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 coatings—A review. J Biomed Mater Res Part A 2016:104A:2628–2641 [2] Wagener V, Virtanen S. 2016 Protective layer formation on magnesium in cell culture medium. Mater. Sci. Eng. C 63, 341–351 [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 coatings—A review. J Biomed Mater Res Part A 2016:104A:2628–2641 [2] Wagener V, Virtanen S. 2016 Protective layer formation on magnesium in cell culture medium. Mater. Sci. Eng. C 63, 341–351 [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$_{\alpha}$ line and the narrow recombination continuum by hydrogenic ions of iron at a redshift $z=1.00\pm0.02$, 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 $\approx 0.1 c$, 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