2,890 research outputs found
On the state of low luminous accreting neutron stars
Observational appearance of a neutron star in the subsonic propeller state
which is a companion of a wind-fed mass-exchange close binary system is
discussed. During the subsonic propeller state the neutron star magnetosphere
is surrounded by a spherical quasi-static plasma envelope, which is extended
from the magnetospheric boundary up to the star accretion radius. The energy
input to the envelope due to the propeller action by the neutron star exceeds
the radiative losses and the plasma temperature in the envelope is of the order
of the free-fall temperature. Under this condition the magnetospheric boundary
is interchange stable. Nevertheless, I find that the rate of plasma penetration
from the envelope into the magnetic field of the neutron star due to diffusion
and magnetic field line reconnection processes is large enough for the
accretion power to dominate the spindown power. I show that the accretion
luminosity of the neutron star in the subsonic propeller state is 5*10**{30} -
10**{33} (dM/dt)_{15} erg/s, where dM/dt is the strength of the normal
companion stellar wind which is parametrized in terms of the maximum possible
mass accretion rate onto the neutron star magnetosphere. On this basis I
suggest that neutron stars in the subsonic propeller state are expected to be
observed as low luminous accretion-powered pulsars. The magnetospheric radius
of the neutron star in this state is determined by the strength of the stellar
wind, (dM/dt)_c, while the accretion luminosity is determined by the rate of
plasma penetration into the star magnetosphere, (dM/dt)_a, which is (dM/dt)_a
<< (dM/dt)_c. That is why the classification of the neutron star state in these
objects using the steady accretion model (i.e. setting (dM/dt)_a = (dM/dt)_c)
can lead to a mistaken conclusion.Comment: 6 pages, accepted for publication in A&
On the duration of the subsonic propeller state of neutron stars in wind-fed mass-exchange close binary systems
The condition for the subsonic propeller - accretor state transition of
neutron stars in wind-fed mass-exchange binary systems is discussed. I show
that the value of the break period, at which the neutron star change its state
to accretor, presented by Davies & Pringle (1981) is underestimated by a factor
of 7.5. The correct value is P_{\rm br} = 450 \mu_{30}^{16/21}
\dot{M}_{15}^{-5/7} (M/M_{\sun})^{-4/21} s. This result forced us to reconsider
some basic conclusions on the efficiency of the propeller spindown mechanism.Comment: 3 pages, published in A&A 368, L
Corrosion Resistance of Steel/Zinc with Silicate Nanoparticles/Polyurethane Paint Systems in NaCl Solution
Surface characteristics and corrosion behaviour of bare electrogalvanized steel coated with polymer/nano-silicate particles added to the electrogalvanizing bath were studied by scanning electron microscopy (SEM), energy dispersive spectrometer (EDXS) and electrochemical impedance spectroscopy (EIS). After applying a barrier polyurethane paint, the paint hardness, porosity, flexibility, colour, gloss, blistering and rusting degrees, and anticorrosive protective properties in 0.05 mol·L-1 NaCl solution were also evaluated. The results correlated well and, being demonstrative of the very slow deterioration rate of the immersed coated electrogalvanized steel, they enabled to assume that if a chemically analogous but thicker coating system was applied; it could be an acceptable alternative in real service conditions.Fil: Célia R. Tomachuk. Energy And Nuclearresearch Institute; BrasilFil: Elsner, Cecilia Ines. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Tecnología de Pinturas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones en Tecnología de Pinturas; ArgentinaFil: Di Sarli, Alejandro Ramón. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Tecnología de Pinturas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones en Tecnología de Pinturas; Argentin
The Anderson model of localization: a challenge for modern eigenvalue methods
We present a comparative study of the application of modern eigenvalue
algorithms to an eigenvalue problem arising in quantum physics, namely, the
computation of a few interior eigenvalues and their associated eigenvectors for
the large, sparse, real, symmetric, and indefinite matrices of the Anderson
model of localization. We compare the Lanczos algorithm in the 1987
implementation of Cullum and Willoughby with the implicitly restarted Arnoldi
method coupled with polynomial and several shift-and-invert convergence
accelerators as well as with a sparse hybrid tridiagonalization method. We
demonstrate that for our problem the Lanczos implementation is faster and more
memory efficient than the other approaches. This seemingly innocuous problem
presents a major challenge for all modern eigenvalue algorithms.Comment: 16 LaTeX pages with 3 figures include
Accretion by Isolated Neutron Stars
Accretion of interstellar material by an isolated neutron star is discussed.
The point I address here is the interaction between the accretion flow and the
stellar magnetosphere. I show that the interchange instabilities of the
magnetospheric boundary under the conditions of interest are basically
suppressed. The entry of the material into the magnetosphere is governed by
diffusion. Due to this reason the persistent accretion luminosity of isolated
neutron stars is limited to < 4E+26 erg/s. These objects can also appear as
X-ray bursters with the burst durations of about 30 minutes and repetition time
of about 1E+5 yr. This indicates that the number of the accreting isolated
neutron stars which could be observed with recent and modern X-ray missions is
a few orders of magnitude smaller than that previously estimated.Comment: 3 pages, to appear in Astrophysics and Space Science, in the
proceedings of "Isolated Neutron Stars: from the Interior to the Surface",
edited by D. Page, R. Turolla and S. Zan
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
A Zeta Function for Juggling Sequences
We give a new generalization of the Riemann zeta function to the set of b-ball juggling sequences. We develop several properties of this zeta function, noting among other things that it is rational in b−s. We provide a meromorphic continuation of the juggling zeta function to the entire complex plane (except for a countable set of singularities) and completely enumerate its zeroes. For most values of b, we are able to show that the zeroes of the b-ball zeta function are located within a critical strip, which is closely analogous to that of the Riemann zeta function
Characterization and corrosion resistance of galvanized steel/passivation composite/ polyurethane paint systems
The corrosion performance of electrogalvanised steel sheets pre-treated with a Cr3+ or Cr+6 -based conversion layer and then covered with polyurethane-waterborne topcoat paint has been studied. The pre-treated metallic panels were coated with one of the three tested polyurethane (PU) topcoat paints, in which the dispersion type was the formulation variable. The pigment was TiO2 (rutile) with a PVC value of 10. Before and after the immersion in 0.05 M NaCl (pH 5.70) or 0.1 M Na2SO4 (pH 6.36) solutions, replicates of the different samples were subjected to standardized tests (porosity, gloss and color, hardness, flexibility). During the immersion, blistering and rusting degrees were evaluated through periodical visual inspections, while the coated steel performance was monitored by Electrochemical Impedance Spectroscopy (EIS) measurements. Initial (dry) and final (wet) paint adhesion was also determined. EIS data were interpreted and discussed in terms of the time dependence of the electrical (paint coating) and electrochemical (steel substrate) parameters associated with interfacial processes describing the metal/paint system deterioration. According to the electrochemical properties, visual inspection and standardized tests results, it was concluded that the studied polyurethane-based polymeric films applied on pretreated electrogalvanised steel provided a very effective protection against corrosion as a result of their excellent barrier properties.Fil: Di Sarli, A. R.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Centro de Investigación y Desarrollo En Tecnología de Pinturas (i); Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; ArgentinaFil: Elsner, Cecilia Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Centro de Investigación y Desarrollo En Tecnología de Pinturas (i); Argentina. Universidad Nacional de La Plata. Facultad de Ingenieria; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; ArgentinaFil: Tomachuk, C. R.. Energy and Nuclear Research Institute; Brasi
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