4,418 research outputs found
On the spreading layer emission in luminous accreting neutron stars
Emission of the neutron star surface potentially contains information about
its size and thus of vital importance for high energy astrophysics. In spite of
the wealth of data on the emission of luminous accreting neutron stars, the
emission of their surfaces is hard to disentangle from their time averaged
spectra. A recent X-ray transient source XTE J1701-462 has provided a unique
dataset covering the largest ever observed luminosity range for a single
source. In this paper, we extract the spectrum of the boundary layer between
the inner part of the accretion disc and the neutron star surface with the help
of maximally spectral model-independent method. We show compelling evidences
that the energy spectrum of the boundary layer stays virtually the same over
factor of 20 variations of the source luminosity. It is rather wide and cannot
be described by a single temperature blackbody spectrum, probably because of
the inhomogeneity of the boundary layer and a spread in the colour temperature.
The observed maximum colour temperature of the boundary/spreading layer
emission of kT~2.4-2.6 keV is very close to the maximum observed colour
temperature in the photospheric radius expansion X-ray bursts, which is set by
the limiting Eddington flux at the neutron star surface. Observed stability of
the boundary layer spectrum and its maximum colour temperature strongly
supports theoretical models of the boundary/spreading layers on surfaces of
luminous accreting neutron stars, which assume the presence of a region
emitting at the local Eddington limit. Variations in the luminosity in that
case lead to changes in the size of this region, but affect less the spectral
shape. Elaboration of this model will provide solid theoretical grounds for
measurements of the neutron star sizes using the emission of the
boundary/spreading layers of luminous accreting neutron stars.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Gravity in Brans-Dicke theory with Born-Infeld scalar field and the Pioneer anomaly
In this paper we discuss a model which can be considered as a generalization
of the well-known scalar-tensor Brans-Dicke theory. This model possesses an
interesting feature: due to Born-Infeld type non-linearity of the scalar field
the properties of the interaction between two test bodies depend significantly
on their masses. It is shown that the model can be interesting in view of the
Pioneer 10, 11 spacecraft anomaly.Comment: 10 pages, 1 figure, partially changed conten
Klein tunneling in carbon nanostructures: a free particle dynamics in disguise
The absence of backscattering in metallic nanotubes as well as perfect Klein
tunneling in potential barriers in graphene are the prominent electronic
characteristics of carbon nanostructures. We show that the phenomena can be
explained by a peculiar supersymmetry generated by a first order Hamiltonian
and zero order supercharge operators. Like the supersymmetry associated with
second order reflectionless finite-gap systems, it relates here the low-energy
behavior of the charge carriers with the free particle dynamics.Comment: 4 pages, 1 fig., typos correcte
Detection of Trace Amounts of Toxic Metals in Environmental Samples by Laser-excited Atomic Fluorescence Spectrometry
Results for the direct determination of trace amounts of Pb and Cd in Antarctic and Greenland ancient ice and recent snow by laser-excited atomic fluorescence spectrometry (LEAFS) are presented. The whole procedure starting from field sampling, mechanical decontamination of the samples in an ultra-clean laboratory and final analysis of the decontaminated samples is described. The measured concentrations varied in the ranges 0.1–3 pg ml^(–1) for Cd and 0.3–30 pg ml^(–1) for Pb. The results for direct analysis by LEAFS agree favourable with those obtained by isotope dilution mass spectrometry and electrothermal atomic absorption spectrometry, which require time-consuming pre-treatment and pre-concentration stages
Factorization Contributions and the Breaking of the Rule in Weak and Couplings
We compute the modified factorization contributions to the
and couplings and
demonstrate that these contributions naturally include terms
which are comparable ( to times) in magnitude to the
corresponding terms. As a consequence, we conclude that models
which treat vector meson exchange contributions to the weak conversion process
assuming such weak couplings to satisfy the rule are unlikely to be reliable.Comment: 13 pages, uses REVTEX Entire manuscript available as a ps file at
http://www.physics.adelaide.edu.au/theory/home.html . Also available via
anonymous ftp at ftp://adelphi.adelaide.edu.au/pub/theory/ADP-95-5.T172.ps To
appear in Physical Review
- …