1,514 research outputs found
Bounds on Compactness for LMXB Neutron Stars from X-ray Burst Oscillations
We have modelled X-ray burst oscillations observed with the Rossi X-ray
Timing Explorer (RXTE) from two low mass X-ray binaries (LMXB): 4U 1636-53 with
a frequency of 580 Hz, and 4U 1728-34 at a frequency of 363 Hz. We have
computed least squares fits to the oscillations observed during the rising
phase of bursts using a model which includes emission from either a single
circular hot spot or a pair of circular antipodal hot spots on the surface of a
neutron star. We model the spreading of the thermonuclear hot spots by assuming
that the hot spot angular size grows linearly with time. We calculate the flux
as a function of rotational phase from the hot spots and take into account
photon deflection in the relativistic gravitational field of the neutron star
assuming the exterior spacetime is the Schwarzschild metric. We find acceptable
fits with our model and we use these to place constraints on the compactness of
the neutron stars in these sources. For 4U 1636-53, in which detection of a 290
Hz sub-harmonic supports the two spot model, we find that the compactness
(i.e., mass/radius ratio) is constrained to be M/R < 0.163 at 90% confidence (G
= c = 1). This requires a relatively stiff equation of state (EOS) for the
stellar interior. For example, if the neutron star has a mass of 1.4 Msun then
its radius must be > 12.8 km. Fits using a single hot spot model are not as
highly constraining. We discuss the implications of our findings for recent
efforts to calculate the EOS of dense nucleon matter and the structure of
neutron stars.Comment: 27 pages, 6 figures, AASTeX. Revised and expanded version.
Resubmitted to Astrophysical Journa
Interferometric measurement of resonance transition wavelengths in C IV, Si IV, Al III, Al II, and Si II
We have made the first interferomeric measurements of the wavelengths of the
important ultraviolet diagnostic lines in the spectra \ion{C}{4} near 155 nm
and \ion{Si}{4} near 139 nm with a vacuum ultraviolet Fourier transform
spectrometer and high-current discharge sources. The wavelength uncertainties
were reduced by one order of magnitude for the \ion{C}{4} lines and by two
orders of magnitude for the \ion{Si}{4} lines. Our measurements also provide
accurate wavelengths for resonance transitions in \ion{Al}{3}, \ion{Al}{2}, and
\ion{Si}{2}.Comment: 6 pages, 2 figures, 1 tabl
XMM-Newton observation of the long-period polar V1309 Ori: The case for pure blobby accretion
Using XMM-Newton we have obtained the first X-ray observation covering a
complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is
dominated by a short, bright phase interval with EPIC pn count rates reaching
up to 15 cts/sec per 30 sec resolution bin. The bright phase emission is well
described by a single blackbody component with kT_bb = (45 +- 3) eV. The
absence of a bremsstrahlung component at photon energies above 1 keV yields a
flux ratio F_bb/F_br > 6700. This represents the most extreme case of a soft
X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray
emission is subdivided into a series of individual flare events supporting the
hypothesis that the soft X-ray excess in V1309 is caused by accretion of dense
blobs. In addition to the bright phase emission, a faint, hard X-ray component
is visible throughout the binary orbit with an almost constant count rate of
0.01 cts/sec. Spectral modelling indicates that this emission originates from a
complex multi-temperature plasma. At least three components of an optically
thin plasma with temperatures kT= 0.065, 0.7, and 2.9 keV are required to fit
the observed flux distribution. The faint phase emission is occulted during the
optical eclipse. Eclipse ingress lasts about 15--20 min and is substantially
prolonged beyond nominal ingress of the white dwarf. This and the comparatively
low plasma temperature provide strong evidence that the faint-phase emission is
not thermal bremsstrahlung from a post-shock accretion column above the white
dwarf. A large fraction of the softer faint-phase emission could be explained
by scattering of photons from the blackbody component in the infalling material
above the accretion region. The remaining hard X-ray flux could be produced in
the coupling region, so far unseen in other polars.Comment: 10 pages, 5 figures, A&A publishe
Voltage gated inter-cation selective ion channels from graphene nanopores
With the ability to selectively control ionic flux, biological protein ion
