4,886 research outputs found
The long-term evolution of the X-ray pulsar XTE J1814-338: a receding jet contribution to the quiescent optical emission?
We present a study of the quiescent optical counterpart of the Accreting
Millisecond X-ray Pulsar XTE J1814-338, carrying out multiband (BVR) orbital
phase-resolved photometry using the ESO VLT/FORS2. The optical light curves are
consistent with a sinusoidal variability modulated with the orbital period,
showing evidence for a strongly irradiated companion star, in agreement with
previous findings. The observed colours cannot be accounted for by the
companion star alone, suggesting the presence of an accretion disc during
quiescence. The system is fainter in all analysed bands compared to previous
observations. The R band light curve displays a possible phase offset with
respect to the B and V band. Through a combined fit of the multi-band light
curves we derive constraints on the companion star and disc fluxes, on the
system distance and on the companion star mass. The irradiation luminosity
required to account for the observed day-side temperature of the companion star
is consistent with the spin-down luminosity of a millisecond radio pulsar. The
flux decrease and spectral evolution of the quiescent optical emission observed
comparing our data with previous observations, collected over 5 years, cannot
be well explained with the contribution of an irradiated companion star and an
accretion disc alone. The progressive flux decrease as the system gets bluer
could be due to a continuum component evolving towards a lower, bluer spectrum.
While most of the continuum component is likely due to the disc, we do not
expect it to become bluer in quiescence. Hence we hypothesize that an
additional component, such as synchrotron emission from a jet was contributing
significantly in the earlier data obtained during quiescence and then
progressively fading or moving its break frequency toward longer wavelengths.Comment: 7 pages, 8 figures, accepted for publication in Section 7. Stellar
structure and evolution of Astronomy and Astrophysic
Stress response function of a two-dimensional ordered packing of frictional beads
We study the stress profile of an ordered two-dimensional packing of beads in
response to the application of a vertical overload localized at its top
surface. Disorder is introduced through the Coulombic friction between the
grains which gives some indeterminacy and allows the choice of one constrained
random number per grain in the calculation of the contact forces. The so-called
`multi-agent' technique we use, lets us deal with systems as large as
grains. We show that the average response profile has a double
peaked structure. At large depth , the position of these peaks grows with
, while their widths scales like . and are analogous to
`propagation' and `diffusion' coefficients. Their values depend on that of the
friction coefficient . At small , we get and , with , which means that the peaks get
closer and wider as the disorder gets larger. This behavior is qualitatively
what was predicted in a model where a stochastic relation between the stress
components is assumed.Comment: 7 pages, 7 figures, accepted version to Europhys. Let
Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface
Although the creation of spin polarization in various non-magnetic media via
electrical spin injection from a ferromagnetic tunnel contact has been
demonstrated, much of the basic behavior is heavily debated. It is reported
here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs
that local magnetostatic fields arising from interface roughness dramatically
alter and even dominate the accumulation and dynamics of spins in the
semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to
reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous
and non-collinear with the injector magnetization. The inverted Hanle effect
serves as experimental signature. This interaction needs to be taken into
account in the analysis of experimental data, particularly in extracting the
spin lifetime and its variation with different parameters (temperature, doping
concentration). It produces a broadening of the standard Hanle curve and
thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si
at room temperature it is shown that the spin lifetime is larger than
previously determined, and a new lower bound of 0.29 ns is obtained. The
results are expected to be general and occur for spins near a magnetic
interface not only in semiconductors but also in metals, organic and
carbon-based materials including graphene, and in various spintronic device
structures.Comment: Final version, with text restructured and appendices added (25 pages,
9 figures). To appear in Phys. Rev.
