346 research outputs found
Magnetic field and unstable accretion during AM Herculis low states
A study of AM Her low states in September 1990 and 1991 and June-July 1997 is
reported from a coordinated campaign with observations obtained at the
Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical
Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory.
Spectra obtained at different dates when the source was in low states at a
comparable V magnitude, show the presence of strong Zeeman absorption features
and marked changes in emission lines with a day-to-day reappearance of the HeII
(4686\AA) emission lines in 1991. Despite this variability, the magnetic field
inferred from the fitting of the absorption spectrum with Zeeman hydrogen
splitting, is remarkably constant with a best value of (12.50.5)MG.
Detailed analysis of the UBVRI light curves shows the presence of repetitive
moderate amplitude ( 0.3-0.5 mag) flares predominantly red in colour.
These flares are attributed to small accretion events and are compared to the
large ( 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We
suggest that the general flaring activity observed during the low states is
generated by accretion events. The different characteristics of the flares
(colour and polarization) are the results of different shock geometries
depending on the net mass accretion flux.Comment: accepted in Astronomy & Astrophysics (Main Journal), 10 pages, 6
Figures, Late
The peculiar source XSS J12270-4859: a LMXB detected by FERMI ?
The X-ray source XSS J12270-4859 has been first suggested to be a magnetic
cataclysmic variable of Intermediate Polar type on the basis of its optical
spectrum and a possible 860 s X-ray periodicity. However further X-ray
observations by the Suzaku and XMM-Newton satellites did not confirm this
periodicity but show a very peculiar variability, including moderate repetitive
flares and numerous absorption dips. These characteristics together with a
suspected 4.3 h orbital period would suggest a possible link with the so-
called "dipping sources", a sub-class of Low-Mass X-ray Binaries (LMXB). Based
on the released FERMI catalogues, the source was also found coincident with a
very high energy (0.1-300 GeV) VHE source 2FGL J1227.7-4853. The good
positional coincidence, together with the lack of any other bright X-ray
sources in the field, makes this identification highly probable. However, none
of the other standard LMXBs have been so far detected by FERMI. Most galactic
VHE sources are associated with rotation-powered pulsars. We present here new
results obtained from a 30 ksec high-time resolution XMM observations in
January 2011 that confirm the flaring-dipping behaviour and provide upper
limits on fast X-ray pulsations. We discuss the possible association of the
source with either a microquasar or an accreting rotation powered pulsar.Comment: To appear in the Proceedings of "The Golden Age of Cataclysmic
Variables (Palermo 2011)", in Mem. Soc. Astron. It. (4 pages, 2 figures
Sensibilité d'Anopheles gambiae aux insecticides en Côte d'Ivoire
Des études sur la sensibilité d'#Anophles gambiae aux insecticides ont été menées dans des zones de riziculture en Côte-d'Ivoire. Les populations larvaires d'#An. gambiae de ce pays sont résistantes au DDT mais sensibles aux insecticides organophosphorés. Les populations adultes issues des environs de Bouaké sont résistantes au DDT et à la perméthrine. Elles présentent une forte présomption de résistance au propoxur. L'effet #knock-down$ de la deltaméthrine et de la lambdacyhalothrine est retardé et considérablement diminué. La souche témoin de Bobo-Dioulasso, ainsi que des populations provenant de la zone de Katiola, éloignée de la ville, sont toujours sensibles aux trois pyréthrinoïdes. La résistance aux pyréthrinoïdes dans Bouaké a peut-être été sélectionnée par l'utilisation intensive des bombes insecticides à usage domestique. (Résumé d'auteur
Analysis of dynamical tunnelling experiments with a Bose-Einstein condensate
Dynamical tunnelling is a quantum phenomenon where a classically forbidden
process occurs, that is prohibited not by energy but by another constant of
motion. The phenomenon of dynamical tunnelling has been recently observed in a
sodium Bose-Einstein condensate. We present a detailed analysis of these
experiments using numerical solutions of the three dimensional Gross-Pitaevskii
equation and the corresponding Floquet theory. We explore the parameter
dependency of the tunnelling oscillations and we move the quantum system
towards the classical limit in the experimentally accessible regime.Comment: accepted for publication in Physical Review
On the nature of the hard X-ray sources SWIFTJ1907.3-2050, IGRJ12123-5802 and IGRJ19552+0044
The INTEGRAL and Swift hard X-ray surveys have identified a large number of
new sources, among which many are proposed as Cataclysmic Variables (CVs). Here
we present the first detailed study of three X-ray selected CVs, Swift
J1907.3-2050, IGRJ12123-5802, and IGRJ19552+0044 based on XMM-Newton, Suzaku,
Swift observations and ground based optical and archival nIR/IR data. Swift
J1907.3-2050 is highly variable from hours to months-years at all wavelengths.
