nb, 64Zn,
65Cu, 63Cu, 59Co, 19F, and 12C samples and in 121 27Al samples. All the samples
were aligned with the proton beam axis inside and outside the demountable 92-cm
thick Pb target of 15-cm diameter assembled of 23 4-cm thick discs. The samples
were placed on 12 target disks to reproduce the long axis distribution of
protons and neutrons. In June 2006, the target was exposed for 18 hours to a
800-MeV proton beam extracted from the ITEP U-10 accelerator. The proton
fluence and the proton beam shape were determined using the 27Al(p,x)7Be
monitor reaction. The reaction rates were determined by the direct
gamma-spectrometry techniques. In total, 1196 gamma-spectra have been measured,
and about 1500 reaction rates determined. The measured reaction rates were
simulated by the MCNPX code using the following databases: ENDF/B6 for neutrons
below 20 MeV, MENDL2 for 20-100 MeV neutrons, and MENDL2P for proton cross
sections up to 200 MeV. An acceptable agreement of simulations with
experimental data has been found.Comment: 4 pages, 5 figures, 3 tables, only pdf file, to be published in Proc.
Int. Conf. on Nucl. Data for Sci. and Technology (ND2007), Nice, France,
April 22-27, 200
Evidence for a Binary Companion to the Central Compact Object 1E 1207.4-5209
Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin
period of 424 ms that contains at least two strong absorption features in its
energy spectrum. This neutron star has been identified as a member of the
radio-quiet compact central objects in supernova remnants. It has been found
that 1E 1207.4-5209 is not spinning down monotonically suggesting that this
neutron star undergoes strong, frequent glitches, contains a fall-back disk, or
possess a binary companion. Here, we report on a sequence of seven XMM-Newton
observations of 1E 1207.4-5209 performed during a 40 day window in June/July
2005. Due to unanticipated variance in the phase measurements beyond the
statistical uncertainties, we could not identify a unique phase-coherent timing
solution. The three most probable timing solutions give frequency time
derivatives of +0.9, -2.6, and +1.6 X 10^(-12) Hz/s (listed in descending order
of significance). We conclude that the local frequency derivative during our
XMM-Newton observing campaign differs from the long-term spin-down rate by more
than an order of magnitude, effectively ruling out glitch models for 1E
1207.4-5209. If the long-term spin frequency variations are caused by timing
noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than
in other pulsars with similar period derivatives. Therefore, it is highly
unlikely that the spin variations are caused by the same physical process that
causes timing noise in other isolated pulsars. The most plausible scenario for
the observed spin irregularities is the presence of a binary companion to 1E
1207.4-5209. We identified a family of orbital solutions that are consistent
with our phase-connected timing solution, archival frequency measurements, and
constraints on the companions mass imposed by deep IR and optical observations.Comment: 8 pages, 4 figures. To be published in the proceedings of "Isolated
Neutron Stars: from the Interior to the Surface" (April 24-28, 2006) - eds.
D. Page, R. Turolla & S. Zan
Detection of Pulsed X-ray Emission from XMM-Newton Observations of PSR J0538+2817
We report on the XMM-Newton observations of the 143 ms pulsar PSR J0538+2817.
We present evidence for the first detections of pulsed X-rays from the source
at a frequency which is consistent with the predicted radio frequency. The
pulse profile is broad and asymmetric, with a pulse fraction of 18 +/- 3%. We
find that the spectrum of the source is well-fit with a blackbody with
T^{infty} = (2.12^{+0.04}_{-0.03}) x 10^6 K and N_{H} = 2.5 x 10^21 cm^{-2}.
The radius determined from the model fit of 1.68 +/- 0.05 km suggests that the
emission is from a heated polar cap. A fit to the spectrum with an atmospheric
model reduces the inferred temperature and hence increases the radius of the
emitting region, however the pulsar distance determined from the fit is then
smaller than the dispersion distance.Comment: 24 pages, 6 figures, 3 tables, accepted for publication in ApJ. Error
in radius calculation corrected, discussion and conclusions remain unchange
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