Pulsar Wind Nebulae in the SKA era
Neutron stars lose the bulk of their rotational energy in the form of a
pulsar wind: an ultra-relativistic outflow of predominantly electrons and
positrons. This pulsar wind significantly impacts the environment and possible
binary companion of the neutron star, and studying the resultant pulsar wind
nebulae is critical for understanding the formation of neutron stars and
millisecond pulsars, the physics of the neutron star magnetosphere, the
acceleration of leptons up to PeV energies, and how these particles impact the
interstellar medium. With the SKA1 and the SKA2, it could be possible to study
literally hundreds of PWNe in detail, critical for understanding the many open
questions in the topics listed above.Comment: Comments: 10 pages, 3 figures, to be published in: "Advancing
Astrophysics with the Square Kilometre Array", Proceedings of Science,
PoS(AASKA14
Phase segregation on electroactive self-assembled monolayers: a numerical approach for describing lateral interactions between redox centers
A numerical method is proposed in order to differentiate a random distribution from a phase segregation of redox centers on (mixed) SAMs. This approach is compared to Laviron’s interactions model and voltammetric data of nitroxylalkanethiolate SAMs
Electrical spin injection and detection in Germanium using three terminal geometry
In this letter, we report on successful electrical spin injection and
detection in \textit{n}-type germanium-on-insulator (GOI) using a
Co/Py/AlO spin injector and 3-terminal non-local measurements. We
observe an enhanced spin accumulation signal of the order of 1 meV consistent
with the sequential tunneling process via interface states in the vicinity of
the AlO/Ge interface. This spin signal is further observable up to
220 K. Moreover, the presence of a strong \textit{inverted} Hanle effect points
at the influence of random fields arising from interface roughness on the
injected spins.Comment: 4 pages, 3 figure
The double pulsar: evolutionary constraints from the system geometry
The double pulsar system PSR J0737-3039A/B is a highly relativistic double
neutron star (DNS) binary, with a 2.4-hour orbital period. The low mass of the
second-formed NS, as well the low system eccentricity and proper motion, point
to a different evolutionary scenario compared to other known DNS systems. We
describe analysis of the pulse profile shape over 6 years of observations, and
present the resulting constraints on the system geometry. We find the recycled
pulsar in this system, PSR J0737-3039A, to have a low misalignment between its
spin and orbital angular momentum axes, with a 68.3% upper limit of 6.1
degrees, assuming emission from both magnetic poles. This tight constraint
lends credence to the idea that the supernova that formed the second pulsar was
relatively symmetric, possibly involving electron-capture onto an O-Ne-Mg core.Comment: 5 page, 2 figures; To appear in the conference proceedings "40 Years
of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007, at
McGill University, Montreal, Canada. Version with full-resolution figures can
be found at http://www.phas.ubc.ca/~ferdman/ferdman.0737_geo.full.ps.gz;
typos corrected, some rewording, and references adde
Electrical and thermal spin accumulation in germanium
In this letter, we first show electrical spin injection in the germanium
conduction band at room temperature and modulate the spin signal by applying a
gate voltage to the channel. The corresponding signal modulation agrees well
with the predictions of spin diffusion models. Then by setting a temperature
gradient between germanium and the ferromagnet, we create a thermal spin
accumulation in germanium without any tunnel charge current. We show that
temperature gradients yield larger spin accumulations than pure electrical spin
injection but, due to competing microscopic effects, the thermal spin
accumulation in germanium remains surprisingly almost unchanged under the
application of a gate voltage to the channel.Comment: 7 pages, 3 figure
Electrochemical Transduction on Self-Assembled Monolayers: Are Covalent Links Essential?
Electrochemical transduction without covalent links between redox and complexant units in a complexing self-assembled monolayer has been established. The results demonstrate that transduction depends on the crown ether/ferrocene ratio and appears to be tunable
Impact of the Nanoscale Organization of Nitroxyl Mixed Self-Assembled Monolayers on their Electrocatalytic Behaviour
Electrocatalysis: The molecular distribution of redox centers on mixed nitroxyl SAMs strongly influences the electrocatalytic reactivity
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