No coherent X-ray pulses are detected but rather transient features. The X-ray
spectrum reveals a multi-temperature optically thin plasma absorbed by complex
neutral material and a soft black body component arising from a small area.
These characteristics are remarkably similar to those observed in magnetic CVs.
A supra-solar abundance of nitrogen could arise from nuclear processed material
from the donor star. Swift J1907.3-2050 could be a peculiar magnetic CV with
the second longest (20.82 h) binary period. IGRJ12123-5802 is variable in the
X-rays on a timescale of ~7.6 h. No coherent pulsations are detected, but its
spectral characteristics suggest that it could be a magnetic CV of the
Intermediate Polar (IP) type. IGRJ19552+0044 shows two X-ray periods, 1.38 h
and 1.69 h and a X-ray spectrum characterized by a multi-temperature plasma
with little absorption.We derive a low accretion rate, consistent with a CV
below the orbital period gap. Its peculiar nIR/IR spectrum suggests a
contribution from cyclotron emission. It could either be a pre-polar or an IP
with the lowest degree of asynchronism.Comment: Accepted for publication in MNRAS. 14 pages, 11 figures, 5 table
X-ray follow-ups of XSSJ12270-4859: a low-mass X-ray binary with gamma ray FERMI-LAT association
XSSJ1227.0-4859 is a peculiar, hard X-ray source recently positionally
associated to the Fermi-LAT source 1FGLJ1227.9-4852/2FGLJ1227.7-4853.
Multi-wavelength observations have added information on this source, indicating
a low-luminosity low-mass X-ray binary (LMXB), but its nature is still unclear.
To progress in our understanding, we present new X-ray data from a monitoring
campaign performed in 2011 with the XMM-Newton, RXTE, and Swift satellites and
combine them with new gamma-ray data from the Fermi and AGILE satellites. We
complement the study with simultaneous near-UV photometry from XMM-Newton and
with previous UV/optical and near-IR data. The X-ray history of XSSJ1227.0-4859
over 7yr shows a persistent and rather stable low-luminosity (~6x10^33
d_{1\,kpc}^2 erg/s) source, with flares and dips being peculiar and permanent
characteristics. The associated Fermi-LAT source 2FGLJ1227.7-4853 is also
stable over an overlapping period of 4.7\,yr. Searches for X-ray fast
pulsations down to msec give upper limits to pulse fractional amplitudes of
15-25% that do not rule out a fast spinning pulsar. The combined
UV/optical/near-IR spectrum reveals a hot component at ~13\,kK and a cool one
at ~4.6\,kK. The latter would suggest a late-type K2-K5 companion star, a
distance range of1.4--3.6kpc and an orbital period of 7--9 h. A near-UV
variability (>6\,h) also suggests a longer orbital period than previously
estimated. The analysis shows that the X-ray and UV/optical/near-IR emissions
are more compatible with an accretion-powered compact object than with a
rotational powered pulsar. The X-ray to UV bolometric luminosity ratio could be
consistent with a binary hosting a neutron star, but the uncertainties in the
radio data may also allow an LMXB black hole with a compact jet. In this case
it would be the first associated with a high-energy gamma-ray source.Comment: 17 pages, 14 figures, 1 table; Accepted for publication in Astronomy
& Astrophysics Main Journa
Instantons revisited: dynamical tunnelling and resonant tunnelling
Starting from trace formulae for the tunnelling splittings (or decay rates)
analytically continued in the complex time domain, we obtain explicit
semiclassical expansions in terms of complex trajectories that are selected
with appropriate complex-time paths. We show how this instanton-like approach,
which takes advantage of an incomplete Wick rotation, accurately reproduces
tunnelling effects not only in the usual double-well potential but also in
situations where a pure Wick rotation is insufficient, for instance dynamical
tunnelling or resonant tunnelling. Even though only one-dimensional autonomous
Hamiltonian systems are quantitatively studied, we discuss the relevance of our
method for multidimensional and/or chaotic tunnelling
Quasi-ordered photonic structures colour the bluespotted ribbontail ray
Due to the scarcity of blue colour exhibited by natural organisms, highlighting the underlying this colour mechanisms is always very impactful for the understanding of the natural world. In this research, the colour of the blue rounded spots occurring in the skin of Taeniura lymma stingray was unveiled by a combination of experimental and numerical techniques. Our results demonstrated that this blue colour arises from coherent scattering in quasi-ordered photonic structures occurring in the skin of this stingray.</p
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