6,629 research outputs found
Linear dynamics of the solar convection zone: excitation of waves in unstably stratified shear flows
In this paper we report on the nonresonant conversion of convectively
unstable linear gravity modes into acoustic oscillation modes in shear flows.
The convectively unstable linear gravity modes can excite acoustic modes with
similar wave-numbers. The frequencies of the excited oscillations may be
qualitatively higher than the temporal variation scales of the source flow,
while the frequency spectra of the generated oscillations should be
intrinsically correlated to the velocity field of the source flow. We
anticipate that this nonresonant phenomenon can significantly contribute to the
production of sound waves in the solar convection zone.Comment: 8 pages. To appear in the proceedings of the conference "Waves in
Dusty, Solar and Space Plasmas", Leuven, Belgium 21-26 May 200
Effect of partial ionization on wave propagation in solar magnetic flux tubes
Observations show that waves are ubiquitous in the solar atmosphere and may
play an important role for plasma heating. The study of waves in the solar
corona is usually based on linear ideal magnetohydrodynamics (MHD) for a fully
ionized plasma. However, the plasma in the photosphere and the chromosphere is
only partially ionized. Here we investigate theoretically the impact of partial
ionization on MHD wave propagation in cylindrical flux tubes in the two-fluid
model. We derive the general dispersion relation that takes into account the
effects of neutral-ion collisions and the neutral gas pressure. We take the
neutral-ion collision frequency as an arbitrary parameter. Particular results
for transverse kink modes and slow magnetoacoustic modes are shown. We find
that the wave frequencies only depend on the properties of the ionized fluid
when the neutral-ion collision frequency is much lower that the wave frequency.
For high collision frequencies realistic of the solar atmosphere ions and
neutrals behave as a single fluid with an effective density corresponding to
the sum of densities of both fluids and an effective sound velocity computed as
the average of the sound velocities of ions and neutrals. The MHD wave
frequencies are modified accordingly. The neutral gas pressure can be neglected
when studying transverse kink waves but it has to be taken into account for a
consistent description of slow magnetoacoustic waves. The MHD waves are damped
due to neutral-ion collisions. The damping is most efficient when the wave
frequency and the collision frequency are of the same order of magnitude. For
high collision frequencies slow magnetoacoustic waves are more efficiently
damped than transverse kink waves. In addition, we find the presence of
cut-offs for certain combinations of parameters that cause the waves to become
non-propagating.Comment: Accepted for publication in A&
Mechanical cleaning of graphene
Contamination of graphene due to residues from nanofabrication often
introduces background doping and reduces charge carrier mobility. For samples
of high electronic quality, post-lithography cleaning treatments are therefore
needed. We report that mechanical cleaning based on contact mode AFM removes
residues and significantly improves the electronic properties. A mechanically
cleaned dual-gated bilayer graphene transistor with hBN dielectrics exhibited a
mobility of ~36,000 cm2/Vs at low temperature.Comment: 4 pages, 4 figure
XMM-Newton and INTEGRAL analysis of the Supergiant Fast X-ray Transient IGR J17354-3255
We present the results of combined INTEGRAL and XMM-Newton observations of
the supergiant fast X-ray transient (SFXT) IGR J173543255. Three XMM-Newton
observations of lengths 33.4 ks, 32.5 ks and 21.9 ks were undertaken, the first
an initial pointing to identify the correct source in the field of view and the
latter two performed around periastron. Simultaneous INTEGRAL observations
across of the orbital cycle were analysed but the source was neither
detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light curves display a
range of moderately bright X-ray activity but there are no particularly strong
flares or outbursts in any of the three observations. We show that the spectral
shape measured by XMM-Newton can be fitted by a consistent model throughout the
observation, suggesting that the observed flux variations are driven by
obscuration from a wind of varying density rather than changes in accretion
mode. The simultaneous INTEGRAL data rule out simple extrapolation of the
simple powerlaw model beyond the XMM-Newton energy range.Comment: 13 pages, 9 figures, This article has been accepted for publication
in Monthly Notices of the Royal Astronomical Society Published by Oxford
University Pres
New insights on accretion in Supergiant Fast X-ray Transients from XMM-Newton and INTEGRAL observations of IGR J175442619
XMM-Newton observations of the supergiant fast X-ray transient
IGRJ175442619 are reported and placed in the context of an analysis of
archival INTEGRAL/IBIS data that provides a refined estimate of the orbital
period at 4.92720.0004 days. A complete outburst history across the
INTEGRAL mission is reported. Although the new XMM-Newton observations (each
lasting 15 ks) targeted the peak flux in the phase-folded hard X-ray
light curve of IGRJ175442619, no bright outbursts were observed, the
source spending the majority of the exposure at intermediate luminosities of
the order of several 10ergs (0.510keV) and
displaying only low level flickering activity. For the final portion of the
exposure, the luminosity of IGRJ175442619 dropped to
410ergs (0.5 - 10 keV), comparable with the
lowest luminosities ever detected from this source, despite the observations
being taken near to periastron. We consider the possible orbital geometry of
IGRJ175442619 and the implications for the nature of the mass transfer
and accretion mechanisms for both IGRJ175442619 and the SFXT population.
We conclude that accretion under the `quasi-spherical accretion' model provides
a good description of the behaviour of IGRJ175442619, and suggest an
additional mechanism for generating outbursts based upon the mass accumulation
rate in the hot shell (atmosphere) that forms around the NS under the
quasi-spherical formulation. Hence we hope to aid in explaining the varied
outburst behaviours observed across the SFXT population with a consistent
underlying physical model.Comment: 12 pages, 5 figures, accepted for publication in MNRA
Allergic contact dermatitis from modified colophonium in wound dressings
This study concerns a 69-year-old female patient with a longstanding history of venous ulcerations on both lower legs and multiple sensitivities, who developed eczematous lesions with the hydrocolloid dressing Combiderm (Convatec Ltd., a Bristol-Myers Squibb division, Ickenham, Middlesex, UK). Epicutaneous tests were positive to this dressing and to a modified colophonium derivative, i.e. glyceryl rosinate, however not to the unmodified colophonium from the standard series. A review of the literature showed several case reports about sensitization to similar hydrocolloids being distributed under various brand names in different countries and which contain the pentaerythritol ester of the hydrogenated rosin as the tackifying agent. Some of the patients described did, while others did not, react to colophonium but only to a modified derivative. In our patient, the reaction to glyceryl rosinate most probably represent cross-sensitivity with the modified colophonium derivative used in Combiderm, the presence (but not the exact nature) of which was showed by the company. In patients where allergic contact dermatitis from hydrocolloid dressings is strongly suspected and colophonium tests negatively, patch testing to modified colophonium derivatives should therefore be performed. As the complete composition of wound dressings is most often unknown, we urgently advocate legal requirements for labelling of those and in fact all medically used devices
- …