channels perform a fundamental role in many physiological processes. For
practical applications that require the functionality of a biological ion
channel, graphene provides a promising solid-state alternative, due to its
atomic thinness and mechanical strength. Here, we demonstrate that nanopores
introduced into graphene membranes, as large as 50 nm in diameter, exhibit
inter-cation selectivity with a ~20x preference for K+ over divalent cations
and can be modulated by an applied gate voltage. Liquid atomic force microscopy
of the graphene devices reveals surface nanobubbles near the pore to be
responsible for the observed selective behavior. Molecular dynamics simulations
indicate that translocation of ions across the pore likely occurs via a thin
water layer at the edge of the pore and the nanobubble. Our results demonstrate
a significant improvement in the inter-cation selectivity displayed by a
solid-state nanopore device and by utilizing the pores in a de-wetted state,
offers an approach to fabricating selective graphene membranes that does not
rely on the fabrication of sub-nm pores
XMM-Newton observation of the 5.25 ms transient millisecond pulsar XTE J1807-294 in outburst
We report on the results obtained for the millisecond transient X-ray pulsar
XTE J1807-294 in a 40 min orbital period system, based on an XMM-Newton ToO
observation carried out during March 2003. The source was found at a luminosity
level of about 2x10^36 erg/s in the 0.5-10 keV range (assuming a distance of 8
kpc). We confirm the presence of the 5.25 ms pulsations (after accounting for
the orbital modulation) and find a pulsed fraction of 5.8% in the 0.3-10 keV
band. The pulse shape in nearly sinusoidal. The spectral continuum of the
source is well fitted by an absorbed Comptonization model plus a soft
component. No emission or absorption lines have been detected in the 0.5-10 keV
range with upper limits of 10-40 eV. The reported analysis represent the first
detailed study of this source, the fourth belonging to the ultra-compact binary
system class hosting an accreting neutron star.Comment: Accepted for publication in ApJ
In vitro characterization of a biodegradable chitosan/bioactive glass coating for Mg alloys
Magnesium and its alloys have already been proposed for biomedical applications in 1878. However, up to date no extended, successful medical Mg product is commercially available. The drawbacks of permanent implants, like stress-shielding or possible release of metal ions through wear, can be avoided with the use of biodegradable metals. Temporary implants as such could make a second surgical process to remove the implant unnecessary, not only decreasing the healthcare costs and associated risks of a surgery, but also reducing the trauma to the patient.
Mg is an abundant cation in the human body and in part physiologically beneficial as the surrounding tissue can absorb and consume the ions. The main problems related to the usage of Mg and its alloys is its high chemical reactivity, a related low corrosion resistance, especially in chloride-containing environments and the accompanying fast hydrogen gas production. [1]
In order to overcome these problems in this study a coating of a natural polymer/bioactive glass composite is applied using electrophoretic deposition. Additionally, functional properties like drug delivery characteristics and antibacterial capacity are added to these coatings.
As natural, cationic polymer chitosan is taken which is the supporting material in the exoskeleton of crustaceans and insects (crab, butterfly) and in cell walls of fungi. It combines biodegradability and biocompatibility with the ability to promote cell adhesion. [1]
To avoid dissolution of the Mg alloy substrate during the deposition, a pretreatment is used. The immersion in DMEM for 24 h is increasing the corrosion resistance to a level that the acidic, aqueous electrolyte during deposition is not corroding the Mg substrate. [2] A comparative study was performed on replacing part of the bioactive glass as ceramic part with silica particles in order to maintain a topography during dissolution of the glass. A constant solid content of 1 g/l was chosen, with 0.5 g/l chitosan in 1 vol% acetic acid, 20 vol% water and 79 vol% ethanol following previous studies. [3] For the cathodic deposition process 0.5 cm electrode distance with stainless steel as the counter electrode was used. The deposition was performed under constant current (50 V) and constant voltage (35 mA) with varying processing times.
[1] Heise S, Virtanen S, Boccaccini AR. 2016. Tackling Mg alloy corrosion by natural polymer coatingsâA review. J Biomed Mater Res Part A 2016:104A:2628â2641
[2] Wagener V, Virtanen S. 2016 Protective layer formation on magnesium in cell culture medium. Mater. Sci. Eng. C 63, 341â351
[3] Cordero-Arias, L. et al. 2013 Electrophoretic deposition of nanostructured-TiO2/Chitosan composite coatings on stainless steel. R. Soc. Chem. 3, 11247-11254
Acknowledgements: This study is supported by the German Science Foundation (DFG)
DEVELOPMENT OF A BIODEGRADABLE NATURAL POLYMER/CERAMIC COATING FOR MG ALLOYS USING ELECTROPHORETIC DEPOSITION
Magnesium and its alloys have already been proposed for biomedical applications in 1878. However, up to date no extended, successful medical Mg product is commercially available. The drawbacks of permanent implants, like stress-shielding or possible release of metal ions through wear, can be avoided with the use of biodegradable metals. Temporary implants as such could make a second surgical process to remove the implant unnecessary, not only decreasing the healthcare costs and associated risks of a surgery, but also reducing the trauma to the patient.
Mg is an abundant cation in the human body and in part physiologically beneficial as the surrounding tissue can absorb and consume the ions. The main problems related to the usage of Mg and its alloys is its high chemical reactivity, a related low corrosion resistance, especially in chloride-containing environments and the accompanying fast hydrogen gas production. [1]
In order to overcome these problems in this study a coating of a natural polymer/bioactive glass composite is applied using electrophoretic deposition. Additionally, functional properties like drug delivery characteristics and antibacterial capacity are added to these coatings.
As natural, cationic polymer chitosan is taken which is the supporting material in the exoskeleton of crustaceans and insects (crab, butterfly) and in cell walls of fungi. It combines biodegradability and biocompatibility with the ability to promote cell adhesion. [1]
To avoid dissolution of the Mg alloy substrate during the deposition, a pretreatment is used. The immersion in DMEM for 24 h is increasing the corrosion resistance to a level that the acidic, aqueous electrolyte during deposition is not corroding the Mg substrate. [2] A comparative study was performed on replacing part of the bioactive glass as ceramic part with silica particles in order to maintain a topography during dissolution of the glass. A constant solid content of 1 g/l was chosen, with 0.5 g/l chitosan in 1 vol% acetic acid, 20 vol% water and 79 vol% ethanol following previous studies. [3] For the cathodic deposition process 0.5 cm electrode distance with stainless steel as the counter electrode was used. The deposition was performed under constant current (50 V) and constant voltage (35 mA) with varying processing times.
[1] Heise S, Virtanen S, Boccaccini AR. 2016. Tackling Mg alloy corrosion by natural polymer coatingsâA review. J Biomed Mater Res Part A 2016:104A:2628â2641
[2] Wagener V, Virtanen S. 2016 Protective layer formation on magnesium in cell culture medium. Mater. Sci. Eng. C 63, 341â351
[3] Cordero-Arias, L. et al. 2013 Electrophoretic deposition of nanostructured-TiO2/Chitosan composite coatings on stainless steel. R. Soc. Chem. 3, 11247-11254
Acknowledgements: This study is supported by the German Science Foundation (DFG)
Observation of X-ray lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor
We report on the discovery of two emission features observed in the X-ray
spectrum of the afterglow of the gamma-ray burst (GRB) of 16 Dec. 1999 by the
Chandra X-Ray Observatory. These features are identified with the Ly
line and the narrow recombination continuum by hydrogenic ions of iron at a
redshift , providing an unambiguous measurement of the distance
of a GRB. Line width and intensity imply that the progenitor of the GRB was a
massive star system that ejected, before the GRB event, \approx 0.01 \Ms of
iron at a velocity , probably by a supernova explosion.Comment: 11 pages,2 fig.s, link to the published paper in Science, 290, 955
(2000) through http://www.ias.rm.cnr.it/grb/gb991216.htm
Recurrent very-long type-I X-ray bursts in the low-mass X-ray binary 4U 1636-53
Two flares with a duration of several hours are reported for the low-mass
X-ray binary 4U 1636-53. The characteristics of these flares (i.e., decay time
scales, spectral softening, fluences) are very similar to the very long type-I
X-ray bursts recently found in several other low-mass X-ray binaries,
suggesting that the flares in 4U 1636-53 are also very long type-I X-ray
bursts. This would make this source the fifth to exhibit this phenomenon and
the first one for which multiple bursts have been found. Interestingly, all
five sources accrete at approximately 10% of the Eddington mass accretion rate.
Although a chance coincidence or a selection effect cannot be ruled-out at
present, this correlation is suggestive and might indicate that only at a
narrow range of mass accretion rate such very long type-I X-ray bursts can
occur.Comment: Accepted for publication in ApJ Letter
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